From: Wincent Colaiuta <win@wincent.com>
Date: Thu, 18 Feb 2010 07:27:28 GMT
Subject: Re: [PATCH] Teach "git add" and friends to be paranoid
Message-ID: <5DDD89A9-900F-40AD-8F3F-F756D6E0AD6C@wincent.com>
In-Reply-To: <7vocjnqf5c.fsf@alter.siamese.dyndns.org>

El 18/02/2010, a las 06:36, Junio C Hamano escribió:

> Nicolas Pitre <nico@fluxnic.net> writes:
> 
>> It is likely to have better performance if the buffer is small enough to 
>> fit in the CPU L1 cache.  There are two sequencial passes over the 
>> buffer: one for the SHA1 computation, and another for the compression, 
>> and currently they're sure to trash the L1 cache on each pass.
> 
> I did a very unscientific test to hash about 14k paths (arch/ and fs/ from
> the kernel source) using "git-hash-object -w --stdin-paths" into an empty
> repository with varying sizes of paranoia buffer (quarter, 1, 4, 8 and
> 256kB) and saw 8-30% overhead.  256kB did hurt and around 4kB seemed to be
> optimal for my this small sample load.
> 
> In any case, with any size of paranoia, this hurts the sane use case, so
> I'd introduce an expert switch to disable it, like this.

Shouldn't a switch that hurts performance and is only needed for insane use cases default to off rather than on?

Cheers,
Wincent