linux-kernel - Re: Big git diff speedup by avoiding x86 "fast string" memcmp

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Message-ID: <20101210042752.GA3144@amd>
Date:	Fri, 10 Dec 2010 15:27:52 +1100
From:	Nick Piggin <npiggin@...nel.dk>
To:	Nick Piggin <npiggin@...nel.dk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-arch@...r.kernel.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: Big git diff speedup by avoiding x86 "fast string" memcmp

On Thu, Dec 09, 2010 at 06:09:38PM +1100, Nick Piggin wrote:
> So replace it with an open-coded byte comparison. This increases code
> size by 24 bytes in the critical __d_lookup_rcu function, but the

Actually, if the loop assumes len is non zero (which is the case for
dentry compare), then the bloat is only 8 bytes, so not a problem.

Also got numbers versus vanilla kernel, out of interest.

> speedup is huge, averaging 10 runs of each:
> 
> git diff st   user   sys   elapsed  CPU
  vanilla       1.19   3.21  4.47      98.0
> before        1.15   2.57  3.82      97.1
> after         1.14   2.35  3.61      96.8
> 
> git diff mt   user   sys   elapsed  CPU
  vanilla       1.57  45.75  3.60    1312
> before        1.27   3.85  1.46     349
> after         1.26   3.54  1.43     333
> 

Single thread elapsed time improvment vanilla vs vfs 19.23%. Not quite
as big as the AMD fam10h speedup, that's probably because Westmere does
atomics so damn quickly.

Multi thread numbers are no surprise.

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