Re: performance problem: "git commit filename"

From: Junio C Hamano <gitster@pobox.com>
Date: Jan 15, 2008, 01:13 UTC
Message-ID: <7vd4s3j3fz.fsf@gitster.siamese.dyndns.org>
In-Reply-To: <alpine.LFD.1.00.0801141611560.2806@woody.linux-foundation.org>

Linus Torvalds <torvalds@linux-foundation.org> writes:

Show 25 quoted lines

> On Sun, 13 Jan 2008, Junio C Hamano wrote:
>> 
>> I've reworked the patch, and in the kernel repository, a
>> single-path commit after touching that path now calls 23k
>> lstat(2).  It used to call 46k lstat(2) after your fix.
>
> Hmm. This part of it looks incorrect:
>
>> diff --git a/diff.c b/diff.c
>> index b18c140..62d0c06 100644
>> --- a/diff.c
>> +++ b/diff.c
>> @@ -1510,6 +1510,10 @@ static int reuse_worktree_file(const char *name, const unsigned char *sha1, int
>>  	if (pos < 0)
>>  		return 0;
>>  	ce = active_cache[pos];
>> +
>> +	if (ce_uptodate(ce))
>> +		return 1;
>> +
>>  	if ((lstat(name, &st) < 0) ||
>>  	    !S_ISREG(st.st_mode) || /* careful! */
>>  	    ce_match_stat(ce, &st, 0) ||
>
> Isn't this wrong?

You are right. We call this with sha1 that may not necessarily be the same as ce->sha1.

The code should probably be something like:

	ce = active_cache[pos];

	/*
         * Even if ce matches the work tree, it is not what we can
	 * reuse for sha1, if the hash is different or not a
         * regular blob.
         */
	if (hashcmp(sha1, ce->sha1) || !S_ISREG(ntohl(ce->st_mode))
		return 0;
	/*
         * Does ce actually match the work tree?  If so we can reuse.
         */
	if (ce_uptodate(ce) ||
	    (!lstat(name, &st) && !ce_match_stat(ce, &st, 0)))
		return 1;
	return 0;

The expression inside the latter if () condition should probably be the new abstraction at the level of ce_modified().

Currently ce_modified() assumes that lstat(2) is cheap and the callers have called it on paths they are interested in already.