# [PATCH 1/4] peel_ref: use faster deref_tag_noverify 14 messages from 2012-10-04 to 2012-10-04. Participants: Jeff King, Nazri Ramliy, Ævar Arnfjörð Bjarmason, Junio C Hamano. Thread: https://gitlist.dev/t/31726 ## Jeff King, 2012-10-04 07:56 Subject: [PATCH 0/4] optimizing upload-pack ref peeling Message-ID: <20121004075609.GA1355@sigill.intra.peff.net> URL: https://gitlist.dev/e/20121004075609.GA1355%40sigill.intra.peff.net In-Reply-To: ``` On Thu, Oct 04, 2012 at 01:54:47AM +0200, Ævar Arnfjörð Bjarmason wrote: > > Hmm. It seems like we should not need to open the tags at all. The main > > reason is to produce the "peeled" advertisement just after it. But for a > > packed ref with a modern version of git that supports the "peeled" > > extension, we should already have that information. > > B.t.w. do you plan to submit this as a non-hack, I'd like to have it > in git.git, so if you're not going to I could pick it up and clean it > up a bit. But I think it would be better coming from you. Here's a cleaned-up version. I realized that peel_ref actually is quite close to what we want; the key thing is that when you are calling it from a for_each_ref loop it will not only use peeled information from packed-refs (if available), but it will also short-circuit the ref lookup for non-packed refs. So it really does what we want. I put some timings into the final patch. The results are very satisfying from a pure numbers standpoint. However, I'm not 100% sure this is all that useful a benchmark. The repos where you get a noticeable benefit are quite big, so I suspect the time to send the advertisements out over the wire will dominate. Still, a CPU second saved is a CPU second earned. And the series actually ends up making the code cleaner and shares some of the optimizations I put into upload-pack with other users of peel_ref. So I think I'd be inclined to use it, even if the speedup doesn't help that much in practice. We'll see what others think. [1/4]: peel_ref: use faster deref_tag_noverify [2/4]: peel_ref: do not return a null sha1 [3/4]: peel_ref: check object type before loading [4/4]: upload-pack: use peel_ref for ref advertisements -Peff ``` ## Jeff King, 2012-10-04 07:58 Subject: [PATCH 1/4] peel_ref: use faster deref_tag_noverify Message-ID: <20121004075815.GA31325@sigill.intra.peff.net> URL: https://gitlist.dev/e/20121004075815.GA31325%40sigill.intra.peff.net In-Reply-To: <20121004075609.GA1355@sigill.intra.peff.net> ``` When we are asked to peel a ref to a sha1, we internally call deref_tag, which will recursively parse each tagged object until we reach a non-tag. This has the benefit that we will verify our ability to load and parse the pointed-to object. However, there is a performance downside: we may not need to load that object at all (e.g., if we are listing peeled simply listing peeled refs), or it may be a large object that should follow a streaming code path (e.g., an annotated tag of a large blob). It makes more sense for peel_ref to choose the fast thing rather than performing the extra check, for two reasons: 1. We will already sometimes short-circuit the tag parsing in favor of a peeled entry from a packed-refs file. So we are already favoring speed in some cases, and it is not wise for a caller to rely on peel_ref to detect corruption. 2. We already silently ignore much larger corruptions, like a ref that points to a non-existent object, or a tag object that exists but is corrupted. 2. peel_ref is not the right place to check for such a database corruption. It is returning only the sha1 anyway, not the actual object. Any callers which use that sha1 to load an object will soon discover the corruption anyway, so we are really just pushing back the discovery to later in the program. Signed-off-by: Jeff King --- This is basically bringing the optimization from 90108a2 (upload-pack: avoid parsing tag destinations, 2012-01-06) to users of peel_ref. refs.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/refs.c b/refs.c index da74a2b..0a916a0 100644 --- a/refs.c +++ b/refs.c @@ -1225,7 +1225,7 @@ fallback: fallback: o = parse_object(base); if (o && o->type == OBJ_TAG) { - o = deref_tag(o, refname, 0); + o = deref_tag_noverify(o); if (o) { hashcpy(sha1, o->sha1); return 0; -- 1.8.0.rc0.10.g8dd2a92 ``` ## Jeff King, 2012-10-04 08:00 Subject: [PATCH 2/4] peel_ref: do not return a null sha1 Message-ID: <20121004080019.GB31325@sigill.intra.peff.net> URL: https://gitlist.dev/e/20121004080019.GB31325%40sigill.intra.peff.net In-Reply-To: <20121004075609.GA1355@sigill.intra.peff.net> ``` The idea of the peel_ref function is to dereference tag objects recursively until we hit a non-tag, and return the sha1. Conceptually, it should return 0 