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

[Vivek Goyal]

On Fri, Jul 09, 2010 at 12:33:36PM +0200, Corrado Zoccolo wrote:
> On Wed, Jul 7, 2010 at 10:06 PM, Jeff Moyer <jmoyer@xxxxxxxxxx> wrote:
> > Corrado Zoccolo <czoccolo@xxxxxxxxx> writes:
> >
> >> On Wed, Jul 7, 2010 at 7:03 PM, Jeff Moyer <jmoyer@xxxxxxxxxx> 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.  Here are the results, up against
> >>> vanilla, deadline, and the blk_yield patch set:
> >>>
> >> Hi Jeff,
> >> can you also add cfq with 8e55063 reverted to the testing mix?
> >
> > Sure, the results now look like this:
> >
> >                 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     110.16   220.6 |   7.0   159.8
> > 8e55063 revert  559.66   198.9 |  16.1   153.3
> >
> > I had accidentally run your patch set (corrado cfq) on ext3, so the
> > numbers were a bit off (everything else was run against ext4).  The
> > corrected numbers above reflect the performance on ext4, which is much
> > better for the sequential reader, but still not great for the fs_mark
> > run.  Reverting 8e55063 definitely gets us into better shape.  However,
> > if we care about the mixed workload, then it won't be enough.
> 
> Wondering why deadline performs so well in the fs_mark workload. Is it
> because it doesn't distinguish between sync and async writes?
> Maybe we can achieve something similar by putting all sync writes
> (that are marked as REQ_NOIDLE) in the noidle tree? This, coupled with
> making jbd(2) perform sync writes, should make the yield automatic,
> since they all live in the same tree for which we don't idle between
> queues, and should be able to provide fairness compared to a
> sequential reader (that lives in the other tree).
> 

This makes sense conceptually at least at CFQ level. By putting
OSYNC/fsync on sync-noidle tree we will not be able to take advantage
of sequential nature of queue but christoph mentioned that all sequential
writes in general should be lumped together and then sent down to CFQ
instead of issuing small writes after some delay. So this probably
is not an issue.

What I am not sure about is impact of switching jbd thread writes from
async to sync (WRITE ---> WRITE_SYNC). Especially if somebody is
journalling the data also (data=journal).

But it is definitely worth trying because then we don't have to idle on
individual queues of WRITE_SYNC as well as fsync performance issue should
also be solved. 

Thanks
Vivek
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