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    SubjectRe: regression introduced by - timers: fix itimer/many thread hang
    On Mon, 10 Nov 2008, Peter Zijlstra wrote:

    > > Can we at least somehow make sure that nothing significantly happens in a
    > > timer interrupt on a processor if the thread has not scheduled any events
    > > or not odone any system calls?
    > Do threads actually scale that far? I thought mmap_sem contention and
    > other shared state would render threads basically useless on these very
    > large machines.

    They scale well. The problem is startup when they concurrently allocate
    memory. That has been solved with distributing the pte locks. mmap_sem is
    taken for read in the fault handler. So you have a wildly bouncing
    cacheline at startup time that causes performance issues but no busy

    > But afaiu this stuff, the per-cpu loop is only done when an itimer is
    > actually active.
    > The detail I've not looked at is, if when this itimer is indeed active
    > and we are running 256 threads of the same application on all cpus do we
    > then do the per-cpu loop for each tick on each cpu?

    I would check how 8p and 16p fare with this? How much potential latency is
    added to thread because it is interrupted and the timer interrupt goes
    through this loop?

     \ /
      Last update: 2008-11-10 16:45    [W:0.026 / U:14.496 seconds]
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