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From: Taylor Blau <me@ttaylorr.com>
To: Jeff King <peff@peff.net>
Cc: "SZEDER Gábor" <szeder.dev@gmail.com>,
	"Colin Stolley" <cstolley@runbox.com>,
	git@vger.kernel.org
Subject: Re: [PATCH] packfile.c: speed up loading lots of packfiles.
Date: Mon, 2 Dec 2019 22:17:44 -0800
Message-ID: <20191203061744.GA74594@syl.local> (raw)
In-Reply-To: <20191202194231.GA10707@sigill.intra.peff.net>

On Mon, Dec 02, 2019 at 02:42:31PM -0500, Jeff King wrote:
> On Mon, Dec 02, 2019 at 06:40:35PM +0100, SZEDER Gábor wrote:
>
> > > When loading packfiles on start-up, we traverse the internal packfile
> > > list once per file to avoid reloading packfiles that have already
> > > been loaded. This check runs in quadratic time, so for poorly
> > > maintained repos with a large number of packfiles, it can be pretty
> > > slow.
> > >
> > > Add a hashmap containing the packfile names as we load them so that
> > > the average runtime cost of checking for already-loaded packs becomes
> > > constant.
> > [...]
> > This patch break test 'gc --keep-largest-pack' in 't6500-gc.sh' when
> > run with GIT_TEST_MULTI_PACK_INDEX=1, because there is a duplicate
> > entry in '.git/objects/info/packs':
>
> Good catch. The issue is that we only add entries to the hashmap in
> prepare_packed_git(), but they may be added to the pack list by other
> callers of install_packed_git(). It probably makes sense to just push
> the hashmap maintenance down into that function, like below. That
> requires an extra strhash() when inserting a new pack, but I don't think
> that's a big deal.

Ah, great catch, and thanks for pointing it out. We have been running
this patch in production at GitHub for a few weeks now, but didn't
notice this because we never run tests with
'GIT_TEST_MULTI_PACK_INDEX=1' in the environment.

Perhaps in the future that might change, but I think that for now that
can explain why the failure wasn't noticed earlier.

> diff --git a/packfile.c b/packfile.c
> index 253559fa87..f0dc63e92f 100644
> --- a/packfile.c
> +++ b/packfile.c
> @@ -757,6 +757,9 @@ void install_packed_git(struct repository *r, struct packed_git *pack)
>
>  	pack->next = r->objects->packed_git;
>  	r->objects->packed_git = pack;
> +
> +	hashmap_entry_init(&pack->packmap_ent, strhash(pack->pack_name));
> +	hashmap_add(&r->objects->pack_map, &pack->packmap_ent);
>  }
>
>  void (*report_garbage)(unsigned seen_bits, const char *path);
> @@ -864,11 +867,8 @@ static void prepare_pack(const char *full_name, size_t full_name_len,
>  		/* Don't reopen a pack we already have. */
>  		if (!hashmap_get(&data->r->objects->pack_map, &hent, pack_name)) {
>  			p = add_packed_git(full_name, full_name_len, data->local);
> -			if (p) {
> -				hashmap_entry_init(&p->packmap_ent, hash);
> -				hashmap_add(&data->r->objects->pack_map, &p->packmap_ent);
> +			if (p)
>  				install_packed_git(data->r, p);
> -			}
>  		}
>  		free(pack_name);
>  	}
>
> -Peff
Thanks,
Taylor

  reply index

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-11-27 22:24 Colin Stolley
2019-11-28  0:42 ` hashmap vs khash? " Eric Wong
2019-11-30 17:36   ` Junio C Hamano
2019-12-02 14:39   ` Jeff King
2019-12-02 17:40 ` SZEDER Gábor
2019-12-02 19:42   ` Jeff King
2019-12-03  6:17     ` Taylor Blau [this message]
2019-12-03 15:34       ` Jeff King
2019-12-03 16:04     ` Junio C Hamano
2019-12-03 17:33       ` Colin Stolley
2019-12-03 22:18         ` Jeff King
2019-12-04 18:15           ` Junio C Hamano
2019-12-03 22:17       ` Jeff King
2019-12-04  4:23         ` Jonathan Nieder
2019-12-03  6:19 ` Taylor Blau

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