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    SubjectRe: [2.4.17/18pre] VM and swap - it's really unusable
    On Sat, Jan 12, 2002 at 12:53:06PM +0100, Roman Zippel wrote:
    > Hi,
    >
    > yodaiken@fsmlabs.com wrote:
    >
    > > I believe that the preempt path leads inexorably to
    > > mutex-with-stupid-priority-trick and that would be very unfortunate indeed.
    > > It's unavoidable because sooner or later someone will find that preempt +
    > > SCHED_FIFO leads to
    > > niced app 1 in K mode gets Sem A
    > > SCHED_FIFO app prempts and blocks on Sem A
    > > whoops! app 2 in K more preempts niced app 1
    >
    > Please explain what's different without the preempt patch.

    See that "preempt" in line 2 . Linux does not
    preempt kernel mode processes otherwise. The beauty of the
    non-preemptive kernel is that "in K mode every process makes progress"
    and even the "niced app" will complete its use of SemA and
    release it in one run. If you have a reasonably fair scheduler you
    can make very useful analysis with Linux now of the form

    Under 50 active proceses in the system means that in every
    2 second interval every process
    will get at least 10ms of time to run.

    That's a very valuable property and it goes away in a preemptive kernel
    to get you something vague.


    >
    > > Hey my DVD player has stalled, lets add sem_with_revolting_priority_trick!
    > > Why the hell is UP Windows XP3 blowing away my Linux box on DVD playing while
    > > Linux now runs with the grace and speed of IRIX?
    >
    > Because the IRIX implementation sucks, every implementation has to suck?
    > Somehow I have the suspicion you're trying to discourage everyone from
    > even trying, because if he'd succeeded you'd loose a big chunk of
    > potential RTLinux customers.

    So your argument is that I'm advocating Andrew Morton's patch which
    reduces latencies more than the preempt patch because I have a
    financial interest in not reducing latencies? Subtle.

    In any case, motive has no bearing on a technical argument.
    Your motive could be to make the 68K look better by reducing
    performance on other processors for all I know.

    --
    ---------------------------------------------------------
    Victor Yodaiken
    Finite State Machine Labs: The RTLinux Company.
    www.fsmlabs.com www.rtlinux.com

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