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

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Jul 07, 2010 at 01:03:08PM -0400, Jeff Moyer wrote:
> Corrado Zoccolo <czoccolo@gmail.com> 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.  Here are the results, up against
> vanilla, deadline, and the blk_yield patch set:
> 
>                  just    just
>                 fs_mark  fio        mixed	
> -------------------------------+--------------
> deadline        529.44   151.4 | 450.0    78.2
> vanilla cfq     107.88   164.4 |   6.6   137.2
> blk_yield cfq   530.82   158.7 | 113.2    78.6
> corrado cfq      80.82   138.1 |   4.5   130.7
> 
> fs_mark results are in files/second, fio results are in MB/s.  All
> results are the average of 5 runs.  In 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).  So, 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.

Thanks
Vivek