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SubjectRe: 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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