if it is successful (and fill in the sha1), or -1 if there was nothing to peel. However, the current behavior is much more confusing. For a regular loose ref, the behavior is as described above. But there is an optimization to reuse the peeled-ref value for a ref that came from a packed-refs file. If we have such a ref, we return its peeled value, even if that peeled value is null (indicating that we know the ref definitely does _not_ peel). It might seem like such information is useful to the caller, who would then know not to bother loading and trying to peel the object. Except that they should not bother loading and trying to peel the object _anyway_, because that fallback is already handled by peel_ref. In other words, the whole point of calling this function is that it handles those details internally, and you either get a sha1, or you know that it is not peel-able. This patch catches the null sha1 case internally and converts it into a -1 return value (i.e., there is nothing to peel). This simplifies callers, which do not need to bother checking themselves. Two callers are worth noting: - in pack-objects, a comment indicates that there is a difference between non-peelable tags and unannotated tags. But that is not the case (before or after this patch). Whether you get a null sha1 has to do with internal details of how peel_ref operated. - in show-ref, if peel_ref returns a failure, the caller tries to decide whether to try peeling manually based on whether the REF_ISPACKED flag is set. But this doesn't make any sense. If the flag is set, that does not necessarily mean the ref came from a packed-refs file with the "peeled" extension. But it doesn't matter, because even if it didn't, there's no point in trying to peel it ourselves, as peel_ref would already have done so. In other words, the fallback peeling is guaranteed to fail. Signed-off-by: Jeff King --- Sorry for the long-winded explanation. I'd love it if somebody could eyeball the code and make sure what I wrote above is correct. I was mightily confused at first by the way that some of the callers treated the return value. I think it is simply a case of code growing crufty as features were added (originally the packed-refs optimization did not exist, and was bolted on later), but I may have mis-read something. builtin/describe.c | 2 +- builtin/pack-objects.c | 1 - builtin/show-ref.c | 23 +++-------------------- refs.c | 2 ++ 4 files changed, 6 insertions(+), 22 deletions(-) diff --git a/builtin/describe.c b/builtin/describe.c index 9fe11ed..04c185b 100644 --- a/builtin/describe.c +++ b/builtin/describe.c @@ -144,7 +144,7 @@ static int get_name(const char *path, const unsigned char *sha1, int flag, void if (!all && !might_be_tag) return 0; - if (!peel_ref(path, peeled) && !is_null_sha1(peeled)) { + if (!peel_ref(path, peeled)) { is_tag = !!hashcmp(sha1, peeled); } else { hashcpy(peeled, sha1); diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c index 5e14064..f069462 100644 --- a/builtin/pack-objects.c +++ b/builtin/pack-objects.c @@ -2033,7 +2033,6 @@ static int add_ref_tag(const char *path, const unsigned char *sha1, int flag, vo if (!prefixcmp(path, "refs/tags/") && /* is a tag? */ !peel_ref(path, peeled) && /* peelable? */ - !is_null_sha1(peeled) && /* annotated tag? */ locate_object_entry(peeled)) /* object packed? */ add_object_entry(sha1, OBJ_TAG, NULL, 0); return 0; diff --git a/builtin/show-ref.c b/builtin/show-ref.c index 4eb016d..8d9b76a 100644 --- a/builtin/show-ref.c +++ b/builtin/show-ref.c @@ -28,7 +28,6 @@ static int show_ref(const char *refname, const unsigned char *sha1, int flag, vo static int show_ref(const char *refname, const unsigned char *sha1, int flag, void *cbdata) { - struct object *obj; const char *hex; unsigned char peeled[20]; @@ -79,25 +78,9 @@ match: if (!deref_tags) return 0; - if ((flag & REF_ISPACKED) && !peel_ref(refname, peeled)) { - if (!is_null_sha1(peeled)) { - hex = find_unique_abbrev(peeled, abbrev); - printf("%s %s^{}\n", hex, refname); - } - } - else { - obj = parse_object(sha1); - if (!obj) - die("git show-ref: bad ref %s (%s)", refname, - sha1_to_hex(sha1)); - if (obj->type == OBJ_TAG) { - obj = deref_tag(obj, refname, 0); - if (!obj) - die("git show-ref: bad tag at ref %s (%s)", refname, - sha1_to_hex(sha1)); - hex = find_unique_abbrev(obj->sha1, abbrev); - printf("%s %s^{}\n", hex, refname); - } + if (!peel_ref(refname, peeled)) { + hex = find_unique_abbrev(peeled, abbrev); + printf("%s %s^{}\n", hex, refname); } return 0; } diff --git a/refs.c b/refs.c index 0a916a0..f672ad9 100644 --- a/refs.c +++ b/refs.c @@ -1202,6 +1202,8 @@ int peel_ref(const char *refname, unsigned char *sha1) if (current_ref && (current_ref->name == refname || !strcmp(current_ref->name, refname))) { if (current_ref->flag & REF_KNOWS_PEELED) { + if (is_null_sha1(current_ref->u.value.peeled)) + return -1; hashcpy(sha1, current_ref->u.value.peeled); return 0; } -- 1.8.0.rc0.10.g8dd2a92 ``` ## Jeff King, 2012-10-04 08:02 Subject: [PATCH 3/4] peel_ref: check object type before loading Message-ID: <20121004080253.GC31325@sigill.intra.peff.net> URL: https://gitlist.dev/e/20121004080253.GC31325%40sigill.intra.peff.net In-Reply-To: <20121004075609.GA1355@sigill.intra.peff.net> ``` The point of peel_ref is to dereference tags; if the base object is not a tag, then we can return early without even loading the object into memory. This patch accomplishes that by checking sha1_object_info for the type. For a packed object, we can get away with just looking in the pack index. For a loose object, we only need to inflate the first couple of header bytes. This is a bit of a gamble; if we do find a tag object, then we will end up loading the content anyway, and the extra lookup will have been wasteful. However, if it is not a tag object, then we save loading the object entirely. Depending on the ratio of non-tags to tags in the input, this can be a minor win or minor loss. However, it does give us one potential major win: if a ref points to a large blob (e.g., via an unannotated tag), then we can avoid looking at it entirely. Signed-off-by: Jeff King --- This optimization is the one that gave me the most pause. While upload-pack does call peel_ref on everything, the other callers all constrain themselves to refs/tags/. So for many projects, we will be calling it mostly on annotated tags, and it may be a very small net loss. But in practice, it will not matter for most projects with a sane number of normal tags, and saving even one accidental giant blob load can have a huge impact. refs.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/refs.c b/refs.c index f672ad9..02e47b1 100644 --- a/refs.c +++ b/refs.c @@ -1225,8 +1225,15 @@ fallback: } fallback: - o = parse_object(base); - if (o && o->type == OBJ_TAG) { + o = lookup_unknown_object(base); + if (o->type == OBJ_NONE) { + int type = sha1_object_info(base, NULL); + if (type < 0) + return -1; + o->type = type; + } + + if (o->type == OBJ_TAG) { o = deref_tag_noverify(o); if (o) { hashcpy(sha1, o->sha1); -- 1.8.0.rc0.10.g8dd2a92 ``` ## Jeff King, 2012-10-04 08:03 Subject: [PATCH 4/4] upload-pack: use peel_ref for ref advertisements Message-ID: <20121004080333.GD31325@sigill.intra.peff.net> URL: https://gitlist.dev/e/20121004080333.GD31325%40sigill.intra.peff.net In-Reply-To: <20121004075609.GA1355@sigill.intra.peff.net> ``` When upload-pack advertises refs, we attempt to peel tags and advertise the peeled version. We currently hand-roll the tag dereferencing, and use as many optimizations as we can to avoid loading non-tag objects into memory. Not only has peel_ref recently learned these optimizations, too, but it also contains an even more important one: it has access to the "peeled" data from the pack-refs file. That means we can avoid not only loading annotated tags entirely, but also avoid doing any kind of object lookup at all. This cut the CPU time to advertise refs by 50% in the linux-2.6 repo, as measured by: echo 0000 | git-upload-pack . >/dev/null best-of-five, warm cache, objects and refs fully packed: [before] [after] real 0m0.026s real 0m0.013s user 0m0.024s user 0m0.008s sys 0m0.000s sys 0m0.000s Those numbers are irrelevantly small compared to an actual fetch. Here's a larger repo (400K refs, of which 12K are unique, and of which only 107 are unique annotated tags): [before] [after] real 0m0.704s real 0m0.596s user 0m0.600s user 0m0.496s sys 0m0.096s sys 0m0.092s This shows only a 15% speedup (mostly because it has fewer actual tags to parse), but a larger absolute value (100ms, which isn't a lot compared to a real fetch, but this advertisement happens on every fetch, even if the client is just finding out they are completely up to date). In truly pathological cases, where you have a large number of unique annotated tags, it can make an even bigger difference. Here are the numbers for a linux-2.6 repository that has had every seventh commit tagged (so about 50K tags): [before] [after] real 0m0.443s real 0m0.097s user 0m0.416s user 0m0.080s sys 0m0.024s sys 0m0.012s Signed-off-by: Jeff King --- upload-pack.c | 14 +++----------- 1 file changed, 3 insertions(+), 11 deletions(-) diff --git a/upload-pack.c b/upload-pack.c index 2e90ccb..6142421 100644 --- a/upload-pack.c +++ b/upload-pack.c @@ -727,12 +727,7 @@ static int send_ref(const char *refname, const unsigned char *sha1, int flag, vo " include-tag multi_ack_detailed"; struct object *o = lookup_unknown_object(sha1); const char *refname_nons = strip_namespace(refname); - - if (o->type == OBJ_NONE) { - o->type = sha1_object_info(sha1, NULL); - if (o->type < 0) - die("git upload-pack: cannot find object %s:", sha1_to_hex(sha1)); - } + unsigned char peeled[20]; if (capabilities) packet_write(1, "%s %s%c%s%s agent=%s\n", @@ -747,11 +742,8 @@ static int send_ref(const char *refname, const unsigned char *sha1, int flag, vo o->flags |= OUR_REF; nr_our_refs++; } - if (o->type == OBJ_TAG) { - o = deref_tag_noverify(o); - if (o) - packet_write(1, "%s %s^{}\n", sha1_to_hex(o->sha1), refname_nons); - } + if (!peel_ref(refname, peeled)) + packet_write(1, "%s %s^{}\n", sha1_to_hex(peeled), refname_nons); return 0; } -- 1.8.0.rc0.10.g8dd2a92 ``` ## Jeff King, 2012-10-04 08:04 Subject: Re: [PATCH 0/4] optimizing upload-pack ref peeling Message-ID: <20121004080438.GA31305@sigill.intra.peff.net> URL: https://gitlist.dev/e/20121004080438.GA31305%40sigill.intra.peff.net In-Reply-To: <20121004075609.GA1355@sigill.intra.peff.net> ``` On Thu, Oct 04, 2012 at 03:56:09AM -0400, Jeff King wrote: > [1/4]: peel_ref: use faster deref_tag_noverify > [2/4]: peel_ref: do not return a null sha1 > [3/4]: peel_ref: check object type before loading > [4/4]: upload-pack: use peel_ref for ref advertisements I included my own timings in the final one, but my "pathological" case at the end is a somewhat made-up attempt to emulate what you described. Can you double-check that this series still has a nice impact on your real-world repository? -Peff ``` ## Ævar Arnfjörð Bjarmason, 2012-10-04 09:01 Subject: Re: [PATCH 0/4] optimizing upload-pack ref peeling Message-ID: URL: https://gitlist.dev/e/CACBZZX6hoXdP5fV8NSmkEOvm1eayyCH%2BvSJYSpd%3DZL0XjC68HQ%40mail.gmail.com In-Reply-To: <20121004080438.GA31305@sigill.intra.peff.net> ``` On Thu, Oct 4, 2012 at 10:04 AM, Jeff King wrote: > On Thu, Oct 04, 2012 at 03:56:09AM -0400, Jeff King wrote: > >> [1/4]: peel_ref: use faster deref_tag_noverify >> [2/4]: peel_ref: do not return a null sha1 >> [3/4]: peel_ref: check object type before loading >> [4/4]: upload-pack: use peel_ref for ref advertisements > > I included my own timings in the final one, but my "pathological" case > at the end is a somewhat made-up attempt to emulate what you described. > Can you double-check that this series still has a nice impact on your > real-world repository? It does, here's best of five for, all compiled with -g -O3: v1.7.8: $ time (echo 0000 | ~/g/git/git-upload-pack . | pv >/dev/null) 3.49MB 0:00:00 [ 5.3MB/s] [ <=> ] real 0m0.660s user 0m0.604s sys 0m0.248s master without your patches: $ time (echo 0000 | ~/g/git/git-upload-pack . | pv >/dev/null) 3.49MB 0:00:00 [10.2MB/s] [ <=> ] real 0m0.344s user 0m0.300s sys 0m0.172s master with your patches: $ time (echo 0000 | ~/g/git/git-upload-pack . | pv >/dev/null) 3.49MB 0:00:00 [31.8MB/s] [ <=> ] real 0m0.113s user 0m0.088s sys 0m0.088s ``` ## Nazri Ramliy, 2012-10-04 12:14 Subject: Re: [PATCH 0/4] optimizing upload-pack ref peeling Message-ID: URL: https://gitlist.dev/e/CAEY4ZpMiQn61ZD8-Gbcr8zKW%3DLRQZKwFb7kwVeAJ3tPrvkRbWQ%40mail.gmail.com In-Reply-To: ``` On Thu, Oct 4, 2012 at 5:01 PM, Ævar Arnfjörð Bjarmason wrote: > $ time (echo 0000 | ~/g/git/git-upload-pack . | pv >/dev/null) Totally off-topic, but ... Thanks for making me look up what pv is. I remember checking it out quiet sometime ago and have forgotten about it altogether. Now I can replace my poorly written, half-assed attempt at a shell script to do something similar to what pv is already capable of! Nazri. ``` ## Junio C Hamano, 2012-10-04 18:24 Subject: Re: [PATCH 1/4] peel_ref: use faster deref_tag_noverify Message-ID: <7vk3v6yu1g.fsf@alter.siamese.dyndns.org> URL: https://gitlist.dev/e/7vk3v6yu1g.fsf%40alter.siamese.dyndns.org In-Reply-To: <20121004075815.GA31325@sigill.intra.peff.net> ``` Jeff King writes: > When we are asked to peel a ref to a sha1, we internally call > deref_tag, which will recursively parse each tagged object > until we reach a non-tag. This has the benefit that we will > verify our ability to load and parse the pointed-to object. > > However, there is a performance downside: we may not need to > load that object at all (e.g., if we are listing peeled > simply listing peeled refs),... Both correct. I checked the existing callsites and nobody seems to assume that it can use the pointed-to object without checking because peel_ref has already checked, so it makes sense. ``` ## Junio C Hamano, 2012-10-04 18:32 Subject: Re: [PATCH 2/4] peel_ref: do not return a null sha1 Message-ID: <7vfw5uytoy.fsf@alter.siamese.dyndns.org> URL: https://gitlist.dev/e/7vfw5uytoy.fsf%40alter.siamese.dyndns.org In-Reply-To: <20121004080019.GB31325@sigill.intra.peff.net> ``` Jeff King writes: > The idea of the peel_ref function is to dereference tag > objects recursively until we hit a non-tag, and return the > sha1. Conceptually, it should return 0 if it is successful > (and fill in the sha1), or -1 if there was nothing to peel. > > However, the current behavior is much more confusing. For a > regular loose ref, the behavior is as described above. But > there is an optimization to reuse the peeled-ref value for a > ref that came from a packed-refs file. If we have such a > ref, we return its peeled value, even if that peeled value > is null (indicating that we know the ref definitely does > _not_ peel). > > It might seem like such information is useful to the caller, > who would then know not to bother loading and trying to peel > the object. Except that they should not bother loading and > trying to peel the object _anyway_, because that fallback is > already handled by peel_ref. In other words, the whole point > of calling this function is that it handles those details > internally, and you either get a sha1, or you know that it > is not peel-able. > > This patch catches the null sha1 case internally and > converts it into a -1 return value (i.e., there is nothing > to peel). This simplifies callers, which do not need to > bother checking themselves. > > Two callers are worth noting: > > - in pack-objects, a comment indicates that there is a > difference between non-peelable tags and unannotated > tags. But that is not the case (before or after this > patch). Whether you get a null sha1 has to do with > internal details of how peel_ref operated. Yeah, this is what I was wondering while reviewing [1/4]. We traditionally said both HEAD^0 and HEAD^0^0 peel to HEAD, but this at least at the internal API level redefines them to "these do not peel at all and is a failure". In other words, peel_ref(ref, sha1) that returns 0 is a way to see if ref points at a real tag object, and if the function returns non-zero, the caller cannot tell if the failure is because it was a valid commit or a corrupt object. The check !is_null_sha1(peeled) always looked fishy to me. Good riddance. ``` ## Junio C Hamano, 2012-10-04 19:06 Subject: Re: [PATCH 3/4] peel_ref: check object type before loading Message-ID: <7vbogiys47.fsf@alter.siamese.dyndns.org> URL: https://gitlist.dev/e/7vbogiys47.fsf%40alter.siamese.dyndns.org In-Reply-To: <20121004080253.GC31325@sigill.intra.peff.net> ``` Jeff King writes: > The point of peel_ref is to dereference tags; if the base > object is not a tag, then we can return early without even > loading the object into memory. > > This patch accomplishes that by checking sha1_object_info > for the type. For a packed object, we can get away with just > looking in the pack index. For a loose object, we only need > to inflate the first couple of header bytes. We look at the pack index and have to follow its delta chain down to the base to find its type; if the object is deeply deltified, this certainly is an overall loss. The only case sha1_object_info() could work well for an object near the tip of a deep delta chain is to find its size, as the diff-delta encodes the size of the base and the size of the result of applying the delta to the base, so you do not have to follow the chain when you are only interested in the final size. But alas nobody calls sha1_object_info() for only size but not type (there are some callers that are interested in only type but not size). > This is a bit of a gamble; if we do find a tag object, then > we will end up loading the content anyway, and the extra > lookup will have been wasteful. However, if it is not a tag > object, then we save loading the object entirely. Depending > on the ratio of non-tags to tags in the input, this can be a > minor win or minor loss. > > However, it does give us one potential major win: if a ref > points to a large blob (e.g., via an unannotated tag), then > we can avoid looking at it entirely. > > Signed-off-by: Jeff King > --- > This optimization is the one that gave me the most pause. While > upload-pack does call peel_ref on everything, the other callers all > constrain themselves to refs/tags/. So for many projects, we will be > calling it mostly on annotated tags, and it may be a very small net > loss. But in practice, it will not matter for most projects with a sane > number of normal tags, and saving even one accidental giant blob load > can have a huge impact. I may be missing something, but the above description is not convincing to me. When was the last time you pointed a blob directly with a ref, whether large or small, and whether within refs/tags or outside? > > refs.c | 11 +++++++++-- > 1 file changed, 9 insertions(+), 