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    SubjectRe: [Kiobuf-io-devel] RFC: Kernel mechanism: Compound event wait
    On Tue, 6 Feb 2001, Ingo Molnar wrote:

    >
    > On Tue, 6 Feb 2001, Ben LaHaise wrote:
    >
    > > This small correction is the crux of the problem: if it blocks, it
    > > takes away from the ability of the process to continue doing useful
    > > work. If it returns -EAGAIN, then that's okay, the io will be
    > > resubmitted later when other disk io has completed. But, it should be
    > > possible to continue servicing network requests or user io while disk
    > > io is underway.
    >
    > typical blocking point is waiting for page completion, not
    > __wait_request(). But, this is really not an issue, NR_REQUESTS can be
    > increased anytime. If NR_REQUESTS is large enough then think of it as the
    > 'absolute upper limit of doing IO', and think of the blocking as 'the
    > kernel pulling the brakes'.

    =) This is what I'm seeing: lots of processes waiting with wchan ==
    __get_request_wait. With async io and a database flushing lots of io
    asynchronously spread out across the disk, the NR_REQUESTS limit is hit
    very quickly.

    > [overhead of 512-byte bhs in the raw IO code is an artificial problem of
    > the raw IO code.]

    True, and in the tests I've run, raw io is using 2KB blocks (same as the
    database).

    -ben

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