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, 21 insertions, 0 deletions

diff --git a/object-store.h b/object-store.h
index 7f7b3cdd80..55ee639350 100644
--- a/object-store.h
+++ b/object-store.h
@@ -60,6 +60,7 @@ struct oid_array *odb_loose_cache(struct object_directory *odb,
 void odb_clear_loose_cache(struct object_directory *odb);
 
 struct packed_git {
+	struct hashmap_entry packmap_ent;
 	struct packed_git *next;
 	struct list_head mru;
 	struct pack_window *windows;
@@ -88,6 +89,20 @@ struct packed_git {
 
 struct multi_pack_index;
 
+static inline int pack_map_entry_cmp(const void *unused_cmp_data,
+				     const struct hashmap_entry *entry,
+				     const struct hashmap_entry *entry2,
+				     const void *keydata)
+{
+	const char *key = keydata;
+	const struct packed_git *pg1, *pg2;
+
+	pg1 = container_of(entry, const struct packed_git, packmap_ent);
+	pg2 = container_of(entry2, const struct packed_git, packmap_ent);
+
+	return strcmp(pg1->pack_name, key ? key : pg2->pack_name);
+}
+
 struct raw_object_store {
 	/*
 	 * Set of all object directories; the main directory is first (and
@@ -132,6 +147,12 @@ struct raw_object_store {
 	struct list_head packed_git_mru;
 
 	/*
+	 * A map of packfiles to packed_git structs for tracking which
+	 * packs have been loaded already.
+	 */
+	struct hashmap pack_map;
+
+	/*
 	 * A fast, rough count of the number of objects in the repository.
 	 * These two fields are not meant for direct access. Use
 	 * approximate_object_count() instead.