threads / patch / 57461

patch, 5 partsfetch: avoid lookup of commits when not appending to FETCH_HEAD

Subject: [PATCH 2/5] fetch: avoid lookup of commits when not appending to FETCH_HEAD

## tl;dr

20 messages between Feb 23, 2022 and Mar 2, 2022. Diffs are folded; open one to read it.

replies: 19people: 3as markdown or json

Patrick Steinhardt· Feb 23, 2022, 12:35 UTC · lore

[PATCH 0/5] fetch: more optimizations for mirror fetches

Hi,

this is another patch series with the aim to speed up mirror fetches. It applies on top of e6ebfd0e8c (The sixth batch, 2022-02-18) with 3824153b23 (Merge branch 'ps/fetch-atomic' into next, 2022-02-18) merged into it to fix a conflict.

The patch series is structured as follows:
    - Patch 1: Another conversion to use the commit-graph, this time to
      look up "want" and "want-ref" lines in git-upload-pack(1).
    - Patch 2: This skips useless commit lookups when we don't write
      FETCH_HEAD. These were only used to compute the correct order in
      which we append to FETCH_HEAD.
    - Patch 3-5: This optimizes the way we look up symbolic refs in the
      files backend. Currently we always search the packed-refs file,
      which cannot ever contain symbolic references.

In total, this patch series brings down the time it takes to replicate one of our biggest repos in numbers of references from 75 seconds to 60 seconds.

Patrick
Patrick Steinhardt (5):
  upload-pack: look up "want" lines via commit-graph
  fetch: avoid lookup of commits when not appending to FETCH_HEAD
  refs: add ability for backends to special-case reading of symbolic
    refs
  remote: read symbolic refs via `refs_read_symbolic_ref()`
  refs/files-backend: optimize reading of symbolic refs
 builtin/fetch.c       | 42 +++++++++++++++++++++++++++---------------
 builtin/remote.c      |  8 +++++---
 refs.c                | 17 +++++++++++++++++
 refs.h                |  3 +++
 refs/debug.c          |  1 +
 refs/files-backend.c  | 33 ++++++++++++++++++++++++++++-----
 refs/packed-backend.c |  1 +
 refs/refs-internal.h  | 16 ++++++++++++++++
 remote.c              | 14 +++++++-------
 upload-pack.c         | 20 +++++++++++++++++---
 10 files changed, 122 insertions(+), 33 deletions(-)
-- 
2.35.1
Patrick Steinhardt· Feb 23, 2022, 12:35 UTC · re: Patrick Steinhardt · lore

When fetching from a remote repository we will by default write what has been fetched into the special FETCH_HEAD reference. The order in which references are written depends on whether the reference is for merge or not, which, despite some other conditions, is also determined based on whether the old object ID the reference is being updated from actually exists in the repository.

To write FETCH_HEAD we thus loop through all references thrice: once for the references that are about to be merged, once for the references that are not for merge, and finally for all references that are ignored. For every iteration, we then look up the old object ID to determine whether the referenced object exists so that we can label it as "not-for-merge" if it doesn't exist. It goes without saying that this can be expensive in case where we are fetching a lot of references.

While this is hard to avoid in the case where we're writing FETCH_HEAD, users can in fact ask us to skip this work via `--no-write-fetch-head`. In that case, we do not care for the result of those lookups at all because we don't have to order writes to FETCH_HEAD in the first place.

Skip this busywork in case we're not writing to FETCH_HEAD. The following benchmark performs a mirror-fetch in a repository with about two million references:

    Benchmark 1: git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD~)
      Time (mean ± σ):     75.388 s ±  1.942 s    [User: 71.103 s, System: 8.953 s]
      Range (min … max):   73.184 s … 76.845 s    3 runs
    Benchmark 2: git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD)
      Time (mean ± σ):     69.486 s ±  1.016 s    [User: 65.941 s, System: 8.806 s]
      Range (min … max):   68.864 s … 70.659 s    3 runs
    Summary
      'git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD)' ran
        1.08 ± 0.03 times faster than 'git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD~)'
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 builtin/fetch.c | 42 +++++++++++++++++++++++++++---------------
 1 file changed, 27 insertions(+), 15 deletions(-)
Show changes to builtin/fetch.c +27 −15
diff --git a/builtin/fetch.c b/builtin/fetch.c
index e8305b6662..4d12c2fd4d 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -1146,7 +1146,6 @@ static int store_updated_refs(const char *raw_url, const char *remote_name,
 	     want_status <= FETCH_HEAD_IGNORE;
 	     want_status++) {
 		for (rm = ref_map; rm; rm = rm->next) {
-			struct commit *commit = NULL;
 			struct ref *ref = NULL;
 
 			if (rm->status == REF_STATUS_REJECT_SHALLOW) {
@@ -1157,21 +1156,34 @@ static int store_updated_refs(const char *raw_url, const char *remote_name,
 			}
 
 			/*
-			 * References in "refs/tags/" are often going to point
-			 * to annotated tags, which are not part of the
-			 * commit-graph. We thus only try to look up refs in
-			 * the graph which are not in that namespace to not
-			 * regress performance in repositories with many
-			 * annotated tags.
+			 * When writing FETCH_HEAD we need to determine whether
+			 * we already have the commit or not. If not, then the
+			 * reference is not for merge and needs to be written
+			 * to the reflog after other commits which we already
+			 * have. We're not interested in this property though
+			 * in case FETCH_HEAD is not to be updated, so we can
+			 * skip the classification in that case.
 			 */
-			if (!starts_with(rm->name, "refs/tags/"))
-				commit = lookup_commit_in_graph(the_repository, &rm->old_oid);
-			if (!commit) {
-				commit = lookup_commit_reference_gently(the_repository,
-									&rm->old_oid,
-									1);
-				if (!commit)
-					rm->fetch_head_status = FETCH_HEAD_NOT_FOR_MERGE;
+			if (fetch_head->fp) {
+				struct commit *commit = NULL;
+
+				/*
+				 * References in "refs/tags/" are often going to point
+				 * to annotated tags, which are not part of the
+				 * commit-graph. We thus only try to look up refs in
+				 * the graph which are not in that namespace to not
+				 * regress performance in repositories with many
+				 * annotated tags.
+				 */
+				if (!starts_with(rm->name, "refs/tags/"))
+					commit = lookup_commit_in_graph(the_repository, &rm->old_oid);
+				if (!commit) {
+					commit = lookup_commit_reference_gently(the_repository,
+										&rm->old_oid,
+										1);
+					if (!commit)
+						rm->fetch_head_status = FETCH_HEAD_NOT_FOR_MERGE;
+				}
 			}
 
 			if (rm->fetch_head_status != want_status)

-- 
2.35.1
Derrick Stolee· Feb 23, 2022, 14:18 UTC · re: Patrick Steinhardt · lore

Re: [PATCH 2/5] fetch: avoid lookup of commits when not appending to FETCH_HEAD

On 2/23/2022 7:35 AM, Patrick Steinhardt wrote:
Show 35 quoted lines
> When fetching from a remote repository we will by default write what has
> been fetched into the special FETCH_HEAD reference. The order in which
> references are written depends on whether the reference is for merge or
> not, which, despite some other conditions, is also determined based on
> whether the old object ID the reference is being updated from actually
> exists in the repository.
> 
> To write FETCH_HEAD we thus loop through all references thrice: once for
> the references that are about to be merged, once for the references that
> are not for merge, and finally for all references that are ignored. For
> every iteration, we then look up the old object ID to determine whether
> the referenced object exists so that we can label it as "not-for-merge"
> if it doesn't exist. It goes without saying that this can be expensive
> in case where we are fetching a lot of references.
> 
> While this is hard to avoid in the case where we're writing FETCH_HEAD,
> users can in fact ask us to skip this work via `--no-write-fetch-head`.
> In that case, we do not care for the result of those lookups at all
> because we don't have to order writes to FETCH_HEAD in the first place.
> 
> Skip this busywork in case we're not writing to FETCH_HEAD. The
> following benchmark performs a mirror-fetch in a repository with about
> two million references:
> 
>     Benchmark 1: git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD~)
>       Time (mean ± σ):     75.388 s ±  1.942 s    [User: 71.103 s, System: 8.953 s]
>       Range (min … max):   73.184 s … 76.845 s    3 runs
> 
>     Benchmark 2: git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD)
>       Time (mean ± σ):     69.486 s ±  1.016 s    [User: 65.941 s, System: 8.806 s]
>       Range (min … max):   68.864 s … 70.659 s    3 runs
> 
>     Summary
>       'git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD)' ran
>         1.08 ± 0.03 times faster than 'git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD~)'
I have a super-small nitpick here.

I see that you are using '-n' to name your experiments. These names are a bit long, especially since they are the same Git command but built at different commits. It would be enough to say the command you are testing before the stats and leave the names as "HEAD" and "HEAD~" (or, I typically use "new" and "old", respectively).

