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    SubjectRe: stochastic fair queueing in the elevator [Re: [BENCHMARK] 2.4.20-ck3 / aa / rmap with contest]
    On Sun, Feb 09, 2003 at 08:33:43PM -0800, Andrew Morton wrote:
    > David Lang <david.lang@digitalinsight.com> wrote:
    > >
    > > note that issuing a fsync should change all pending writes to 'syncronous'
    > > as should writes to any partition mounted with the sync option, or writes
    > > to a directory with the S flag set.
    >
    > We know, at I/O submission time, whether a write is to be waited upon.
    > That's in writeback_control.sync_mode.
    >
    > That, combined with an assumption that "all reads are synchronous" would
    > allow the outgoing BIOs to be appropriately tagged.

    This may be a terribly stupid question, if so pls. just tell me :)

    I assume read-ahead requests go elsewhere? Or do we assume that someone
    is waiting for them as well?

    If we assume they are synchronous, that would be rather unfair
    especially on multi-user systems - and the 90% accuracy that Rik
    suggested would seem exaggerated to say the least (accuracy would be
    more like 10% on a good day).

    > It's still approximate. An exact solution would involve only marking I/O as
    > synchronous when some process actually waits on its completion. I do not
    > believe that all the extra lookup and locking infrastructure and storage
    > which this would require is justified. Certainly not in a first iteration.

    Just a quirk; NFS file servers.

    The client does read-ahead - the nfsd on the server-side will wait for
    the read request, thus the read is synchronous... But it's not.

    --
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    :.........................:............{Konkhra}...............:
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