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    SubjectRe: [ltt-dev] [RFC PATCH] block: Fix bio merge induced high I/O latency
    On Tue, 2009-01-20 at 23:54 -0500, Mathieu Desnoyers wrote:
    > * Ben Gamari ( wrote:
    > > On Tue, Jan 20, 2009 at 7:25 PM, Mathieu Desnoyers
    > > <> wrote:
    > > > * Mathieu Desnoyers ( wrote:
    > > >
    > > > As a side-note : I'd like to have my results confirmed by others.
    > >
    > > Well, I think the (fixed) patch did help to some degree (I haven't
    > > done fio benchmarks to compare against yet). Unfortunately, the I/O
    > > wait time problem still remains. I have been waiting 3 minutes now for
    > > evolution to start with 88% I/O wait time yet no visible signs of
    > > progress. I've confirmed I'm using the CFQ scheduler, so that's not
    > > the problem.
    > >
    > Did you also
    > echo 1 > /sys/block/sd{a,b}/device/queue_depth
    I have been using this in some of my measurements (this is recorded, of

    > echo 1 > /sys/block/sd{a,b}/queue/iosched/slice_async_rq
    > echo 1 > /sys/block/sd{a,b}/queue/iosched/quantum
    I haven't been doing this although I will collect a data set with these
    parameters set. It would be to compare the effect of this to the default

    > (replacing sd{a,b} with your actual drives) ?
    > It seems to have been part of the factors that helped (along with the
    > patch).
    > And hopefully you don't have a recent Seagate hard drive like me ? :-)
    Thankfully, no.

    > So you test case is :
    > - start a large dd with 1M block size
    > - time evolution
    I've been using evolution to get a rough idea of the performance of the
    configurations but not as a benchmark per se. I have some pretty
    good-sized maildirs, so launching evolution for the first time can be
    quite a task, IO-wise. Also, switching between folders used to be quite
    time consuming. It seems like the patch did help a bit on this front

    For a quantitative benchmark I've been using the fio job that you posted
    earlier. I've been collecting results and should have a pretty good data
    set soon.

    I'll send out a compilation of all the data I've collected as soon as
    I've finished.

    - Ben

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