linux-kernel - Re: [patch 00/52] vfs scalability patches updated

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Date:	Wed, 7 Jul 2010 03:49:36 +1000
From:	Nick Piggin <npiggin@...e.de>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	John Stultz <johnstul@...ibm.com>,
	Frank Mayhar <fmayhar@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 00/52] vfs scalability patches updated

On Fri, Jul 02, 2010 at 03:23:17AM +1000, Nick Piggin wrote:
> On Wed, Jun 30, 2010 at 10:40:49PM +1000, Nick Piggin wrote:
> > But actually it's not all for scalability. I have some follow on patches
> > (that require RCU inodes, among other things) that actually improve
> > single threaded performance significnatly. git diff workload IIRC was
> > several % improved from speeding up stat(2).
> 
> I rewrote the store-free path walk patch that goes on top of this
> patchset (it's now much cleaner and more optimised, I'll post a patch
> soonish). It is quicker than I remembered.
> 
> A single thread running stat(2) in a loop on a file "./file" has the
> following cost (on an 2s8c Barcelona):
> 
> 2.6.35-rc3	595 ns/op
> patched		336 ns/op
> 
> stat(2) takes 56% the time with patches. It's something like 13 fewer
> atomic operations per syscall.
> 
> What's that good for? A single threaded, cached `git diff` on the linux
> kernel tree takes just 81% of the time after the vfs patches (0.27s vs
> 0.33s).

At the other end of the scale, I tried dbench on ramfs on the little
32n64c Altix. Dbench actually has statfs() call completely removed from
the workload -- it's still a little problematic and patched kernel
throughput is ~halved with statfs().

dbench procs        1         64
2.6.35-rc3    235MB/s     95MB/s ( 0.6% scaling)
patched       245MB/s  14870MB/s (94.8% scaling)

(note all these numbers are with store-free path walking patches on top
of the posted patchset -- dbench procs do path walking from common cwds
so it will never scale this well if we have to take refcounts on common
dentries)

Thanks,
Nick

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