2 deletions(-) > > diff --git a/refs.c b/refs.c > index f672ad9..02e47b1 100644 > --- a/refs.c > +++ b/refs.c > @@ -1225,8 +1225,15 @@ fallback: > } > > fallback: > - o = parse_object(base); > - if (o && o->type == OBJ_TAG) { > + o = lookup_unknown_object(base); > + if (o->type == OBJ_NONE) { > + int type = sha1_object_info(base, NULL); > + if (type < 0) > + return -1; > + o->type = type; > + } > + > + if (o->type == OBJ_TAG) { > o = deref_tag_noverify(o); > if (o) { > hashcpy(sha1, o->sha1); ``` ## Jeff King, 2012-10-04 19:41 Subject: Re: [PATCH 3/4] peel_ref: check object type before loading Message-ID: <20121004194150.GA13955@sigill.intra.peff.net> URL: https://gitlist.dev/e/20121004194150.GA13955%40sigill.intra.peff.net In-Reply-To: <7vbogiys47.fsf@alter.siamese.dyndns.org> ``` On Thu, Oct 04, 2012 at 12:06:16PM -0700, Junio C Hamano wrote: > Jeff King writes: > > > The point of peel_ref is to dereference tags; if the base > > object is not a tag, then we can return early without even > > loading the object into memory. > > > > This patch accomplishes that by checking sha1_object_info > > for the type. For a packed object, we can get away with just > > looking in the pack index. For a loose object, we only need > > to inflate the first couple of header bytes. > > We look at the pack index and have to follow its delta chain down to > the base to find its type; if the object is deeply deltified, this > certainly is an overall loss. Yes. Although in practice I would expect most things pointed to by refs to not be delta'd. And indeed, comparing "git index-pack -v" to "git for-each-ref" in my git.git repository yields 950 non-delta tips and only 42 delta (and those are mostly rebased commit objects). In my linux-2.6 repo, I have 0 delta ref tips (probably because I do not actually do kernel work, but keep it around for querying). The numbers I mentioned in 926f1dd (which did this exact same optimization for upload-pack) showed a slight improvement in practice. Because keep in mind that the alternative to this multi-step delta lookup for the type is to load the object. Which will do the _same_ lookup, but will also have to actually compute the delta. So the only real loss is that you have to repeat the lookup (or in the case of a loose object, re-open and re-inflate the first 32 bytes). The best way to do this would be to have sha1_object_info return some kind of handle to the object where we could get the meta info cheaply, and then reuse the handle to get the actual object data later (so for a packed object, cache the actual pack location; for a loose object, keep the descriptor and zlib stream open). Of course, the speed up in 926f1dd probably won't matter anymore because the whole point of this series is to avoid touching the object at all. So hitting sha1_object_info in peel_ref is already the slow path. On most objects, an optimization like this will not be noticeable either way. The interesting thing is that we can avoid touching a large or expensive object at all. > > This optimization is the one that gave me the most pause. While > > upload-pack does call peel_ref on everything, the other callers all > > constrain themselves to refs/tags/. So for many projects, we will be > > calling it mostly on annotated tags, and it may be a very small net > > loss. But in practice, it will not matter for most projects with a sane > > number of normal tags, and saving even one accidental giant blob load > > can have a huge impact. > > I may be missing something, but the above description is not > convincing to me. When was the last time you pointed a blob > directly with a ref, whether large or small, and whether within > refs/tags or outside? Not often (though you have such a ref in git.git). I started down this road because just applying patch 4/4 produces a regression in the test suite. t1050 tries to clone a tag pointing to a large object with GIT_ALLOC_LIMIT set. Using peel_ref regresses this to actually load the whole object into memory. That particular regression is fixed by 1/1 (because it's an annotated tag pointing to the large object). This would fix the same case if it were an unannotated tag. Now is that a real-world test? No, I'm not sure it is. But I consider it a win for large-object git[1] every time we can avoid loading an object, because it only takes one unnecessary load to ruin the user experience. So even if it's unlikely, if it isn't hurting the regular code path, I think it's worth doing (and I'm not sure it is hurting; my numbers showed that it be helping, though I suspect it is doing nothing after my 4/4). -Peff [1] One thing I've been toying is with "external alternates"; dumping your large objects in some realtively