Re: [PATCH] packfile.c: speed up loading lots of packfiles.
From: Taylor Blau <hidden>
Date: 2019-12-03 06:17:49
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:quoted
quoted
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.
quoted hunk ↗ jump to hunk
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