Show 44 quoted lines
>  			/*
> -			 * References in "refs/tags/" are often going to point
> -			 * to annotated tags, which are not part of the
> -			 * commit-graph. We thus only try to look up refs in
> -			 * the graph which are not in that namespace to not
> -			 * regress performance in repositories with many
> -			 * annotated tags.
> +			 * When writing FETCH_HEAD we need to determine whether
> +			 * we already have the commit or not. If not, then the
> +			 * reference is not for merge and needs to be written
> +			 * to the reflog after other commits which we already
> +			 * have. We're not interested in this property though
> +			 * in case FETCH_HEAD is not to be updated, so we can
> +			 * skip the classification in that case.
>  			 */
> -			if (!starts_with(rm->name, "refs/tags/"))
> -				commit = lookup_commit_in_graph(the_repository, &rm->old_oid);
> -			if (!commit) {
> -				commit = lookup_commit_reference_gently(the_repository,
> -									&rm->old_oid,
> -									1);
> -				if (!commit)
> -					rm->fetch_head_status = FETCH_HEAD_NOT_FOR_MERGE;
> +			if (fetch_head->fp) {
> +				struct commit *commit = NULL;
> +
> +				/*
> +				 * References in "refs/tags/" are often going to point
> +				 * to annotated tags, which are not part of the
> +				 * commit-graph. We thus only try to look up refs in
> +				 * the graph which are not in that namespace to not
> +				 * regress performance in repositories with many
> +				 * annotated tags.
> +				 */
> +				if (!starts_with(rm->name, "refs/tags/"))
> +					commit = lookup_commit_in_graph(the_repository, &rm->old_oid);
> +				if (!commit) {
> +					commit = lookup_commit_reference_gently(the_repository,
> +										&rm->old_oid,
> +										1);
> +					if (!commit)
> +						rm->fetch_head_status = FETCH_HEAD_NOT_FOR_MERGE;
> +				}
>  			}
Looks good. Most of the diff is whitespace.

Thanks, -Stolee

Patrick Steinhardt· Mar 1, 2022, 08:44 UTC · re: Derrick Stolee · lore

Re: [PATCH 2/5] fetch: avoid lookup of commits when not appending to FETCH_HEAD

On Wed, Feb 23, 2022 at 09:18:05AM -0500, Derrick Stolee wrote:
Show 44 quoted lines
> On 2/23/2022 7:35 AM, Patrick Steinhardt wrote:
> > When fetching from a remote repository we will by default write what has
> > been fetched into the special FETCH_HEAD reference. The order in which
> > references are written depends on whether the reference is for merge or
> > not, which, despite some other conditions, is also determined based on
> > whether the old object ID the reference is being updated from actually
> > exists in the repository.
> > 
> > To write FETCH_HEAD we thus loop through all references thrice: once for
> > the references that are about to be merged, once for the references that
> > are not for merge, and finally for all references that are ignored. For
> > every iteration, we then look up the old object ID to determine whether
> > the referenced object exists so that we can label it as "not-for-merge"
> > if it doesn't exist. It goes without saying that this can be expensive
> > in case where we are fetching a lot of references.
> > 
> > While this is hard to avoid in the case where we're writing FETCH_HEAD,
> > users can in fact ask us to skip this work via `--no-write-fetch-head`.
> > In that case, we do not care for the result of those lookups at all
> > because we don't have to order writes to FETCH_HEAD in the first place.
> > 
> > Skip this busywork in case we're not writing to FETCH_HEAD. The
> > following benchmark performs a mirror-fetch in a repository with about
> > two million references:
> > 
> >     Benchmark 1: git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD~)
> >       Time (mean ± σ):     75.388 s ±  1.942 s    [User: 71.103 s, System: 8.953 s]
> >       Range (min … max):   73.184 s … 76.845 s    3 runs
> > 
> >     Benchmark 2: git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD)
> >       Time (mean ± σ):     69.486 s ±  1.016 s    [User: 65.941 s, System: 8.806 s]
> >       Range (min … max):   68.864 s … 70.659 s    3 runs
> > 
> >     Summary
> >       'git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD)' ran
> >         1.08 ± 0.03 times faster than 'git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD~)'
> 
> I have a super-small nitpick here.
> 
> I see that you are using '-n' to name your experiments. These names
> are a bit long, especially since they are the same Git command but
> built at different commits. It would be enough to say the command
> you are testing before the stats and leave the names as "HEAD" and
> "HEAD~" (or, I typically use "new" and "old", respectively).
Fair enough, will change.
Patrick
Show 48 quoted lines
> >  			/*
> > -			 * References in "refs/tags/" are often going to point
> > -			 * to annotated tags, which are not part of the
> > -			 * commit-graph. We thus only try to look up refs in
> > -			 * the graph which are not in that namespace to not
> > -			 * regress performance in repositories with many
> > -			 * annotated tags.
> > +			 * When writing FETCH_HEAD we need to determine whether
> > +			 * we already have the commit or not. If not, then the
> > +			 * reference is not for merge and needs to be written
> > +			 * to the reflog after other commits which we already
> > +			 * have. We're not interested in this property though
> > +			 * in case FETCH_HEAD is not to be updated, so we can
> > +			 * skip the classification in that case.
> >  			 */
> > -			if (!starts_with(rm->name, "refs/tags/"))
> > -				commit = lookup_commit_in_graph(the_repository, &rm->old_oid);
> > -			if (!commit) {
> > -				commit = lookup_commit_reference_gently(the_repository,
> > -									&rm->old_oid,
> > -									1);
> > -				if (!commit)
> > -					rm->fetch_head_status = FETCH_HEAD_NOT_FOR_MERGE;
> > +			if (fetch_head->fp) {
> > +				struct commit *commit = NULL;
> > +
> > +				/*
> > +				 * References in "refs/tags/" are often going to point
> > +				 * to annotated tags, which are not part of the
> > +				 * commit-graph. We thus only try to look up refs in
> > +				 * the graph which are not in that namespace to not
> > +				 * regress performance in repositories with many
> > +				 * annotated tags.
> > +				 */
> > +				if (!starts_with(rm->name, "refs/tags/"))
> > +					commit = lookup_commit_in_graph(the_repository, &rm->old_oid);
> > +				if (!commit) {
> > +					commit = lookup_commit_reference_gently(the_repository,
> > +										&rm->old_oid,
> > +										1);
> > +					if (!commit)
> > +						rm->fetch_head_status = FETCH_HEAD_NOT_FOR_MERGE;
> > +				}
> >  			}
> Looks good. Most of the diff is whitespace.
> 
> Thanks,
> -Stolee
Patrick Steinhardt· Feb 23, 2022, 12:35 UTC · re: Patrick Steinhardt · lore

[PATCH 1/5] upload-pack: look up "want" lines via commit-graph

During packfile negotiation the client will send "want" and "want-ref" lines to the server to tell it which objects it is interested in. The server-side parses each of those and looks them up to see whether it actually has requested objects. This lookup is performed by calling `parse_object()` directly, which thus hits the object database. In the general case though most of the objects the client requests will be commits. We can thus try to look up the object via the commit-graph opportunistically, which is much faster than doing the same via the object database.

Refactor parsing of both "want" and "want-ref" lines to do so.

The following benchmark is executed in a repository with a huge number of references. It uses cached request from git-fetch(1) as input and contains about 876,000 "want" lines:

    Benchmark 1: git-upload-pack (HEAD~)
      Time (mean ± σ):      7.113 s ±  0.028 s    [User: 6.900 s, System: 0.662 s]
      Range (min … max):    7.072 s …  7.168 s    10 runs
    Benchmark 2: git-upload-pack (HEAD)
      Time (mean ± σ):      6.622 s ±  0.061 s    [User: 6.452 s, System: 0.650 s]
      Range (min … max):    6.535 s …  6.727 s    10 runs
    Summary
      'git-upload-pack (HEAD)' ran
        1.07 ± 0.01 times faster than 'git-upload-pack (HEAD~)'
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 upload-pack.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)
Show changes to upload-pack.c +17 −3
diff --git a/upload-pack.c b/upload-pack.c
index 8acc98741b..3a851b3606 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -1400,13 +1400,19 @@ static int parse_want(struct packet_writer *writer, const char *line,
 	const char *arg;
 	if (skip_prefix(line, "want ", &arg)) {
 		struct object_id oid;
+		struct commit *commit;
 		struct object *o;
 
 		if (get_oid_hex(arg, &oid))
 			die("git upload-pack: protocol error, "
 			    "expected to get oid, not '%s'", line);
 
-		o = parse_object(the_repository, &oid);
+		commit = lookup_commit_in_graph(the_repository, &oid);
+		if (commit)
+			o = &commit->object;
+		else
+			o = parse_object(the_repository, &oid);
+
 		if (!o) {
 			packet_writer_error(writer,
 					    "upload-pack: not our ref %s",
@@ -1434,7 +1440,7 @@ static int parse_want_ref(struct packet_writer *writer, const char *line,
 	if (skip_prefix(line, "want-ref ", &refname_nons)) {
 		struct object_id oid;
 		struct string_list_item *item;
-		struct object *o;
+		struct object *o = NULL;
 		struct strbuf refname = STRBUF_INIT;
 
 		strbuf_addf(&refname, "%s%s", get_git_namespace(), refname_nons);
@@ -1448,7 +1454,15 @@ static int parse_want_ref(struct packet_writer *writer, const char *line,
 		item = string_list_append(wanted_refs, refname_nons);
 		item->util = oiddup(&oid);
 
-		o = parse_object_or_die(&oid, refname_nons);
+		if (!starts_with(refname_nons, "refs/tags/")) {
+			struct commit *commit = lookup_commit_in_graph(the_repository, &oid);
+			if (commit)
+				o = &commit->object;
+		}
+
+		if (!o)
+			o = parse_object_or_die(&oid, refname_nons);
+
 		if (!(o->flags & WANTED)) {
 			o->flags |= WANTED;
 			add_object_array(o, NULL, want_obj);

-- 
2.35.1
Derrick Stolee· Feb 23, 2022, 14:13 UTC · re: Patrick Steinhardt · lore