slow data store (e.g., a RESTful HTTP service). You could cache and cheaply query a list of "sha1 / size / type" for each object from the store, but getting the actual objects would be much more expensive. But again, it would depend on whether you would actually have such a store directly accessible by a ref. ``` ## Junio C Hamano, 2012-10-04 20:41 Subject: Re: [PATCH 3/4] peel_ref: check object type before loading Message-ID: <7vd30yx94r.fsf@alter.siamese.dyndns.org> URL: https://gitlist.dev/e/7vd30yx94r.fsf%40alter.siamese.dyndns.org In-Reply-To: <20121004194150.GA13955@sigill.intra.peff.net> ``` Jeff King writes: > [1] One thing I've been toying is with "external alternates"; dumping > your large objects in some realtively slow data store (e.g., a > RESTful HTTP service). You could cache and cheaply query a list of > "sha1 / size / type" for each object from the store, but getting the > actual objects would be much more expensive. But again, it would > depend on whether you would actually have such a store directly > accessible by a ref. Yeah, that actually has been another thing we were discussing locally, without coming to something concrete enough to present to the list. The basic idea is to mark such paths with attributes, and use a variant of smudge/clean filter that is _not_ a filter (as we do not want to have the interface to this external helper to be "we feed the whole big blob to you"). Instead, these smudgex/cleanx things work on a pathname. - Your in-tree objects store a blob that records a description of the large thing. Call such a blob a surrogate. "clone", "fetch" and "push" all deal only with surrogates so your in-history data will stay small. - When checking out, the attributes mechanism kicks in and runs the "not filter" variant of smudge with the data in the surrogate. The surrogate records how to get the real thing from where, and how to validate what you got is correct. A hand-wavy example may look like this: get: download http://cdn.example.com/67def20 sha1sum: f84667def209e4a84e37e8488a08e9eca3f208c1 to tell you to download a single URL with whatever means suitable for your platform (perhaps curl or wget), and verify the result by running sha1sum. Or it may involve get: git-fetch git://git.example.com/images.git/ master object: 85a094f22f02c54c740448f6716da608a5e89a80 to tell you to "git fetch" from the given git-reachable resource into some place and grab the object via "git cat-file", possibly streaming it out. The details do not matter at this point in the design process. The smudgex helper is responsible for caching previously fetched large contents, maintaining association between the surrogate blob and its real data, so that once the real thing is downloaded, and the contents of the path needs to change to something else (e.g. user checks out a different branch) and then change to the previous thing again (e.g. user comes back to the original branch), it does not download it again. - When checking if the working tree is clean relative to the index, the smudgex/cleanx helper will be consulted. It will be given the surrogate data in the index and the path in the working tree. We may want to allow the helper implementation to give a read-only hardlink directly into helper's cache storage, so that it can consult its database of surrogate-to-real mapping and perform this verification cheaply by inode comparison, or something. - When running "git add" a modified large stuff prepared in the working tree, cleanx helper is called to prepare a new surrogate, and that is what is registered in the index. The helper is also responsible for storing the new large stuff away and arrange it to be retrievable when others see and use this surrogate. The initial scope of supporting something like that in core-git would be to add the necessary infrastracture to arrange such smudgex and cleanx helpers are called when a path is marked as a surrogate in the attribute system, and supply a sample helper. ``` ## Jeff King, 2012-10-04 21:59 Subject: Re: [PATCH 3/4] peel_ref: check object type before loading Message-ID: <20121004215933.GA17358@sigill.intra.peff.net> URL: https://gitlist.dev/e/20121004215933.GA17358%40sigill.intra.peff.net In-Reply-To: <7vd30yx94r.fsf@alter.siamese.dyndns.org> ``` On Thu, Oct 04, 2012 at 01:41:40PM -0700, Junio C Hamano wrote: > Jeff King writes: > > > [1] One thing I've been toying is with "external alternates"; dumping > > your large objects in some realtively slow data store (e.g., a > > RESTful HTTP service). You could cache and cheaply query a list of > > "sha1 / size / type" for each object from the store, but getting the > > actual objects would be much more expensive. But again, it would > > depend on