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    SubjectRe: elevator algorithm bug in ll_rw_blk.c
    Hi,

    On Sun, 15 Nov 1998 14:13:31 -0500, Chip Salzenberg
    <chip@perlsupport.com> said:

    > According to Alan Cox:
    >> Chip:
    >> > But that's a *one-way* elevator. Ideal elevators are two-way,
    >> > aren't they?
    >>
    >> If you go for the shortest movement alone you starve the edges of the disks.

    > I said "two-way", not "shortest movement".

    Two-way elevators _do_ have this problem. A block in the middle
    cylinder of the disk gets serviced twice per full two-way pass, once in
    each direction. Maximum wait time is half a pass; average is a
    quarter. For a block at the edge of the disk, it will get serviced only
    when the two-way pass approaches close to the edge of the disk, and will
    not get another chance for a full pass, average wait half a pass.

    It's not a total stavation issue, but it _does_ give you a significant
    unfairness.

    --Stephen


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