linux-kernel - Re: 2.6.22 -mm merge plans -- vm bugfixes

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Message-ID: <46385699.4090201@yahoo.com.au>
Date:	Wed, 02 May 2007 19:15:05 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Nick Piggin <nickpiggin@...oo.com.au>
CC:	Hugh Dickins <hugh@...itas.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Andrea Arcangeli <andrea@...e.de>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: 2.6.22 -mm merge plans -- vm bugfixes

Nick Piggin wrote:
> Hugh Dickins wrote:
> 
>> On Tue, 1 May 2007, Nick Piggin wrote:
> 
> 
>>> There were concerns that we could do this more cheaply, but I think it
>>> is important to start with a base that is simple and more likely to
>>> be correct and build on that. My testing didn't show any obvious
>>> problems with performance.
>>
>>
>>
>> I don't see _problems_ with performance, but I do consistently see the
>> same kind of ~5% degradation in lmbench fork, exec, sh, mmap latency
>> and page fault tests on SMP, several machines, just as I did last year.
> 
> 
> OK. I did run some tests at one stage which didn't show a regression
> on my P4, however I don't know that they were statistically significant.
> I'll try a couple more runs and post numbers.

I didn't have enough time tonight to get means/stddev, etc, but the runs
are pretty stable.

Patch tested was just the lock page one.

SMP kernel, tasks bound to 1 CPU:

P4 Xeon
          pagefault   fork          exec
2.6.21   1.67-1.69   140.7-142.0   449.5-460.8
+patch   1.75-1.77   144.0-145.5   456.2-463.0

So it's taken on nearly 5% on pagefault, but looks like less than 2% on
fork, so not as bad as your numbers (phew).

G5
          pagefault   fork          exec
2.6.21   1.49-1.51   164.6-170.8   741.8-760.3
+patch   1.71-1.73   175.2-180.8   780.5-794.2

Bigger hit there.

Page faults can be improved a tiny bit by not using a test and clear op
in unlock_page (less barriers for the G5).

I don't think that's really a blocker problem for a merge, but I wonder
what we can do to improve it. Lockless pagecache shaves quite a bit of
straight line find_get_page performance there.

Going to a non-sleeping lock might be one way to go in the long term, but
it would require quite a lot of restructuring.

-- 
SUSE Labs, Novell Inc.
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