whether you would actually have such a store directly > > accessible by a ref. > > Yeah, that actually has been another thing we were discussing > locally, without coming to something concrete enough to present to > the list. > > The basic idea is to mark such paths with attributes, and use a > variant of smudge/clean filter that is _not_ a filter (as we do not > want to have the interface to this external helper to be "we feed > the whole big blob to you"). Instead, these smudgex/cleanx things > work on a pathname. There are a few ways that smudge/clean filters are not up to the task currently. One is definitely the efficiency of the interface. Another is that this distinction is not "canonical in-repo versus working tree representation". Yes, part of it is about what you store in the repo versus what you checkout. But you may also want to diff or merge these large items. Almost certainly not with the internal text tools, but you might want to feed the _real_ blobs (not the fake surrogates) to external tools, and smudge and clean are not a natural fit for that. If you teach the external tools to dereference the surrogates, it would work. I think you could also get around it by giving these smudgex/cleanx filters slightly different semantics. But the thing I most dislike about a solution like this is that the representation pollutes git's data model. That is, git stores a blob representing the surrogate, and that is the official sha1 that goes into the tree. That means your surrogate decisions are locked into history forever, which has a few implications: 1. If I store a surrogate and you store the real blob, we do not get the same tree (and in fact get conflicts). 2. Once I store a blob, I can never revise my decision not to store that blob in every subsequent clone without rewriting history. I can convert it into a surrogate, but old versions of that blob will always be required for object connectivity. 3. If your surrogate contains more than just the sha1 (e.g., if it points to http://cdn.example.com), your history is forever tied to that (and you are stuck with configuring run-time redirects if your URL changes). My thinking was to make it all happen below the git object level, at the same level as packs and loose objects. This is more invasive to git, but much more flexible. The basic implementation is pretty straightforward. You configure one or more helpers which have two operations: provide a list of sha1s, and fetch a single sha1. Whenever an object lookup fails in the packs and loose objects, we check the helper lists. We can cache the lists in some mmap'able format similar to a pack index, so we can do quick checks for object existence, type, and size. And if the lookup actually needs the object, we fetch and cache (where the caching policy would all be determined by the external script). The real challenges are: 1. It is expensive to meet normal reachability guarantees. So you would not want a remote to delta against a blob that you _could_ get, but do not have. 2. You need to tell remotes that you can access some objects in a different way, and not to send them as part of an object transfer. 3. Many commands need to be taught not to load objects carelessly. For the most part, we do well with this because it's already expensive to load large objects from disk. I think we've got most of them on the diff code path, but I wouldn't be surprised if one or two more crop up. Fsck would need to learn to handle these objects differently. Item (3) is really just about trying it and seeing where the problems come up. For items (1) and (2), we'd need a protocol extension. Having the receiver send something like "I have these blobs from my external source, don't send them" is a nice idea, but it doesn't scale well (you have to advertise the whole list for each transfer, because the receiver doesn't know which ones are actually referenced). Having the receiver say "I have access to external database $FOO, go check the list of objects there and don't send anything it has" unnecessarily ties the sender to the external database (i.e., they have to implement $FOO for it to work). Something simple like "do not send blobs larger than N bytes, nor make deltas against such blobs" would work. It does mean that your value of N really needs to match up with what goes into your external database, but I don't think that will be a huge problem in practice (after the transfer you can do a consistency check that between the fetched objects and the external db, you have everything). > [details on smudgex/cleanx] All of what you wrote seems very sane; I think the real question is whether this should be "above" or "below" git's user-visible data model layer. -Peff ```