Re: Xterm Hangs - Possible scheduler defect?

From: Ingo Oeser (ioe-lkml_at_axxeo.de)
Date: 02/25/05

  • Next message: Christian Borntraeger: "Re: Invalid module format in Fedora core2" 
    To: Chris Friesen <cfriesen@nortel.com>
Date:	Fri, 25 Feb 2005 16:39:50 +0100

    Chris Friesen wrote:
    > Ingo Oeser wrote:
    > > Stupid applications can starve other applications for a while, but not
    > > forever, because the kernel is still running and deciding.
    >
    > Not so.
    >
    >
    >
    > task 1: sched_rr, priority 1, takes mutex
    > task 2: sched_rr, priority 2, cpu hog, infinite loop
    > task 3: sched_rr, priority 99, tries to get mutex
    >
    > And now tasks 1 and 3 are starved forever. Arguably bad application
    > design, but it demonstrates a case where applications can starve other
    > applications.

    You are right.
     
    In "If a SCHED_RR process has been running for a time period equal to or
    longer than the time quantum, it will be put at the end of the list for
    its priority" I missed the "for its priority" part.

    You would need to change the priority of task 1 until it releases the
    mutex. Ideally the owner gets the maximum priority of
    his and all the waiters on it, until it releases his mutex, where he regains
    its old priority after release of mutex. But this priority elevation happens
    only, if he is runnable. If not, he gets his old priority back, until he is
    runnable.

    But then again you just need to grab a mutex shared with a high priority
    task and consume CPU.

    Since this behavior is not defined in POSIX AFAIK, you just have
    to write your applications properly or use SCHED_OTHER for CPU hogging.

    Regards

    Ingo Oeser

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  • Next message: Christian Borntraeger: "Re: Invalid module format in Fedora core2"

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