Re: [PATCH 0/2] cfq-iosched: fixing RQ_NOIDLE handling.

[Corrado Zoccolo]

On Thu, Jul 8, 2010 at 4:35 PM, Vivek Goyal <vgoyal@xxxxxxxxxx> wrote:
> On Wed, Jul 07, 2010 at 01:03:08PM -0400, Jeff Moyer wrote:
>> Corrado Zoccolo <czoccolo@xxxxxxxxx> writes:
>>
>> > Hi Jens,
>> > patch 8e55063 "cfq-iosched: fix corner cases in idling logic", is
>> > suspected for some regressions on high end hardware.
>> > The two patches from this series:
>> > - [PATCH 1/2] cfq-iosched: fix tree-wide handling of rq_noidle
>> > - [PATCH 2/2] cfq-iosched: RQ_NOIDLE enabled for SYNC_WORKLOAD
>> > fix two issues that I have identified, related to how RQ_NOIDLE is
>> > used by the upper layers.
>> > First patch makes sure that a RQ_NOIDLE coming after a sequence of
>> > possibly idling requests from the same queue on the no-idle tree will
>> > clear the noidle_tree_requires_idle flag.
>> > Second patch enables RQ_NOIDLE for queues in the idling tree,
>> > restoring the behaviour pre-8e55063 patch.
>>
>> Hi, Corrado,
>>
>> I ran your kernel through my tests. ÂHere are the results, up against
>> vanilla, deadline, and the blk_yield patch set:
>>
>>         Âjust  Âjust
>>         fs_mark Âfio    Âmixed
>> -------------------------------+--------------
>> deadline    Â529.44  151.4 | 450.0  Â78.2
>> vanilla cfq   107.88  164.4 |  6.6  137.2
>> blk_yield cfq  530.82  158.7 | 113.2  Â78.6
>> corrado cfq   Â80.82  138.1 |  4.5  130.7
>>
>> fs_mark results are in files/second, fio results are in MB/s. ÂAll
>> results are the average of 5 runs. ÂIn order to get results for the
>> mixed workload for both vanilla and Corrado's kernels, I had to extend
>> the runtime from 30s to 300s.
>>
>> So, the changes proposed in this thread actually make performance worse
>> across the board.
>>
>> I re-ran my tests against a RHEL 5 kernel (which is based on 2.6.18),
>> and it shows that fs_mark performance is much better than stock CFQ in
>> 2.6.35-rc3, and the mixed workload results are much the same as they are
>> now (which is to say, the fs_mark process is completely starved by the
>> sequential reader). ÂSo, that problem has existed for a long time.
>>
>> I'm still in the process of collecting data from production servers and
>> will report back with my findings there.
>
> Hi Jeff and all,
>
> How about if we simply get rid of idling on RQ_NOIDLE threads (as
> corrado's patch series does) and not try to solve the problem of fsync
> being starved in the presence of sequential readers. I mean it might just
> be a theoritical problem and not many people are running into it. That's
> how CFQ has been behaving for long-2 time and if nobody is complaining
> then we probably don't have to fix it.

8e55063 was done to fix theoretical problems as well :)
I think, instead, that Jeff's approach of yielding the queue when a
better knowledge is present is good, and this set of patches is not
intended as a replacement. It is intended just to fix some regressions
introduced by a previous commit, and I hope it could work together
with Jeff's patch.
Clearly, if RQ_NOIDLE is used only in the places that Jeff is already
handling, then it is better to completely remove RQ_NOIDLE handling,
so my patch set becomes obsolete.

Thanks,
Corrado
>
> Thanks
> Vivek
>



-- 
__________________________________________________________________________

dott. Corrado Zoccolo                          mailto:czoccolo@xxxxxxxxx
PhD - Department of Computer Science - University of Pisa, Italy
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The self-confidence of a warrior is not the self-confidence of the average
man. The average man seeks certainty in the eyes of the onlooker and calls
that self-confidence. The warrior seeks impeccability in his own eyes and
calls that humbleness.
                               Tales of Power - C. Castaneda
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