Re: [PATCH 3/3] Avoid doing extra 'lstat()'s for d_type if we have an up-to-date cache entry

From: Junio C Hamano <gitster@pobox.com>
Date: Jul 9, 2009, 18:34 UTC
Message-ID: <7vbpnt3dth.fsf@alter.siamese.dyndns.org>
In-Reply-To: <alpine.LFD.2.01.0907090954090.3352@localhost.localdomain>

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

Show 13 quoted lines

>> > +		if (!ce_uptodate(ce))
>> > +			break;	/* continue? */
>> 
>> I think this should be continue, as the directory D you are interested in
>> may have two files, one modified, the other uptodate.
>
> The thing is, the directory may have subdirectories, and there may be 
> tens of thousands of files there. And maybe this gets called by code that 
> hasn't done any cache preloading at all, so nothing will be up-to-date.
>
> Do we want to loop over thousands of entries? Or do we want to loop as 
> little as possible, and just say "most of the time the first entry will be 
> representative".

Ah, I see.

It depends on how expensive it is to iterate over an in-core array to check a single bit (that may not even have been updated) in the cache entries, compared to an extra lstat(). Perhaps we could autotune that, but it probably is not worth it. ;-)