Re: [PATCH] packfile.c: speed up loading lots of packfiles.

[Jeff King <peff@peff.net>]

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.

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