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    SubjectRe: Scheduling of low-priority background processes
    On Mon, 5 Nov 2001, Thomas Koeller wrote:

    > On those systems, you could assign a scheduling priority lower than
    > the one nomally used by interactive processes to CPU-hogging,
    > numbercrunching tasks. These tasks would then use up any CPU time left
    > over by interactive processes without otherwise interfering with them.
    > I always found this feature very useful (think of SETI@home!).

    > But that idea is so obvious, and since nobody did it so far, I am
    > probably missing something. What is it?

    Priority inversion. I did a patch which does exactly
    what you describe, around the 2.1 timeframe. It worked
    fine most of the time, but occasionally the following

    1) a SCHED_IDLE process got hold of some kernel lock
    2) a normal, low-priority process started eating CPU
    for a number of seconds
    3) a high-priority normal process wanted the lock the
    SCHED_IDLE task had, but had to wait several seconds,
    at times up to a minute, before the SCHED_IDLE task
    got a chance to run and release the lock

    This wasn't too much of a problem on my own system, but
    of course this is an easily exploitable vulnerability for

    For me, this just means we should improve the scheduler so
    nice levels are stronger ... say that a nice +20 process
    only gets 1% of the CPU of a normal priority process ;)


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