Re: [PATCH 0/8] Reduce latencies and improve overall reclaim efficiency v2

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On 10/18/2010 03:55 PM, Mel Gorman wrote:
> On Thu, Oct 14, 2010 at 05:28:33PM +0200, Christian Ehrhardt wrote:
> 
>> Seing the patches Mel sent a few weeks ago I realized that this series
>> might be at least partially related to my reports in 1Q 2010 - so I ran my
>> testcase on a few kernels to provide you with some more backing data.
> 
> Thanks very much for revisiting this.
> 
>> Results are always the average of three iozone runs as it is known to be somewhat noisy - especially when affected by the issue I try to show here.
>> As discussed in detail in older threads the setup uses 16 disks and scales the number of concurrent iozone processes.
>> Processes are evenly distributed so that it always is one process per disk.
>> In the past we reported 40% to 80% degradation for the sequential read case based on 2.6.32 which can still be seen.
>> What we found was that the allocations for page cache with GFP_COLD flag loop a long time between try_to_free, get_page, reclaim as free makes some progress and due to that GFP_COLD allocations can loop and retry.
>> In addition my case had no writes at all, which forced congestion_wait to wait the full timeout all the time.
>>
>> Kernel (git)                   4          8         16   deviation #16 case                           comment
>> linux-2.6.30              902694    1396073    1892624                 base                              base
>> linux-2.6.32              752008     990425     932938               -50.7%     impact as reported in 1Q 2010
>> linux-2.6.35               63532      71573      64083               -96.6%                    got even worse
>> linux-2.6.35.6            176485     174442     212102               -88.8%  fixes useful, but still far away
>> linux-2.6.36-rc4-trace    119683     188997     187012               -90.1%                         still bad
>> linux-2.6.36-rc4-fix      884431    1114073    1470659               -22.3%            Mels fixes help a lot!
>>
[...]
> If all goes according to plan,
> kernel 2.6.37-rc1 will be of interest. Thanks again.

Here a measurement with 2.6.37-rc1 as confirmation of progress:
   linux-2.6.37-rc1          876588    1161876    1643430               -13.1%       even better than 2.6.36-fix

That means 2.6.37-rc1 really shows what we hoped for.
And it eventually even turned out a little bit better than 2.6.36 + your fixes.

 

-- 

Grüsse / regards, Christian Ehrhardt
IBM Linux Technology Center, System z Linux Performance 

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