Re: [PATCH 1/5] upload-pack: look up "want" lines via commit-graph

On 2/23/2022 7:35 AM, Patrick Steinhardt wrote:
Show 15 quoted lines
> The following benchmark is executed in a repository with a huge number
> of references. It uses cached request from git-fetch(1) as input and
> contains about 876,000 "want" lines:
> 
>     Benchmark 1: git-upload-pack (HEAD~)
>       Time (mean ± σ):      7.113 s ±  0.028 s    [User: 6.900 s, System: 0.662 s]
>       Range (min … max):    7.072 s …  7.168 s    10 runs
> 
>     Benchmark 2: git-upload-pack (HEAD)
>       Time (mean ± σ):      6.622 s ±  0.061 s    [User: 6.452 s, System: 0.650 s]
>       Range (min … max):    6.535 s …  6.727 s    10 runs
> 
>     Summary
>       'git-upload-pack (HEAD)' ran
>         1.07 ± 0.01 times faster than 'git-upload-pack (HEAD~)'
Nice!
Show 7 quoted lines
> -		o = parse_object(the_repository, &oid);
> +		commit = lookup_commit_in_graph(the_repository, &oid);
> +		if (commit)
> +			o = &commit->object;
> +		else
> +			o = parse_object(the_repository, &oid);
> +

This is a neat trick. I see that we've also done this trick in revision.c:get_reference(). Perhaps it is worth creating a helper, maybe named parse_probably_commit()?

Show 26 quoted lines
>  		if (!o) {
>  			packet_writer_error(writer,
>  					    "upload-pack: not our ref %s",
> @@ -1434,7 +1440,7 @@ static int parse_want_ref(struct packet_writer *writer, const char *line,
>  	if (skip_prefix(line, "want-ref ", &refname_nons)) {
>  		struct object_id oid;
>  		struct string_list_item *item;
> -		struct object *o;
> +		struct object *o = NULL;
>  		struct strbuf refname = STRBUF_INIT;
>  
>  		strbuf_addf(&refname, "%s%s", get_git_namespace(), refname_nons);
> @@ -1448,7 +1454,15 @@ static int parse_want_ref(struct packet_writer *writer, const char *line,
>  		item = string_list_append(wanted_refs, refname_nons);
>  		item->util = oiddup(&oid);
>  
> -		o = parse_object_or_die(&oid, refname_nons);
> +		if (!starts_with(refname_nons, "refs/tags/")) {
> +			struct commit *commit = lookup_commit_in_graph(the_repository, &oid);
> +			if (commit)
> +				o = &commit->object;
> +		}
> +
> +		if (!o)
> +			o = parse_object_or_die(&oid, refname_nons);
> +

Even here, we _could_ use a parse_probably_commit() helper inside the if (!starts_with(...)) block, even though we would still need the if (!o) check later.

Thanks, -Stolee

Patrick Steinhardt· Mar 1, 2022, 08:43 UTC · re: Derrick Stolee · lore

Re: [PATCH 1/5] upload-pack: look up "want" lines via commit-graph

On Wed, Feb 23, 2022 at 09:13:53AM -0500, Derrick Stolee wrote:
Show 31 quoted lines
> On 2/23/2022 7:35 AM, Patrick Steinhardt wrote:
> 
> > The following benchmark is executed in a repository with a huge number
> > of references. It uses cached request from git-fetch(1) as input and
> > contains about 876,000 "want" lines:
> > 
> >     Benchmark 1: git-upload-pack (HEAD~)
> >       Time (mean ± σ):      7.113 s ±  0.028 s    [User: 6.900 s, System: 0.662 s]
> >       Range (min … max):    7.072 s …  7.168 s    10 runs
> > 
> >     Benchmark 2: git-upload-pack (HEAD)
> >       Time (mean ± σ):      6.622 s ±  0.061 s    [User: 6.452 s, System: 0.650 s]
> >       Range (min … max):    6.535 s …  6.727 s    10 runs
> > 
> >     Summary
> >       'git-upload-pack (HEAD)' ran
> >         1.07 ± 0.01 times faster than 'git-upload-pack (HEAD~)'
> 
> Nice!
> 
> > -		o = parse_object(the_repository, &oid);
> > +		commit = lookup_commit_in_graph(the_repository, &oid);
> > +		if (commit)
> > +			o = &commit->object;
> > +		else
> > +			o = parse_object(the_repository, &oid);
> > +
> 
> This is a neat trick. I see that we've also done this trick in
> revision.c:get_reference(). Perhaps it is worth creating a helper,
> maybe named parse_probably_commit()?

That might be a good idea, thanks. I'll have a look at what the end result would look like.

Patrick
Show 33 quoted lines
> >  		if (!o) {
> >  			packet_writer_error(writer,
> >  					    "upload-pack: not our ref %s",
> > @@ -1434,7 +1440,7 @@ static int parse_want_ref(struct packet_writer *writer, const char *line,
> >  	if (skip_prefix(line, "want-ref ", &refname_nons)) {
> >  		struct object_id oid;
> >  		struct string_list_item *item;
> > -		struct object *o;
> > +		struct object *o = NULL;
> >  		struct strbuf refname = STRBUF_INIT;
> >  
> >  		strbuf_addf(&refname, "%s%s", get_git_namespace(), refname_nons);
> > @@ -1448,7 +1454,15 @@ static int parse_want_ref(struct packet_writer *writer, const char *line,
> >  		item = string_list_append(wanted_refs, refname_nons);
> >  		item->util = oiddup(&oid);
> >  
> > -		o = parse_object_or_die(&oid, refname_nons);
> > +		if (!starts_with(refname_nons, "refs/tags/")) {
> > +			struct commit *commit = lookup_commit_in_graph(the_repository, &oid);
> > +			if (commit)
> > +				o = &commit->object;
> > +		}
> > +
> > +		if (!o)
> > +			o = parse_object_or_die(&oid, refname_nons);
> > +
> 
> Even here, we _could_ use a parse_probably_commit() helper
> inside the if (!starts_with(...)) block, even though we would
> still need the if (!o) check later.
> 
> Thanks,
> -Stolee
Patrick Steinhardt· Mar 1, 2022, 09:24 UTC · re: Patrick Steinhardt · lore

Re: [PATCH 1/5] upload-pack: look up "want" lines via commit-graph

On Tue, Mar 01, 2022 at 09:43:19AM +0100, Patrick Steinhardt wrote:
Show 37 quoted lines
> On Wed, Feb 23, 2022 at 09:13:53AM -0500, Derrick Stolee wrote:
> > On 2/23/2022 7:35 AM, Patrick Steinhardt wrote:
> > 
> > > The following benchmark is executed in a repository with a huge number
> > > of references. It uses cached request from git-fetch(1) as input and
> > > contains about 876,000 "want" lines:
> > > 
> > >     Benchmark 1: git-upload-pack (HEAD~)
> > >       Time (mean ± σ):      7.113 s ±  0.028 s    [User: 6.900 s, System: 0.662 s]
> > >       Range (min … max):    7.072 s …  7.168 s    10 runs
> > > 
> > >     Benchmark 2: git-upload-pack (HEAD)
> > >       Time (mean ± σ):      6.622 s ±  0.061 s    [User: 6.452 s, System: 0.650 s]
> > >       Range (min … max):    6.535 s …  6.727 s    10 runs
> > > 
> > >     Summary
> > >       'git-upload-pack (HEAD)' ran
> > >         1.07 ± 0.01 times faster than 'git-upload-pack (HEAD~)'
> > 
> > Nice!
> > 
> > > -		o = parse_object(the_repository, &oid);
> > > +		commit = lookup_commit_in_graph(the_repository, &oid);
> > > +		if (commit)
> > > +			o = &commit->object;
> > > +		else
> > > +			o = parse_object(the_repository, &oid);
> > > +
> > 
> > This is a neat trick. I see that we've also done this trick in
> > revision.c:get_reference(). Perhaps it is worth creating a helper,
> > maybe named parse_probably_commit()?
> 
> That might be a good idea, thanks. I'll have a look at what the end
> result would look like.
> 
> Patrick

I had a look at existing callsites which use `lookup_commit_in_graph()`, but I found that it wasn't easily possible to convert them all to use a new helper like you propose. Most of them have some custom logic like skipping `parse_object()` if it's part of a promisor pack, so I really only found two locations where such a new helper could be used without also adding and supporting flags. I don't really think that's worth it for now.

