Re: [PATCH] fetch-pack.c: use oidset to check existence of loose object
- From
Junio C Hamano <gitster@pobox.com>
- Date
- Mar 8, 2018, 18:42 UTC
- Message-ID
- <xmqqr2ouwgsd.fsf@gitster-ct.c.googlers.com>
- In-Reply-To
- <20180308120639.109438-1-tikuta@chromium.org>
Takuto Ikuta <tikuta@chromium.org> writes:
> In repository having large number of refs, lstat for non-existing loose > objects makes `git fetch` slow.
It is not immediately clear how "large number of refs" and "lstat for loose objects" interact with each other to create a problem. "In repository having large number of refs, because such and such processing needs to do this and that, 'git fetch' ends up doing a lot of lstat(2) calls to see if many objects exist in the loose form, which makes it slow". Please fill in the blanks.
Show 8 quoted lines
> This patch stores existing loose objects in hashmap beforehand and use > it to check existence instead of using lstat. > > With this patch, the number of lstat calls in `git fetch` is reduced > from 411412 to 13794 for chromium repository. > > I took time stat of `git fetch` disabling quickfetch for chromium > repository 3 time on linux with SSD.
Now you drop a clue that would help to fill in the blanks above, but I am not sure what the significance of your having to disable quickfetch in order to take measurements---it makes it sound as if it is an articificial problem that does not exist in real life (i.e. when quickfetch is not disabled), but I am getting the feeling that it is not what you wanted to say here.
In any case, do_fetch_pack() tries to see if all of the tip commits we are going to fetch exist locally, so when you are trying a fetch that grabs huge number of refs (by the way, it means that the first sentence of the proposed log message is not quite true---it is "When fetching a large number of refs", as it does not matter how many refs _we_ have, no?), everything_local() ends up making repeated calls to has_object_file_with_flags() to all of the refs.
I like the idea---this turns "for each of these many things, check if it exists with lstat(2)" into "enumerate what exists with lstat(2), and then use that for the existence test"; if you need to try N objects for existence, and you only have M objects loose where N is vastly larger than M, it will be a huge win. If you have very many loose objects and checking only a handful of objects for existence check, you would lose big, though, no?
Show 13 quoted lines
> diff --git a/fetch-pack.c b/fetch-pack.c
> index d97461296..1658487f7 100644
> --- a/fetch-pack.c
> +++ b/fetch-pack.c
> @@ -711,6 +711,15 @@ static void mark_alternate_complete(struct object *obj)
> mark_complete(&obj->oid);
> }
>
> +static int add_loose_objects_to_set(const struct object_id *oid,
> + const char *path,
> + void *data)
> +{
> + struct oidset* set = (struct oidset*)(data);Style: in our codebase, asterisk does not stick to the type.
struct oidset *set = (struct oidset *)(data);
Show 7 quoted lines
> @@ -719,16 +728,21 @@ static int everything_local(struct fetch_pack_args *args, > int retval; > int old_save_commit_buffer = save_commit_buffer; > timestamp_t cutoff = 0; > + struct oidset loose_oid_set = OIDSET_INIT; > + > + for_each_loose_object(add_loose_objects_to_set, &loose_oid_set, 0);
OK, so this is the "enumerate all loose objects" phase.
Show 5 quoted lines
> save_commit_buffer = 0;
>
> for (ref = *refs; ref; ref = ref->next) {
> struct object *o;
> + unsigned int flag = OBJECT_INFO_QUICK;Hmm, OBJECT_INFO_QUICK optimization was added in dfdd4afc ("sha1_file: teach sha1_object_info_extended more flags", 2017-06-21), but since 8b4c0103 ("sha1_file: support lazily fetching missing objects", 2017-12-08) it appears that passing OBJECT_INFO_QUICK down the codepath does not do anything interesting. Jonathan (cc'ed), are all remaining hits from "git grep OBJECT_INFO_QUICK" all dead no-ops these days?
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> > - if (!has_object_file_with_flags(&ref->old_oid, > - OBJECT_INFO_QUICK)) > - continue; > + if (!oidset_contains(&loose_oid_set, &ref->old_oid)) > + flag |= OBJECT_INFO_SKIP_LOOSE; > > + if (!has_object_file_with_flags(&ref->old_oid, flag)) > + continue;
Here, you want a way to say "I know this does not exist in the loose form, so check if it exists in a non-loose form", and that is why you invented the new flag.
Show 11 quoted lines
> diff --git a/sha1_file.c b/sha1_file.c > index 1b94f39c4..c903cbcec 100644 > --- a/sha1_file.c > +++ b/sha1_file.c > @@ -1262,6 +1262,9 @@ int sha1_object_info_extended(const unsigned char *sha1, struct object_info *oi, > if (find_pack_entry(real, &e)) > break; > > + if (flags & OBJECT_INFO_SKIP_LOOSE) > + return -1; > +
I cannot quite convince myself that this is done at the right layer; it smells to be at a bit too low a layer. This change makes sense only to a caller that is interested in the existence test. If the flag is named after what it does, i.e. "ignore loose object", then it does sort-of make sense, though.
> /* Most likely it's a loose object. */ > if (!sha1_loose_object_info(real, oi, flags)) > return 0;