linux-kernel - Re: PERF: performance tests with the split LRU VM in -mm

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Date:	Mon, 28 Jul 2008 08:30:34 -0700
From:	"Ray Lee" <ray-lk@...rabbit.org>
To:	"Rik van Riel" <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	akpm@...ux-foundation.org
Subject: Re: PERF: performance tests with the split LRU VM in -mm

On Mon, Jul 28, 2008 at 7:57 AM, Rik van Riel <riel@...hat.com> wrote:
> On Thu, 24 Jul 2008 22:25:10 -0400
> Rik van Riel <riel@...hat.com> wrote:
>
>>   TEST 1: dd if=/dev/sda of=/dev/null bs=1M
>>
>> kernel  speed    swap used
>>
>> 2.6.26  111MB/s  500kB
>> -mm     110MB/s  59MB     (ouch, system noticably slower)
>> noforce       111MB/s  128kB
>> stream  108MB/s  0        (slight regression, not sure why yet)
>>
>> This patch shows that the split LRU VM in -mm has a problem
>> with large streaming IOs: the working set gets pushed out of
>> memory, which makes doing anything else during the big streaming
>> IO kind of painful.
>>
>> However, either of the two patches posted fixes that problem,
>> though at a slight performance penalty for the "stream" patch.
>
> OK, the throughput number with this test turns out not to mean
> nearly as much as I thought.
>
> Switching off CPU frequency scaling, pinning the CPUs at the
> highest speed, resulted in a throughput of only 102MB/s.
>
> My suspicion is that faster running code on the CPU results
> in IOs being sent down to the device faster, resulting in
> smaller IOs and lower throughput.

Or the IOs are getting sent in a different order, and so
coalescing/merging isn't occurring as often. Getting some
instrumentation (something as simple as a histogram) on the IO sizes
could be useful.
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