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

[Jeff Moyer <jmoyer@redhat.com>]

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.

Cheers,
Jeff