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    SubjectRe: disk head scheduling

    On Thu, 18 Mar 1999, Arvind Sankar wrote:

    > I have a currently working algo which is greedy instead. i.e. for a new
    > request, it computes the place where its insertion will produce the least
    > incremental cost (where cost is the number of additional sectors to seek).
    > It seems to work for me. No idea how to compare the two algos, though. Can
    > somebody help me out here...

    be careful. These are all well-known problems, and your algorithm (also
    called SSF (smallest-seek-first) scheduling) is not fair enough. It has to
    be coupled with a mechanizm that provides fairness. (ie. guarantees that a
    request will not 'hang around' for too long time, unfairly blocking a
    process just because the position of the sector is unfortunate)

    > Another point is that IN_ORDER seems to be called only for two requests
    > on the same device, so no idea why it compares the device numbers.

    no, all requests (for all devices) are in a single 'queue'. (Per-major
    device queues is candidate 2.3 feature, it's really simple)

    -- mingo


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