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    SubjectRe: 2.6.20-rc2+: CFQ halving disk throughput.
    On Mon, Jan 01 2007, Mark Lord wrote:
    > Rene Herman wrote:
    > >Tejun Heo wrote:
    > >
    > >>Everything seems fine in the dmesg. Performance degradation is
    > >>probably some other issue in -rc kernel. I'm suspecting recently
    > >>fixed block layer bug. If it's still the same in the next -rc,
    > >>please report.
    > >
    > >In fact, it's CFQ. The PATA thing was a red herring. 2.6.20-rc2 and 3
    > >give me ~ 24 MB/s from "hdparm t /dev/hda" while 2.6.20-rc1 and below
    > >give me ~ 50 MB/s.
    > >
    > >Jens: this is due to "[PATCH] cfq-iosched: tighten allow merge
    > >criteria", 719d34027e1a186e46a3952e8a24bf91ecc33837:
    > >
    > >http://www2.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=719d34027e1a186e46a3952e8a24bf91ecc33837
    > >
    > >
    > >If I revert that one, I have my 50 M/s back. config and dmesg attached
    > >in case they're useful.
    >
    > Wow.. same deal here -- sequential throughput drops from 40MB/sec to
    > 28MB/sec
    > with CFQ -- whereas the anticipatory scheduler maintains the 40MB/sec.
    >
    > Jens.. I wonder if the new merging test is a bit too strict?
    >
    > There are four possible combinations, and the new code
    > allows merging for two of them: sync+sync and async+async.
    >
    > But surely one of (not sure which) sync+async or async+sync may also be
    > okay?
    > Or would it?

    Async merge to sync request should be ok. But I wonder what happens with
    hdparm, since it seems to trigger one of these tests. Very puzzling.
    I'll dive in and take a look.

    > This is a huge performance hit.

    Indeed, not acceptable of course. And not intentional :-)

    --
    Jens Axboe

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