Patrick
Show 33 quoted lines
> > >  		if (!o) {
> > >  			packet_writer_error(writer,
> > >  					    "upload-pack: not our ref %s",
> > > @@ -1434,7 +1440,7 @@ static int parse_want_ref(struct packet_writer *writer, const char *line,
> > >  	if (skip_prefix(line, "want-ref ", &refname_nons)) {
> > >  		struct object_id oid;
> > >  		struct string_list_item *item;
> > > -		struct object *o;
> > > +		struct object *o = NULL;
> > >  		struct strbuf refname = STRBUF_INIT;
> > >  
> > >  		strbuf_addf(&refname, "%s%s", get_git_namespace(), refname_nons);
> > > @@ -1448,7 +1454,15 @@ static int parse_want_ref(struct packet_writer *writer, const char *line,
> > >  		item = string_list_append(wanted_refs, refname_nons);
> > >  		item->util = oiddup(&oid);
> > >  
> > > -		o = parse_object_or_die(&oid, refname_nons);
> > > +		if (!starts_with(refname_nons, "refs/tags/")) {
> > > +			struct commit *commit = lookup_commit_in_graph(the_repository, &oid);
> > > +			if (commit)
> > > +				o = &commit->object;
> > > +		}
> > > +
> > > +		if (!o)
> > > +			o = parse_object_or_die(&oid, refname_nons);
> > > +
> > 
> > Even here, we _could_ use a parse_probably_commit() helper
> > inside the if (!starts_with(...)) block, even though we would
> > still need the if (!o) check later.
> > 
> > Thanks,
> > -Stolee
Derrick Stolee· Mar 2, 2022, 18:53 UTC · re: Patrick Steinhardt · lore

Re: [PATCH 1/5] upload-pack: look up "want" lines via commit-graph

On 3/1/2022 4:24 AM, Patrick Steinhardt wrote:
> On Tue, Mar 01, 2022 at 09:43:19AM +0100, Patrick Steinhardt wrote:
>> On Wed, Feb 23, 2022 at 09:13:53AM -0500, Derrick Stolee wrote:
>>> On 2/23/2022 7:35 AM, Patrick Steinhardt wrote:
Show 24 quoted lines
>>>> -		o = parse_object(the_repository, &oid);
>>>> +		commit = lookup_commit_in_graph(the_repository, &oid);
>>>> +		if (commit)
>>>> +			o = &commit->object;
>>>> +		else
>>>> +			o = parse_object(the_repository, &oid);
>>>> +
>>>
>>> This is a neat trick. I see that we've also done this trick in
>>> revision.c:get_reference(). Perhaps it is worth creating a helper,
>>> maybe named parse_probably_commit()?
>>
>> That might be a good idea, thanks. I'll have a look at what the end
>> result would look like.
>>
>> Patrick
> 
> I had a look at existing callsites which use `lookup_commit_in_graph()`,
> but I found that it wasn't easily possible to convert them all to use a
> new helper like you propose. Most of them have some custom logic like
> skipping `parse_object()` if it's part of a promisor pack, so I really
> only found two locations where such a new helper could be used without
> also adding and supporting flags. I don't really think that's worth it
> for now.
Thanks for taking the time to look into it.
-Stolee
Patrick Steinhardt· Feb 23, 2022, 12:35 UTC · re: Patrick Steinhardt · lore

[PATCH 3/5] refs: add ability for backends to special-case reading of symbolic refs

Reading of symbolic and non-symbolic references is currently treated the same in reference backends: we always call `refs_read_raw_ref()` and then decide based on the returned flags what type it is. This has one downside though: symbolic references may be treated different from normal references in a backend from normal references. The packed-refs backend for example doesn't even know about symbolic references, and as a result it is pointless to even ask it for one.

There are cases where we really only care about whether a reference is symbolic or not, but don't care about whether it exists at all or may be a non-symbolic reference. But it is not possible to optimize for this case right now, and as a consequence we will always first check for a loose reference to exist, and if it doesn't, we'll query the packed-refs backend for a known-to-not-be-symbolic reference. This is inefficient and requires us to search all packed references even though we know to not care for the result at all.

Introduce a new function `refs_read_symbolic_ref()` which allows us to fix this case. This function will only ever return symbolic references and can thus optimize for the scenario layed out above. By default, if the backend doesn't provide an implementation for it, we just use the old code path and fall back to `read_raw_ref()`. But in case the backend provides its own, more efficient implementation, we will use that one instead.

Note that this function is explicitly designed to not distinguish between missing references and non-symbolic references. If it did, we'd be forced to always search the packed-refs backend to see whether the symbolic reference the user asked for really doesn't exist, or if it exists as a non-symbolic reference.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs.c                | 17 +++++++++++++++++
 refs.h                |  3 +++
 refs/debug.c          |  1 +
 refs/files-backend.c  |  1 +
 refs/packed-backend.c |  1 +
 refs/refs-internal.h  | 16 ++++++++++++++++
 6 files changed, 39 insertions(+)
Show changes to 6 files +39 −0

refs.c, refs.h, refs/debug.c, refs/files-backend.c, refs/packed-backend.c, refs/refs-internal.h

diff --git a/refs.c b/refs.c
index 1598fb13e2..0b79bdd7c3 100644
--- a/refs.c
+++ b/refs.c
@@ -1673,6 +1673,23 @@ int refs_read_raw_ref(struct ref_store *ref_store, const char *refname,
 					   type, failure_errno);
 }
 
+int refs_read_symbolic_ref(struct ref_store *ref_store, const char *refname,
+			   struct strbuf *referent)
+{
+	struct object_id oid;
+	int ret, failure_errno = 0;
+	unsigned int type = 0;
+
+	if (ref_store->be->read_symbolic_ref)
+		return ref_store->be->read_symbolic_ref(ref_store, refname, referent);
+
+	ret = refs_read_raw_ref(ref_store, refname, &oid, referent, &type, &failure_errno);
+	if (ret || !(type & REF_ISSYMREF))
+		return -1;
+
+	return 0;
+}
+
 const char *refs_resolve_ref_unsafe(struct ref_store *refs,
 				    const char *refname,
 				    int resolve_flags,
diff --git a/refs.h b/refs.h
index 1ae12c410a..23479c7ee0 100644
--- a/refs.h
+++ b/refs.h
@@ -82,6 +82,9 @@ int read_ref_full(const char *refname, int resolve_flags,
 		  struct object_id *oid, int *flags);
 int read_ref(const char *refname, struct object_id *oid);
 
+int refs_read_symbolic_ref(struct ref_store *ref_store, const char *refname,
+			   struct strbuf *referent);
+
 /*
  * Return 0 if a reference named refname could be created without
  * conflicting with the name of an existing reference. Otherwise,
diff --git a/refs/debug.c b/refs/debug.c
index 2b0771ca53..c590d37720 100644
--- a/refs/debug.c
+++ b/refs/debug.c
@@ -435,6 +435,7 @@ struct ref_storage_be refs_be_debug = {
 
 	debug_ref_iterator_begin,
 	debug_read_raw_ref,
+	NULL,
 
 	debug_reflog_iterator_begin,
 	debug_for_each_reflog_ent,
diff --git a/refs/files-backend.c b/refs/files-backend.c
index f59589d6cc..f3428a9f12 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -3286,6 +3286,7 @@ struct ref_storage_be refs_be_files = {
 
 	files_ref_iterator_begin,
 	files_read_raw_ref,
+	NULL,
 
 	files_reflog_iterator_begin,
 	files_for_each_reflog_ent,
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 27dd8c3922..f56e2cc635 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -1684,6 +1684,7 @@ struct ref_storage_be refs_be_packed = {
 
 	packed_ref_iterator_begin,
 	packed_read_raw_ref,
+	NULL,
 
 	packed_reflog_iterator_begin,
 	packed_for_each_reflog_ent,
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index 6e15db3ca4..001ef15835 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -649,6 +649,21 @@ typedef int read_raw_ref_fn(struct ref_store *ref_store, const char *refname,
 			    struct object_id *oid, struct strbuf *referent,
 			    unsigned int *type, int *failure_errno);
 
+/*
+ * Read a symbolic reference from the specified reference store. This function
+ * is optional: if not implemented by a backend, then `read_raw_ref_fn` is used
+ * to read the symbolcic reference instead. It is intended to be implemented
+ * only in case the backend can optimize the reading of symbolic references.
+ *
+ * Return 0 on success, or -1 on failure. `referent` will be set to the target
+ * of the symbolic reference on success. This function explicitly does not
+ * distinguish between error cases and the reference not being a symbolic
+ * reference to allow backends to optimize this operation in case symbolic and
+ * non-symbolic references are treated differently.
+ */
+typedef int read_symbolic_ref_fn(struct ref_store *ref_store, const char *refname,
+				 struct strbuf *referent);
+
 struct ref_storage_be {
 	struct ref_storage_be *next;
 	const char *name;
@@ -668,6 +683,7 @@ struct ref_storage_be {
 
 	ref_iterator_begin_fn *iterator_begin;
 	read_raw_ref_fn *read_raw_ref;
+	read_symbolic_ref_fn *read_symbolic_ref;
 
 	reflog_iterator_begin_fn *reflog_iterator_begin;
 	for_each_reflog_ent_fn *for_each_reflog_ent;

-- 
2.35.1
Patrick Steinhardt· Feb 23, 2022, 12:35 UTC · re: Patrick Steinhardt · lore

[PATCH 4/5] remote: read symbolic refs via `refs_read_symbolic_ref()`

We have two cases in the remote code where we check whether a reference is symbolic or not, but don't mind in case it doesn't exist or in case it exists but is a non-symbolic reference. Convert these two callsites to use the new `refs_read_symbolic_ref()` function, whose intent is to implement exactly that usecase.

