Re: scheduler (sched_4bsd) questions

From: Julian Elischer (julian_at_elischer.org)
Date: 09/19/04

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    Date: Sun, 19 Sep 2004 00:10:07 -0700
To: John Baldwin <jhb@FreeBSD.org>

    John Baldwin wrote:
    > On Saturday 18 September 2004 01:42 pm, Stephan Uphoff wrote:
    >
    >>On Fri, 2004-09-17 at 21:20, Julian Elischer wrote:
    >>
    >>>Stephan Uphoff wrote:
    >>>
    >>>>If this is true kernel threads can be preempted while holding
    >>>>for example the root vnode lock (or other important kernel
    >>>>resources) while not getting a chance to run until there are no more
    >>>>user processes with better priority.
    >>>
    >>>This is also true, though it is a slightly more complicated thing than
    >>>that.
    >>>Preempting threads are usually interrupt threads and are thus usually
    >>>short lived,.
    >>
    >>But interrupt threads often wake up other threads ...
    >
    >
    > That are lower priority and thus won't be preempted to. Instead, they run
    > when the interrupt thread goes back to sleep after it finishes.

    though that may still be before the original preempted thread gets run..

    that reminds me..
    John, we should add a flag to setrunqueue to add a preempted thread back at the
    FRONT of it's runqueue.. So that it gets a chance to use the rest of its slot..

    the alternative is to keep a special "stack" (per cpu) of preempted threads so
    that we com back to the thread that we preempted.

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