Re: [PATCH v3] fetch-pack.c: use oidset to check existence of loose object

From: Junio C Hamano <gitster@pobox.com>
Date: Mar 9, 2018, 19:54 UTC
Message-ID: <xmqq606558k2.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <20180309132655.224308-1-tikuta@chromium.org>

Takuto Ikuta <tikuta@chromium.org> writes:

> In repository having large number of remote refs, because to check

Isn't this "When fetching from a repository with large number of refs,"? The number of refs (whether it is local or remote-tracking) the local side has has nothing to do with the issue you are addressing, no?

> existence of each refs in local repository, 'git fetch' ends up doing a
> lot of lstat(2) calls to see if it exists in loose form, which makes it
> slow.

Other than that, the above description reads much better and makes the result easier to understand.

> This patch enumerates loose objects in hashmap beforehand and uses it to
> check existence instead of using lstat(2) to improve performance of
> fetch-pack for repositories having large number of remote refs compared
> to the number of loose objects.

We'd rather write this paragraph as if giving an order to the codebase "to be like so", e.g.

	Instead of making as many lstat(2) calls as the refs the
	remote side advertised to see if these objects exist in the
	loose form, first enumerate all the existing loose objects
	in hashmap beforehand and use it to check existence of
	them...

> I took time stat of `git fetch` disabling quickfetch, so that fetch-pack

I still do not know if a benchmark with quickfetch disabled gives relevant numbers, for reasons I gave earlier. The relative numbers between Linux and MacBook look quite convincing, as they illustrate differences of lstat(2) performance on these platforms.

Show 11 quoted lines

>  	for (ref = *refs; ref; ref = ref->next) {
>  		struct object *o;
> +		unsigned int flags = OBJECT_INFO_QUICK;
>  
> -		if (!has_object_file_with_flags(&ref->old_oid,
> -						OBJECT_INFO_QUICK))
> -			continue;
> +		if (!oidset_contains(&loose_oid_set, &ref->old_oid)) {
> +			/* I know this does not exist in the loose form,
> +			 * so check if it exists in a non-loose form.
> +			 */

	/*
	 * Our multi-line comment looks like this,
	 * with opening slash-asterisk and closing
	 * asterisk-slash on their own lines.
	 */

Thanks.