No change in behaviour is expected from this change.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 builtin/remote.c |  8 +++++---
 remote.c         | 14 +++++++-------
 2 files changed, 12 insertions(+), 10 deletions(-)
Show changes to 2 files +12 −10

builtin/remote.c, remote.c

diff --git a/builtin/remote.c b/builtin/remote.c
index 6f27ddc47b..0142ed09b5 100644
--- a/builtin/remote.c
+++ b/builtin/remote.c
@@ -766,13 +766,15 @@ static int mv(int argc, const char **argv)
 	for_each_ref(read_remote_branches, &rename);
 	for (i = 0; i < remote_branches.nr; i++) {
 		struct string_list_item *item = remote_branches.items + i;
-		int flag = 0;
+		struct strbuf referent = STRBUF_INIT;
 
-		read_ref_full(item->string, RESOLVE_REF_READING, NULL, &flag);
-		if (!(flag & REF_ISSYMREF))
+		if (refs_read_symbolic_ref(get_main_ref_store(the_repository), item->string,
+					   &referent))
 			continue;
 		if (delete_ref(NULL, item->string, NULL, REF_NO_DEREF))
 			die(_("deleting '%s' failed"), item->string);
+
+		strbuf_release(&referent);
 	}
 	for (i = 0; i < remote_branches.nr; i++) {
 		struct string_list_item *item = remote_branches.items + i;
diff --git a/remote.c b/remote.c
index c97c626eaa..42a4e7106e 100644
--- a/remote.c
+++ b/remote.c
@@ -1945,13 +1945,9 @@ const char *branch_get_push(struct branch *branch, struct strbuf *err)
 	return branch->push_tracking_ref;
 }
 
-static int ignore_symref_update(const char *refname)
+static int ignore_symref_update(const char *refname, struct strbuf *scratch)
 {
-	int flag;
-
-	if (!resolve_ref_unsafe(refname, 0, NULL, &flag))
-		return 0; /* non-existing refs are OK */
-	return (flag & REF_ISSYMREF);
+	return !refs_read_symbolic_ref(get_main_ref_store(the_repository), refname, scratch);
 }
 
 /*
@@ -1964,6 +1960,7 @@ static int ignore_symref_update(const char *refname)
 static struct ref *get_expanded_map(const struct ref *remote_refs,
 				    const struct refspec_item *refspec)
 {
+	struct strbuf scratch = STRBUF_INIT;
 	const struct ref *ref;
 	struct ref *ret = NULL;
 	struct ref **tail = &ret;
@@ -1971,11 +1968,13 @@ static struct ref *get_expanded_map(const struct ref *remote_refs,
 	for (ref = remote_refs; ref; ref = ref->next) {
 		char *expn_name = NULL;
 
+		strbuf_reset(&scratch);
+
 		if (strchr(ref->name, '^'))
 			continue; /* a dereference item */
 		if (match_name_with_pattern(refspec->src, ref->name,
 					    refspec->dst, &expn_name) &&
-		    !ignore_symref_update(expn_name)) {
+		    !ignore_symref_update(expn_name, &scratch)) {
 			struct ref *cpy = copy_ref(ref);
 
 			cpy->peer_ref = alloc_ref(expn_name);
@@ -1987,6 +1986,7 @@ static struct ref *get_expanded_map(const struct ref *remote_refs,
 		free(expn_name);
 	}
 
+	strbuf_release(&scratch);
 	return ret;
 }
 

-- 
2.35.1
Patrick Steinhardt· Feb 23, 2022, 12:35 UTC · re: Patrick Steinhardt · lore

[PATCH 5/5] refs/files-backend: optimize reading of symbolic refs

When reading references via `files_read_raw_ref()` we always consult both the loose reference, and if that wasn't found, we also consult the packed-refs file. While this makes sense to read a generic reference, it is wasteful in the case where we only care about symbolic references: the packed-refs backend does not support them, and thus it cannot ever return one for us.

Special-case reading of symbolic references for the files backend such that we always skip asking the packed-refs backend.

We use `refs_read_symbolic_ref()` extensively to determine whether we need to skip updating local symbolic references during a fetch, which is why the change results in a significant speedup when doing fetches in repositories with huge numbers of references. The following benchmark executes a mirror-fetch in a repository with about 2 million references:

    Benchmark 1: git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD~)
      Time (mean ± σ):     68.372 s ±  2.344 s    [User: 65.629 s, System: 8.786 s]
      Range (min … max):   65.745 s … 70.246 s    3 runs
    Benchmark 2: git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD)
      Time (mean ± σ):     60.259 s ±  0.343 s    [User: 61.019 s, System: 7.245 s]
      Range (min … max):   60.003 s … 60.649 s    3 runs
    Summary
      'git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD)' ran
        1.13 ± 0.04 times faster than 'git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD~)'
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs/files-backend.c | 34 ++++++++++++++++++++++++++++------
 1 file changed, 28 insertions(+), 6 deletions(-)
Show changes to refs/files-backend.c +28 −6
diff --git a/refs/files-backend.c b/refs/files-backend.c
index f3428a9f12..0457ecdb42 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -338,9 +338,9 @@ static struct ref_cache *get_loose_ref_cache(struct files_ref_store *refs)
 	return refs->loose;
 }
 
-static int files_read_raw_ref(struct ref_store *ref_store, const char *refname,
-			      struct object_id *oid, struct strbuf *referent,
-			      unsigned int *type, int *failure_errno)
+static int read_ref_internal(struct ref_store *ref_store, const char *refname,
+			     struct object_id *oid, struct strbuf *referent,
+			     unsigned int *type, int *failure_errno, int skip_packed_refs)
 {
 	struct files_ref_store *refs =
 		files_downcast(ref_store, REF_STORE_READ, "read_raw_ref");
@@ -381,7 +381,7 @@ static int files_read_raw_ref(struct ref_store *ref_store, const char *refname,
 	if (lstat(path, &st) < 0) {
 		int ignore_errno;
 		myerr = errno;
-		if (myerr != ENOENT)
+		if (myerr != ENOENT || skip_packed_refs)
 			goto out;
 		if (refs_read_raw_ref(refs->packed_ref_store, refname, oid,
 				      referent, type, &ignore_errno)) {
@@ -425,7 +425,8 @@ static int files_read_raw_ref(struct ref_store *ref_store, const char *refname,
 		 * ref is supposed to be, there could still be a
 		 * packed ref:
 		 */
-		if (refs_read_raw_ref(refs->packed_ref_store, refname, oid,
+		if (skip_packed_refs ||
+		    refs_read_raw_ref(refs->packed_ref_store, refname, oid,
 				      referent, type, &ignore_errno)) {
 			myerr = EISDIR;
 			goto out;
@@ -470,6 +471,27 @@ static int files_read_raw_ref(struct ref_store *ref_store, const char *refname,
 	return ret;
 }
 
+static int files_read_raw_ref(struct ref_store *ref_store, const char *refname,
+			      struct object_id *oid, struct strbuf *referent,
+			      unsigned int *type, int *failure_errno)
+{
+	return read_ref_internal(ref_store, refname, oid, referent, type, failure_errno, 0);
+}
+
+static int files_read_symbolic_ref(struct ref_store *ref_store, const char *refname,
+				   struct strbuf *referent)
+{
+	struct object_id oid;
+	int failure_errno, ret;
+	unsigned int type;
+
+	ret = read_ref_internal(ref_store, refname, &oid, referent, &type, &failure_errno, 1);
+	if (ret)
+		return ret;
+
+	return !(type & REF_ISSYMREF);
+}
+
 int parse_loose_ref_contents(const char *buf, struct object_id *oid,
 			     struct strbuf *referent, unsigned int *type,
 			     int *failure_errno)
@@ -3286,7 +3308,7 @@ struct ref_storage_be refs_be_files = {
 
 	files_ref_iterator_begin,
 	files_read_raw_ref,
-	NULL,
+	files_read_symbolic_ref,
 
 	files_reflog_iterator_begin,
 	files_for_each_reflog_ent,

-- 
2.35.1
Patrick Steinhardt· Mar 1, 2022, 09:33 UTC · re: Patrick Steinhardt · lore

[PATCH v2 0/5] fetch: more optimizations for mirror fetches

Hi,

this is another patch series with the aim to speed up mirror fetches. It applies on top of e6ebfd0e8c (The sixth batch, 2022-02-18) with 3824153b23 (Merge branch 'ps/fetch-atomic' into next, 2022-02-18) merged into it to fix a conflict.

The only change compared to v2 is an update to the benchmarks so that they're less verbose, as proposed by Derrick. I also had a look at introducing a new helper `parse_object_probably_commit()`, but I didn't find the end result to be much of an improvement compared to the ad-hoc `lookup_commit_in_graph() || parse_object()` dance we do right now.

Thanks!
Patrick
Patrick Steinhardt (5):
  upload-pack: look up "want" lines via commit-graph
  fetch: avoid lookup of commits when not appending to FETCH_HEAD
  refs: add ability for backends to special-case reading of symbolic
    refs
  remote: read symbolic refs via `refs_read_symbolic_ref()`
  refs/files-backend: optimize reading of symbolic refs
 builtin/fetch.c       | 42 +++++++++++++++++++++++++++---------------
 builtin/remote.c      |  8 +++++---
 refs.c                | 17 +++++++++++++++++
 refs.h                |  3 +++
 refs/debug.c          |  1 +
 refs/files-backend.c  | 33 ++++++++++++++++++++++++++++-----
 refs/packed-backend.c |  1 +
 refs/refs-internal.h  | 16 ++++++++++++++++
 remote.c              | 14 +++++++-------
 upload-pack.c         | 20 +++++++++++++++++---
 10 files changed, 122 insertions(+), 33 deletions(-)
Range-diff against v1:
1:  ca5e136cca ! 1:  b5c696bd8e upload-pack: look up "want" lines via commit-graph
    @@ Commit message
         Refactor parsing of both "want" and "want-ref" lines to do so.
     
