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    Subjectlow-latency scheduling patch for 2.4.0

    A patch against kernel 2.4.0 final which provides low-latency
    scheduling is at

    Some notes:

    - Worst-case scheduling latency with *very* intense workloads is now
    0.8 milliseconds on a 500MHz uniprocessor.

    For normal workloads you can expect to achieve better than 0.5
    milliseconds for ever. For example, worst-case latency between entry
    to an interrupt routine and activation of a usermode process during a
    `make clean && make bzImage' is 0.35 milliseconds. This is one to
    three orders of magnitude better than BeOS, MacOS and the Windowses.

    - Low latency is enabled from the `Processor type and features'
    kernel configuration menu for all architectures. It would be nice to
    hear from non-x86 users.

    - The SMP problem hasn't been addressed. Enabling low-latency for
    SMP works well under normal workloads but comes unstuck under very
    heavy workloads. I'll be taking a further look at this.

    - The supporting tools `rtc_debug' and `amlat' have been updated.
    These are quite useful tools for providing accurate measurement of
    latencies. They may also be used to identify the causes of poor
    latency in the kernel.

    - Remaining problem areas (the Don't Do That list) is pretty small:

    - Scrolling the fb console.
    - Running hdparm.
    - Using LILO
    - Starting the X server

    - Low latency will probably only be achieved when using the ext2 and
    NFS filesystems.

    - If you care about latency, be *very* cautious about upgrading to
    XFree86 4.x. I'll cover this issue in a separate email, copied
    to the XFree team.

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