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Re: [PATCH] fetch-pack.c: use oidset to check existence of loose object

Takuto Ikuta <tikuta@xxxxxxxxxxxx> 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.

> 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?

> 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);

> @@ -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.

>  	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?

> -		if (!has_object_file_with_flags(&ref->old_oid,
> -			continue;
> +		if (!oidset_contains(&loose_oid_set, &ref->old_oid))
> +		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.

> 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;