         The following benchmark is executed in a repository with a huge number
    -    of references. It uses cached request from git-fetch(1) as input and
    -    contains about 876,000 "want" lines:
    +    of references. It uses cached request from git-fetch(1) as input to
    +    git-upload-pack(1) that contains about 876,000 "want" lines:
     
    -        Benchmark 1: git-upload-pack (HEAD~)
    +        Benchmark 1: HEAD~
               Time (mean ± σ):      7.113 s ±  0.028 s    [User: 6.900 s, System: 0.662 s]
               Range (min … max):    7.072 s …  7.168 s    10 runs
     
    -        Benchmark 2: git-upload-pack (HEAD)
    +        Benchmark 2: HEAD
               Time (mean ± σ):      6.622 s ±  0.061 s    [User: 6.452 s, System: 0.650 s]
               Range (min … max):    6.535 s …  6.727 s    10 runs
     
             Summary
    -          'git-upload-pack (HEAD)' ran
    -            1.07 ± 0.01 times faster than 'git-upload-pack (HEAD~)'
    +          'HEAD' ran
    +            1.07 ± 0.01 times faster than 'HEAD~'
     
         Signed-off-by: Patrick Steinhardt <ps@pks.im>
     
2:  80f993dddd ! 2:  fbe76b78c3 fetch: avoid lookup of commits when not appending to FETCH_HEAD
    @@ Commit message
     
         Skip this busywork in case we're not writing to FETCH_HEAD. The
         following benchmark performs a mirror-fetch in a repository with about
    -    two million references:
    +    two million references via `git fetch --prune --no-write-fetch-head
    +    +refs/*:refs/*`:
     
    -        Benchmark 1: git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD~)
    +        Benchmark 1: HEAD~
               Time (mean ± σ):     75.388 s ±  1.942 s    [User: 71.103 s, System: 8.953 s]
               Range (min … max):   73.184 s … 76.845 s    3 runs
     
    -        Benchmark 2: git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD)
    +        Benchmark 2: HEAD
               Time (mean ± σ):     69.486 s ±  1.016 s    [User: 65.941 s, System: 8.806 s]
               Range (min … max):   68.864 s … 70.659 s    3 runs
     
             Summary
    -          'git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD)' ran
    -            1.08 ± 0.03 times faster than 'git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD~)'
    +          'HEAD' ran
    +            1.08 ± 0.03 times faster than 'HEAD~'
     
         Signed-off-by: Patrick Steinhardt <ps@pks.im>
     
3:  28cacbdbe2 = 3:  29eb81d37c refs: add ability for backends to special-case reading of symbolic refs
4:  1d24101fe4 = 4:  0489380e00 remote: read symbolic refs via `refs_read_symbolic_ref()`
5:  7213ffdbdd ! 5:  b6eca63d3b refs/files-backend: optimize reading of symbolic refs
    @@ Commit message
         need to skip updating local symbolic references during a fetch, which is
         why the change results in a significant speedup when doing fetches in
         repositories with huge numbers of references. The following benchmark
    -    executes a mirror-fetch in a repository with about 2 million references:
    +    executes a mirror-fetch in a repository with about 2 million references
    +    via `git fetch --prune --no-write-fetch-head +refs/*:refs/*`:
     
    -        Benchmark 1: git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD~)
    +        Benchmark 1: HEAD~
               Time (mean ± σ):     68.372 s ±  2.344 s    [User: 65.629 s, System: 8.786 s]
               Range (min … max):   65.745 s … 70.246 s    3 runs
     
    -        Benchmark 2: git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD)
    +        Benchmark 2: HEAD
               Time (mean ± σ):     60.259 s ±  0.343 s    [User: 61.019 s, System: 7.245 s]
               Range (min … max):   60.003 s … 60.649 s    3 runs
     
             Summary
    -          'git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD)' ran
    -            1.13 ± 0.04 times faster than 'git fetch --prune --no-write-fetch-head +refs/*:refs/* (HEAD~)'
    +          'HEAD' ran
    +            1.13 ± 0.04 times faster than 'HEAD~'
     
         Signed-off-by: Patrick Steinhardt <ps@pks.im>
     
-- 
2.35.1
Patrick Steinhardt· Mar 1, 2022, 09:33 UTC · re: Patrick Steinhardt · lore

[PATCH v2 1/5] upload-pack: look up "want" lines via commit-graph

During packfile negotiation the client will send "want" and "want-ref" lines to the server to tell it which objects it is interested in. The server-side parses each of those and looks them up to see whether it actually has requested objects. This lookup is performed by calling `parse_object()` directly, which thus hits the object database. In the general case though most of the objects the client requests will be commits. We can thus try to look up the object via the commit-graph opportunistically, which is much faster than doing the same via the object database.

Refactor parsing of both "want" and "want-ref" lines to do so.

The following benchmark is executed in a repository with a huge number of references. It uses cached request from git-fetch(1) as input to git-upload-pack(1) that contains about 876,000 "want" lines:

    Benchmark 1: HEAD~
      Time (mean ± σ):      7.113 s ±  0.028 s    [User: 6.900 s, System: 0.662 s]
      Range (min … max):    7.072 s …  7.168 s    10 runs
    Benchmark 2: HEAD
      Time (mean ± σ):      6.622 s ±  0.061 s    [User: 6.452 s, System: 0.650 s]
      Range (min … max):    6.535 s …  6.727 s    10 runs
    Summary
      'HEAD' ran
        1.07 ± 0.01 times faster than 'HEAD~'
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 upload-pack.c | 20 +++++++++++++++++---
 1 file changed, 17 insertions(+), 3 deletions(-)
Show changes to upload-pack.c +17 −3
diff --git a/upload-pack.c b/upload-pack.c
index 8acc98741b..3a851b3606 100644
--- a/upload-pack.c
+++ b/upload-pack.c
@@ -1400,13 +1400,19 @@ static int parse_want(struct packet_writer *writer, const char *line,
 	const char *arg;
 	if (skip_prefix(line, "want ", &arg)) {
 		struct object_id oid;
+		struct commit *commit;
 		struct object *o;
 
 		if (get_oid_hex(arg, &oid))
 			die("git upload-pack: protocol error, "
 			    "expected to get oid, not '%s'", line);
 
-		o = parse_object(the_repository, &oid);
+		commit = lookup_commit_in_graph(the_repository, &oid);
+		if (commit)
+			o = &commit->object;
+		else
+			o = parse_object(the_repository, &oid);
+
 		if (!o) {
 			packet_writer_error(writer,
 					    "upload-pack: not our ref %s",
@@ -1434,7 +1440,7 @@ static int parse_want_ref(struct packet_writer *writer, const char *line,
 	if (skip_prefix(line, "want-ref ", &refname_nons)) {
 		struct object_id oid;
 		struct string_list_item *item;
-		struct object *o;
+		struct object *o = NULL;
 		struct strbuf refname = STRBUF_INIT;
 
 		strbuf_addf(&refname, "%s%s", get_git_namespace(), refname_nons);
@@ -1448,7 +1454,15 @@ static int parse_want_ref(struct packet_writer *writer, const char *line,
 		item = string_list_append(wanted_refs, refname_nons);
 		item->util = oiddup(&oid);
 
-		o = parse_object_or_die(&oid, refname_nons);
+		if (!starts_with(refname_nons, "refs/tags/")) {
+			struct commit *commit = lookup_commit_in_graph(the_repository, &oid);
+			if (commit)
+				o = &commit->object;
+		}
+
+		if (!o)
+			o = parse_object_or_die(&oid, refname_nons);
+
 		if (!(o->flags & WANTED)) {
 			o->flags |= WANTED;
 			add_object_array(o, NULL, want_obj);

-- 
2.35.1
Patrick Steinhardt· Mar 1, 2022, 09:33 UTC · re: Patrick Steinhardt · lore

[PATCH v2 2/5] fetch: avoid lookup of commits when not appending to FETCH_HEAD

When fetching from a remote repository we will by default write what has been fetched into the special FETCH_HEAD reference. The order in which references are written depends on whether the reference is for merge or not, which, despite some other conditions, is also determined based on whether the old object ID the reference is being updated from actually exists in the repository.

To write FETCH_HEAD we thus loop through all references thrice: once for the references that are about to be merged, once for the references that are not for merge, and finally for all references that are ignored. For every iteration, we then look up the old object ID to determine whether the referenced object exists so that we can label it as "not-for-merge" if it doesn't exist. It goes without saying that this can be expensive in case where we are fetching a lot of references.

While this is hard to avoid in the case where we're writing FETCH_HEAD, users can in fact ask us to skip this work via `--no-write-fetch-head`. In that case, we do not care for the result of those lookups at all because we don't have to order writes to FETCH_HEAD in the first place.

