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

Ray Lee
2008-07-28T15:30:34Z

On Mon, Jul 28, 2008 at 7:57 AM, Rik van Riel <riel@redhat.com> wrote:
> On Thu, 24 Jul 2008 22:25:10 -0400
> Rik van Riel <riel@redhat.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.