[PATCH] find_containing_dir(): allocate strbuf less extravagantly

From: mhagger@alum.mit.edu <mhagger@alum.mit.edu>
Date: May 22, 2012, 13:16 UTC
Message-ID: <1337692566-3718-1-git-send-email-mhagger@alum.mit.edu>

From: Michael Haggerty <mhagger@alum.mit.edu>

It might seem like allocating a fixed-length buffer of uninitialized memory should be pretty cheap even if the buffer is of size PATH_MAX. But empirically, it is measurably faster to allocated only the strlen of the input string.

Thanks to Peff for pointing out a performance regression in this area that might be fixed by this patch.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
---
I am not able to reproduce performance regressions as bad as those
observed by Peff.  It seems to depend on the amount of memory
pressure.  The smaller regression that I *did* see is fixed by this
patch, reducing the time for "git fetch . refs/*:refs/*" from 10.1 s
to 9.3 s.  The change is sensible in any case, but we will have to
wait for Peff's verdict about whether it fixes the whole problem for
him, too.

 refs.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/refs.c b/refs.c
index d6bdb47..fffbb17 100644
--- a/refs.c
+++ b/refs.c
@@ -383,7 +383,7 @@ static struct ref_dir *find_containing_dir(struct ref_dir *dir,
 {
 	struct strbuf dirname;
 	const char *slash;
-	strbuf_init(&dirname, PATH_MAX);
+	strbuf_init(&dirname, strlen(refname));
 	for (slash = strchr(refname, '/'); slash; slash = strchr(slash + 1, '/')) {
 		struct ref_dir *subdir;
 		strbuf_add(&dirname,

-- 
1.7.10