Skip this busywork in case we're not writing to FETCH_HEAD. The following benchmark performs a mirror-fetch in a repository with about two million references via `git fetch --prune --no-write-fetch-head +refs/*:refs/*`:

    Benchmark 1: HEAD~
      Time (mean ± σ):     75.388 s ±  1.942 s    [User: 71.103 s, System: 8.953 s]
      Range (min … max):   73.184 s … 76.845 s    3 runs
    Benchmark 2: HEAD
      Time (mean ± σ):     69.486 s ±  1.016 s    [User: 65.941 s, System: 8.806 s]
      Range (min … max):   68.864 s … 70.659 s    3 runs
    Summary
      'HEAD' ran
        1.08 ± 0.03 times faster than 'HEAD~'
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 builtin/fetch.c | 42 +++++++++++++++++++++++++++---------------
 1 file changed, 27 insertions(+), 15 deletions(-)
Show changes to builtin/fetch.c +27 −15
diff --git a/builtin/fetch.c b/builtin/fetch.c
index e8305b6662..4d12c2fd4d 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -1146,7 +1146,6 @@ static int store_updated_refs(const char *raw_url, const char *remote_name,
 	     want_status <= FETCH_HEAD_IGNORE;
 	     want_status++) {
 		for (rm = ref_map; rm; rm = rm->next) {
-			struct commit *commit = NULL;
 			struct ref *ref = NULL;
 
 			if (rm->status == REF_STATUS_REJECT_SHALLOW) {
@@ -1157,21 +1156,34 @@ static int store_updated_refs(const char *raw_url, const char *remote_name,
 			}
 
 			/*
-			 * References in "refs/tags/" are often going to point
-			 * to annotated tags, which are not part of the
-			 * commit-graph. We thus only try to look up refs in
-			 * the graph which are not in that namespace to not
-			 * regress performance in repositories with many
-			 * annotated tags.
+			 * When writing FETCH_HEAD we need to determine whether
+			 * we already have the commit or not. If not, then the
+			 * reference is not for merge and needs to be written
+			 * to the reflog after other commits which we already
+			 * have. We're not interested in this property though
+			 * in case FETCH_HEAD is not to be updated, so we can
+			 * skip the classification in that case.
 			 */
-			if (!starts_with(rm->name, "refs/tags/"))
-				commit = lookup_commit_in_graph(the_repository, &rm->old_oid);
-			if (!commit) {
-				commit = lookup_commit_reference_gently(the_repository,
-									&rm->old_oid,
-									1);
-				if (!commit)
-					rm->fetch_head_status = FETCH_HEAD_NOT_FOR_MERGE;
+			if (fetch_head->fp) {
+				struct commit *commit = NULL;
+
+				/*
+				 * References in "refs/tags/" are often going to point
+				 * to annotated tags, which are not part of the
+				 * commit-graph. We thus only try to look up refs in
+				 * the graph which are not in that namespace to not
+				 * regress performance in repositories with many
+				 * annotated tags.
+				 */
+				if (!starts_with(rm->name, "refs/tags/"))
+					commit = lookup_commit_in_graph(the_repository, &rm->old_oid);
+				if (!commit) {
+					commit = lookup_commit_reference_gently(the_repository,
+										&rm->old_oid,
+										1);
+					if (!commit)
+						rm->fetch_head_status = FETCH_HEAD_NOT_FOR_MERGE;
+				}
 			}
 
 			if (rm->fetch_head_status != want_status)

-- 
2.35.1
Patrick Steinhardt· Mar 1, 2022, 09:33 UTC · re: Patrick Steinhardt · lore

[PATCH v2 3/5] refs: add ability for backends to special-case reading of symbolic refs

Reading of symbolic and non-symbolic references is currently treated the same in reference backends: we always call `refs_read_raw_ref()` and then decide based on the returned flags what type it is. This has one downside though: symbolic references may be treated different from normal references in a backend from normal references. The packed-refs backend for example doesn't even know about symbolic references, and as a result it is pointless to even ask it for one.

There are cases where we really only care about whether a reference is symbolic or not, but don't care about whether it exists at all or may be a non-symbolic reference. But it is not possible to optimize for this case right now, and as a consequence we will always first check for a loose reference to exist, and if it doesn't, we'll query the packed-refs backend for a known-to-not-be-symbolic reference. This is inefficient and requires us to search all packed references even though we know to not care for the result at all.

Introduce a new function `refs_read_symbolic_ref()` which allows us to fix this case. This function will only ever return symbolic references and can thus optimize for the scenario layed out above. By default, if the backend doesn't provide an implementation for it, we just use the old code path and fall back to `read_raw_ref()`. But in case the backend provides its own, more efficient implementation, we will use that one instead.

Note that this function is explicitly designed to not distinguish between missing references and non-symbolic references. If it did, we'd be forced to always search the packed-refs backend to see whether the symbolic reference the user asked for really doesn't exist, or if it exists as a non-symbolic reference.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs.c                | 17 +++++++++++++++++
 refs.h                |  3 +++
 refs/debug.c          |  1 +
 refs/files-backend.c  |  1 +
 refs/packed-backend.c |  1 +
 refs/refs-internal.h  | 16 ++++++++++++++++
 6 files changed, 39 insertions(+)
Show changes to 6 files +39 −0

refs.c, refs.h, refs/debug.c, refs/files-backend.c, refs/packed-backend.c, refs/refs-internal.h

diff --git a/refs.c b/refs.c
index 1598fb13e2..0b79bdd7c3 100644
--- a/refs.c
+++ b/refs.c
@@ -1673,6 +1673,23 @@ int refs_read_raw_ref(struct ref_store *ref_store, const char *refname,
 					   type, failure_errno);
 }
 
+int refs_read_symbolic_ref(struct ref_store *ref_store, const char *refname,
+			   struct strbuf *referent)
+{
+	struct object_id oid;
+	int ret, failure_errno = 0;
+	unsigned int type = 0;
+
+	if (ref_store->be->read_symbolic_ref)
+		return ref_store->be->read_symbolic_ref(ref_store, refname, referent);
+
+	ret = refs_read_raw_ref(ref_store, refname, &oid, referent, &type, &failure_errno);
+	if (ret || !(type & REF_ISSYMREF))
+		return -1;
+
+	return 0;
+}
+
 const char *refs_resolve_ref_unsafe(struct ref_store *refs,
 				    const char *refname,
 				    int resolve_flags,
diff --git a/refs.h b/refs.h
index 1ae12c410a..23479c7ee0 100644
--- a/refs.h
+++ b/refs.h
@@ -82,6 +82,9 @@ int read_ref_full(const char *refname, int resolve_flags,
 		  struct object_id *oid, int *flags);
 int read_ref(const char *refname, struct object_id *oid);
 
+int refs_read_symbolic_ref(struct ref_store *ref_store, const char *refname,
+			   struct strbuf *referent);
+
 /*
  * Return 0 if a reference named refname could be created without
  * conflicting with the name of an existing reference. Otherwise,
diff --git a/refs/debug.c b/refs/debug.c
index 2b0771ca53..c590d37720 100644
--- a/refs/debug.c
+++ b/refs/debug.c
@@ -435,6 +435,7 @@ struct ref_storage_be refs_be_debug = {
 
 	debug_ref_iterator_begin,
 	debug_read_raw_ref,
+	NULL,
 
 	debug_reflog_iterator_begin,
 	debug_for_each_reflog_ent,
diff --git a/refs/files-backend.c b/refs/files-backend.c
index f59589d6cc..f3428a9f12 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -3286,6 +3286,7 @@ struct ref_storage_be refs_be_files = {
 
 	files_ref_iterator_begin,
 	files_read_raw_ref,
+	NULL,
 
 	files_reflog_iterator_begin,
 	files_for_each_reflog_ent,
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 27dd8c3922..f56e2cc635 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -1684,6 +1684,7 @@ struct ref_storage_be refs_be_packed = {
 
 	packed_ref_iterator_begin,
 	packed_read_raw_ref,
+	NULL,
 
 	packed_reflog_iterator_begin,
 	packed_for_each_reflog_ent,
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index 6e15db3ca4..001ef15835 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -649,6 +649,21 @@ typedef int read_raw_ref_fn(struct ref_store *ref_store, const char *refname,
 			    struct object_id *oid, struct strbuf *referent,
 			    unsigned int *type, int *failure_errno);
 
+/*
+ * Read a symbolic reference from the specified reference store. This function
+ * is optional: if not implemented by a backend, then `read_raw_ref_fn` is used
+ * to read the symbolcic reference instead. It is intended to be implemented
+ * only in case the backend can optimize the reading of symbolic references.
+ *
+ * Return 0 on success, or -1 on failure. `referent` will be set to the target
+ * of the symbolic reference on success. This function explicitly does not
+ * distinguish between error cases and the reference not being a symbolic
+ * reference to allow backends to optimize this operation in case symbolic and
+ * non-symbolic references are treated differently.
+ */
+typedef int read_symbolic_ref_fn(struct ref_store *ref_store, const char *refname,
+				 struct strbuf *referent);
+
 struct ref_storage_be {
 	struct ref_storage_be *next;
 	const char *name;
@@ -668,6 +683,7 @@ struct ref_storage_be {
 
 	ref_iterator_begin_fn *iterator_begin;
 	read_raw_ref_fn *read_raw_ref;
+	read_symbolic_ref_fn *read_symbolic_ref;
 
 	reflog_iterator_begin_fn *reflog_iterator_begin;
 	for_each_reflog_ent_fn *for_each_reflog_ent;

-- 
2.35.1
Patrick Steinhardt· Mar 1, 2022, 09:33 UTC · re: Patrick Steinhardt · lore

[PATCH v2 4/5] remote: read symbolic refs via `refs_read_symbolic_ref()`

We have two cases in the remote code where we check whether a reference is symbolic or not, but don't mind in case it doesn't exist or in case it exists but is a non-symbolic reference. Convert these two callsites to use the new `refs_read_symbolic_ref()` function, whose intent is to implement exactly that usecase.

