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    SubjectRe: [GIT PULL] Ext3 latency fixes


    On Mon, 6 Apr 2009, Jens Axboe wrote:
    >
    > Well, either you are submitting a single piece of IO (in which case you
    > just want to unplug or directly submit as part of the submit_bio()), or
    > you are submitting several IOS (in which case you just want to unplug at
    > the end of the IO submission, before waiting).

    That's not true.

    The plugging is often across multiple threads. It didn't _use_ to be (we
    always unplugged at wait), but it is now. Nothing else explains why that
    patch by Ted makes such a big throughput thing, because the code did

    ret = submit_bh(WRITE_SYNC, bh);
    wait_on_buffer(bh);

    ie it very much submits a _single_ IO, and waits on it. If plugging made a
    difference, that means that unplugging was delayed so long that somebody
    else does IO too - ie it gets delayed past a wait event.

    So according to your own rules, that submit_bh() _should_ use WRITE_SYNC,
    but something bad happens if it does. I'm not quite seeing _what_, though,
    unless there are multiple processes trying to dirty the _same_ buffer, and
    they win if they all can dirty it without doing IO on it in between (and
    then the write turns into just one write).

    Linus



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