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packfile.c: speed up loading lots of packfiles

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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.

Add a perf test to p5303 to show speed-up.

The existing p5303 test runtimes are dominated by other factors and do
not show an appreciable speed-up. The new test in p5303 clearly exposes
a speed-up in bad cases. In this test we create 10,000 packfiles and
measure the start-up time of git rev-parse, which does little else
besides load in the packs.

Here are the numbers for the new p5303 test:

Test                         HEAD^             HEAD
---------------------------------------------------------------------
5303.12: load 10,000 packs   1.03(0.92+0.10)   0.12(0.02+0.09) -88.3%

Signed-off-by: Colin Stolley <cstolley@runbox.com>
Helped-by: Jeff King <peff@peff.net>
[jc: squashed the change to call hashmap in install_packed_git() by peff]
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Colin Stolley
2019-11-27 16:24:53 -06:00
committed by Junio C Hamano
parent d9f6f3b619
commit ec48540fe8
4 changed files with 52 additions and 9 deletions
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3
object.c
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@ -479,6 +479,7 @@ struct raw_object_store *raw_object_store_new(void)
memset(o, 0, sizeof(*o));
INIT_LIST_HEAD(&o->packed_git_mru);
hashmap_init(&o->pack_map, pack_map_entry_cmp, NULL, 0);
return o;
}
@ -518,6 +519,8 @@ void raw_object_store_clear(struct raw_object_store *o)
INIT_LIST_HEAD(&o->packed_git_mru);
close_object_store(o);
o->packed_git = NULL;
hashmap_free(&o->pack_map);
}
void parsed_object_pool_clear(struct parsed_object_pool *o)