No change in behaviour is expected from this change.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 builtin/remote.c |  8 +++++---
 remote.c         | 14 +++++++-------
 2 files changed, 12 insertions(+), 10 deletions(-)
Show changes to 2 files +12 −10

builtin/remote.c, remote.c

diff --git a/builtin/remote.c b/builtin/remote.c
index 6f27ddc47b..0142ed09b5 100644
--- a/builtin/remote.c
+++ b/builtin/remote.c
@@ -766,13 +766,15 @@ static int mv(int argc, const char **argv)
 	for_each_ref(read_remote_branches, &rename);
 	for (i = 0; i < remote_branches.nr; i++) {
 		struct string_list_item *item = remote_branches.items + i;
-		int flag = 0;
+		struct strbuf referent = STRBUF_INIT;
 
-		read_ref_full(item->string, RESOLVE_REF_READING, NULL, &flag);
-		if (!(flag & REF_ISSYMREF))
+		if (refs_read_symbolic_ref(get_main_ref_store(the_repository), item->string,
+					   &referent))
 			continue;
 		if (delete_ref(NULL, item->string, NULL, REF_NO_DEREF))
 			die(_("deleting '%s' failed"), item->string);
+
+		strbuf_release(&referent);
 	}
 	for (i = 0; i < remote_branches.nr; i++) {
 		struct string_list_item *item = remote_branches.items + i;
diff --git a/remote.c b/remote.c
index c97c626eaa..42a4e7106e 100644
--- a/remote.c
+++ b/remote.c
@@ -1945,13 +1945,9 @@ const char *branch_get_push(struct branch *branch, struct strbuf *err)
 	return branch->push_tracking_ref;
 }
 
-static int ignore_symref_update(const char *refname)
+static int ignore_symref_update(const char *refname, struct strbuf *scratch)
 {
-	int flag;
-
-	if (!resolve_ref_unsafe(refname, 0, NULL, &flag))
-		return 0; /* non-existing refs are OK */
-	return (flag & REF_ISSYMREF);
+	return !refs_read_symbolic_ref(get_main_ref_store(the_repository), refname, scratch);
 }
 
 /*
@@ -1964,6 +1960,7 @@ static int ignore_symref_update(const char *refname)
 static struct ref *get_expanded_map(const struct ref *remote_refs,
 				    const struct refspec_item *refspec)
 {
+	struct strbuf scratch = STRBUF_INIT;
 	const struct ref *ref;
 	struct ref *ret = NULL;
 	struct ref **tail = &ret;
@@ -1971,11 +1968,13 @@ static struct ref *get_expanded_map(const struct ref *remote_refs,
 	for (ref = remote_refs; ref; ref = ref->next) {
 		char *expn_name = NULL;
 
+		strbuf_reset(&scratch);
+
 		if (strchr(ref->name, '^'))
 			continue; /* a dereference item */
 		if (match_name_with_pattern(refspec->src, ref->name,
 					    refspec->dst, &expn_name) &&
-		    !ignore_symref_update(expn_name)) {
+		    !ignore_symref_update(expn_name, &scratch)) {
 			struct ref *cpy = copy_ref(ref);
 
 			cpy->peer_ref = alloc_ref(expn_name);
@@ -1987,6 +1986,7 @@ static struct ref *get_expanded_map(const struct ref *remote_refs,
 		free(expn_name);
 	}
 
+	strbuf_release(&scratch);
 	return ret;
 }
 

-- 
2.35.1
Patrick Steinhardt· Mar 1, 2022, 09:33 UTC · re: Patrick Steinhardt · lore

[PATCH v2 5/5] refs/files-backend: optimize reading of symbolic refs

When reading references via `files_read_raw_ref()` we always consult both the loose reference, and if that wasn't found, we also consult the packed-refs file. While this makes sense to read a generic reference, it is wasteful in the case where we only care about symbolic references: the packed-refs backend does not support them, and thus it cannot ever return one for us.

Special-case reading of symbolic references for the files backend such that we always skip asking the packed-refs backend.

We use `refs_read_symbolic_ref()` extensively to determine whether we need to skip updating local symbolic references during a fetch, which is why the change results in a significant speedup when doing fetches in repositories with huge numbers of references. The following benchmark executes a mirror-fetch in a repository with about 2 million references via `git fetch --prune --no-write-fetch-head +refs/*:refs/*`:

    Benchmark 1: HEAD~
      Time (mean ± σ):     68.372 s ±  2.344 s    [User: 65.629 s, System: 8.786 s]
      Range (min … max):   65.745 s … 70.246 s    3 runs
    Benchmark 2: HEAD
      Time (mean ± σ):     60.259 s ±  0.343 s    [User: 61.019 s, System: 7.245 s]
      Range (min … max):   60.003 s … 60.649 s    3 runs
    Summary
      'HEAD' ran
        1.13 ± 0.04 times faster than 'HEAD~'
Signed-off-by: Patrick Steinhardt <ps@pks.im>
---
 refs/files-backend.c | 34 ++++++++++++++++++++++++++++------
 1 file changed, 28 insertions(+), 6 deletions(-)
Show changes to refs/files-backend.c +28 −6
diff --git a/refs/files-backend.c b/refs/files-backend.c
index f3428a9f12..0457ecdb42 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -338,9 +338,9 @@ static struct ref_cache *get_loose_ref_cache(struct files_ref_store *refs)
 	return refs->loose;
 }
 
-static int files_read_raw_ref(struct ref_store *ref_store, const char *refname,
-			      struct object_id *oid, struct strbuf *referent,
-			      unsigned int *type, int *failure_errno)
+static int read_ref_internal(struct ref_store *ref_store, const char *refname,
+			     struct object_id *oid, struct strbuf *referent,
+			     unsigned int *type, int *failure_errno, int skip_packed_refs)
 {
 	struct files_ref_store *refs =
 		files_downcast(ref_store, REF_STORE_READ, "read_raw_ref");
@@ -381,7 +381,7 @@ static int files_read_raw_ref(struct ref_store *ref_store, const char *refname,
 	if (lstat(path, &st) < 0) {
 		int ignore_errno;
 		myerr = errno;
-		if (myerr != ENOENT)
+		if (myerr != ENOENT || skip_packed_refs)
 			goto out;
 		if (refs_read_raw_ref(refs->packed_ref_store, refname, oid,
 				      referent, type, &ignore_errno)) {
@@ -425,7 +425,8 @@ static int files_read_raw_ref(struct ref_store *ref_store, const char *refname,
 		 * ref is supposed to be, there could still be a
 		 * packed ref:
 		 */
-		if (refs_read_raw_ref(refs->packed_ref_store, refname, oid,
+		if (skip_packed_refs ||
+		    refs_read_raw_ref(refs->packed_ref_store, refname, oid,
 				      referent, type, &ignore_errno)) {
 			myerr = EISDIR;
 			goto out;
@@ -470,6 +471,27 @@ static int files_read_raw_ref(struct ref_store *ref_store, const char *refname,
 	return ret;
 }
 
+static int files_read_raw_ref(struct ref_store *ref_store, const char *refname,
+			      struct object_id *oid, struct strbuf *referent,
+			      unsigned int *type, int *failure_errno)
+{
+	return read_ref_internal(ref_store, refname, oid, referent, type, failure_errno, 0);
+}
+
+static int files_read_symbolic_ref(struct ref_store *ref_store, const char *refname,
+				   struct strbuf *referent)
+{
+	struct object_id oid;
+	int failure_errno, ret;
+	unsigned int type;
+
+	ret = read_ref_internal(ref_store, refname, &oid, referent, &type, &failure_errno, 1);
+	if (ret)
+		return ret;
+
+	return !(type & REF_ISSYMREF);
+}
+
 int parse_loose_ref_contents(const char *buf, struct object_id *oid,
 			     struct strbuf *referent, unsigned int *type,
 			     int *failure_errno)
@@ -3286,7 +3308,7 @@ struct ref_storage_be refs_be_files = {
 
 	files_ref_iterator_begin,
 	files_read_raw_ref,
-	NULL,
+	files_read_symbolic_ref,
 
 	files_reflog_iterator_begin,
 	files_for_each_reflog_ent,

-- 
2.35.1
Junio C Hamano· Mar 1, 2022, 22:02 UTC · re: Patrick Steinhardt · lore

Re: [PATCH v2 0/5] fetch: more optimizations for mirror fetches

Patrick Steinhardt <ps@pks.im> writes:
> this is another patch series with the aim to speed up mirror fetches. It
> applies on top of e6ebfd0e8c (The sixth batch, 2022-02-18) with
> 3824153b23 (Merge branch 'ps/fetch-atomic' into next, 2022-02-18) merged
> into it to fix a conflict.

Thanks for clearly describing a base. Except that a merge on 'next' (e.g. 3824153b23) is not what we want a new topic to depend on; use the tip(s) of individual topic(s), i.e. 583bc419 (fetch: make `--atomic` flag cover pruning of refs, 2022-02-17), instead.

Derrick Stolee· Mar 2, 2022, 18:54 UTC · re: Patrick Steinhardt · lore

Re: [PATCH v2 0/5] fetch: more optimizations for mirror fetches

On 3/1/2022 4:33 AM, Patrick Steinhardt wrote:
Show 12 quoted lines
> Hi,
> 
> this is another patch series with the aim to speed up mirror fetches. It
> applies on top of e6ebfd0e8c (The sixth batch, 2022-02-18) with
> 3824153b23 (Merge branch 'ps/fetch-atomic' into next, 2022-02-18) merged
> into it to fix a conflict.
> 
> The only change compared to v2 is an update to the benchmarks so that
> they're less verbose, as proposed by Derrick. I also had a look at
> introducing a new helper `parse_object_probably_commit()`, but I didn't
> find the end result to be much of an improvement compared to the ad-hoc
> `lookup_commit_in_graph() || parse_object()` dance we do right now.

I'm satisfied that you tried the helper idea. This version looks good to me.

Thanks, -Stolee

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