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    SubjectRe: Status of the IO scheduler fixes for 2.4


    On Wed, 2 Jul 2003, Chris Mason wrote:

    > On Wed, 2003-07-02 at 18:28, Marcelo Tosatti wrote:
    > > On Wed, 2 Jul 2003, Marcelo Tosatti wrote:
    > >
    > > >
    > > > Hello people,
    > > >
    > > > What is the status of the IO scheduler fixes for increased fairness for
    > > > 2.4 ?
    > > >
    > > > I haven't had time to read and think about everything you guys discussed,
    > > > so a brief summary would be very helpful for me.
    > > >
    > > > Danke
    > >
    > > Ah, we all want that the fairness issues to be fixed in 2.4.22, right ?
    >
    > My current code is attached, it's basically a merge of these 3 patches,
    > with modifications based on benchmarks and latency measurements here.
    >
    > fix_pausing: From Andrea, it fixes a few corner case races where
    > wakeups can be missed in wait_on_buffer, wait_on_page, and
    > __get_request_wait.
    >
    > elevator-low-latency: From Andrea, it keeps the amount of io on a given
    > queue to a reasonable number. This prevents a small number of huge
    > requests from introducing large latencies on smaller requests.
    >
    > q->full: From Nick, it reduces latency in __get_request_wait by making
    > sure new io can't come in and steal requests before old waiters are
    > served.
    >
    > Those represent the big 3 areas I believe the latencies are coming
    > from. The q->full patch can hurt throughput badly as the number of
    > writers increases (50% of what 2.4.21 gets for 10 or more concurrent
    > streaming writers), but it really seems to help desktop workloads here.

    Chris,

    Would you please separate those tree fixes in separate diffs?

    For me it seems low latency and fix-pausing patches should be enough for
    "better" IO fairness. I might be wrong about that, though.

    Lets try this: Include elevator-low-latency in -pre3 (which I'm trying to
    release today), then fix pausing in -pre4. If the IO fairness still doesnt
    get somewhat better for general use (well get isolated user reports and
    benchmarks for both the two patches), then I might consider the q->full
    patch (it has throughtput drawbacks and I prefer avoiding a tunable
    there).


    Sounds good?
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