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    SubjectRe: Unresponiveness of 2.4.16
    Marcelo Tosatti wrote:
    > On Tue, 27 Nov 2001, Andrew Morton wrote:
    > > Torrey Hoffman wrote:
    > > >
    > > > I've running 2.4.16 with this VM patch combined with your
    > > > 2.4.15-pre7-low-latency patch from (it applied with a
    > > > little fuzz, no rejects). Is this a combination that you would feel
    > > > comfortable with?
    > >
    > > Should be OK. There is a possibility of livelock when you have
    > > a lot of dirty buffers against multiple devices.
    > Could you please describe this one ?

    It's a recurring problem with the low-latency patch. Basically:

    for (lots of data) {
    if (current->need_resched) {
    goto restart;
    if (something_which_is_often_true)

    If there is a realtime task which wants to be scheduled at,
    say, one kilohertz, and the execution of that loop takes
    more than one millisecond before it actually hits other_stuff()
    and does any actual work, we make no progress at all, and we lock
    up until the 1 kHz scheduling pressure is stopped.

    In the 2.4.15-pre low-latency patch this can happen if we're
    running fsync_dev(devA) and there are heaps of buffers for
    devB on a list.

    It's not a problem in your kernel ;)

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