packfile.c: speed up loading lots of packfiles

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>

1 files changed, 3 insertions, 0 deletions

diff --git a/object.c b/object.c
index 3b8b8c55c9..142ef69399 100644
--- a/object.c
+++ b/object.c
@@ -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)