Subject: Re: Some rough edges of core.fsmonitor

From: Duy Nguyen <[email-masked]>
Date: Mon, 29 Jan 2018 16:40:34 +0700

On Sat, Jan 27, 2018 at 12:43:41PM +0100, Ævar Arnfjörð Bjarmason wrote:
> b) with fsmonitor
> 
>     $ time GIT_TRACE_PERFORMANCE=1 ~/g/git/git-status
>     12:34:23.833625 read-cache.c:1890       performance: 0.049485685 s: read cache .git/index

This is sort of off topic but may be interesting for big repo guys. It
looks like read cache's time is partially dominated by malloc().

This is the performance breakdown of do_read_index()

$ GIT_TRACE_PERFORMANCE=2 ~/w/git/t/helper/test-read-cache
0.000078489 s: open/mmap/close
0.038915239 s: main entries
0.018983150 s: ext TREE
0.012667080 s: ext UNTR
0.000005372 s: ext FSMN
0.001473470 s: munmap
0.072386911 s: read cache .git/index

Reading main index entries takes like half of the time (0.038 vs
0.072). With the below patch to take out hundred thousands of malloc()
we have this, loading main entries now only takes 0.012s:

$ GIT_TRACE_PERFORMANCE=2 ~/w/git/t/helper/test-read-cache
0.000046587 s: open/mmap/close
0.012077300 s: main entries
0.020477683 s: ext TREE
0.010259998 s: ext UNTR
0.000010250 s: ext FSMN
0.000753854 s: munmap
0.043906473 s: read cache .git/index

We used to do less malloc until debed2a629 (read-cache.c: allocate
index entries individually - 2011-10-24) but I don't think we can
simply revert that (not worth the extra complexity of the old way).

Now "TREE" and "UNTR" extensions become a bigger problem.

-- 8< --
diff --git a/read-cache.c b/read-cache.c
index d60e0a8480..88f4213c99 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1622,7 +1622,12 @@ static struct cache_entry *cache_entry_from_ondisk(struct ondisk_cache_entry *on
 						   const char *name,
 						   size_t len)
 {
+#if 0
 	struct cache_entry *ce = xmalloc(cache_entry_size(len));
+#else
+	static char buf[1024];
+	struct cache_entry *ce = (struct cache_entry *)buf;
+#endif
 
 	ce->ce_stat_data.sd_ctime.sec = get_be32(&ondisk->ctime.sec);
 	ce->ce_stat_data.sd_mtime.sec = get_be32(&ondisk->mtime.sec);
diff --git a/t/helper/test-read-cache.c b/t/helper/test-read-cache.c
index 48255eef31..e1d21d17a3 100644
--- a/t/helper/test-read-cache.c
+++ b/t/helper/test-read-cache.c
@@ -8,7 +8,9 @@ int cmd_main(int argc, const char **argv)
 	setup_git_directory();
 	for (i = 0; i < cnt; i++) {
 		read_cache();
+#if 0
 		discard_cache();
+#endif
 	}
 	return 0;
 }
-- 8< --