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    SubjectRe: elevator algorithm bug in ll_rw_blk.c
    On Fri, 13 Nov 1998, Chris Wedgwood wrote:

    > On Mon, Nov 09, 1998 at 11:37:43PM +0000, Stephen C. Tweedie wrote:
    >
    > > <Wolfgang.Oertl@uibk.ac.at> said:
    > >
    > > > as v2.2 is expected soon I better bring this topic up now. As you
    > > > know the "elevator algorihm" is employed for disk I/O, but -- it
    > > > *looks* broken to me.
    > >
    > > It isn't!
    >
    > How long has it not been broken for?
    >
    > I ask this because back aroun 2.1.26 I made a small char device which
    > allowed to to log and then dump the sequences of disk update produced
    > during a `sync' operation.
    >
    > Anyhow, the net result of this was that they were not optimally
    > order, not even close.

    Optimal order is not possible unless you know the future ;-)

    The request *should* be optimally ordered within the current request queue
    state. I.e. if the queue has request for the blocks [4, 10000, 10005] and
    the 'elevator' is going 'up', you issue 4, then 10000. If you then get a
    new request for block 5, your're out of luck. This is the classical
    elevator alg. from OS course, I assume that's what Linux uses.

    Cheers,

    Rafael



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