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    SubjectRe: scheduling problem?
    Date
    Hi,

    I have played around with this code previously.
    This is my current understanding.
    [yield problem?]

    On Tuesday 02 January 2001 09:27, Mike Galbraith wrote:
    > Hi,
    >
    > I am seeing (what I believe is;) severe process CPU starvation in
    > 2.4.0-prerelease. At first, I attributed it to semaphore troubles
    > as when I enable semaphore deadlock detection in IKD and set it to
    > 5 seconds, it triggers 100% of the time on nscd when I do sequential
    > I/O (iozone eg). In the meantime, I've done a slew of tracing, and
    > I think the holder of the semaphore I'm timing out on just flat isn't
    > being scheduled so it can release it. In the usual case of nscd, I
    > _think_ it's another nscd holding the semaphore. In no trace can I
    > go back far enough to catch the taker of the semaphore or any user
    > task other than iozone running between __down() time and timeout 5
    > seconds later. (trace buffer covers ~8 seconds of kernel time)
    >
    > I think the snippet below captures the gist of the problem.
    >
    > c012f32e nr_free_pages +<e/4c> (0.16) pid(256)
    > c012f37a nr_inactive_clean_pages +<e/44> (0.22) pid(256)

    wakeup_bdflush (from beginning of __alloc_pages; page_alloc.c:324 )
    > c01377f2 wakeup_bdflush +<12/a0> (0.14) pid(256)
    > c011620a wake_up_process +<e/58> (0.29) pid(256)

    > c012eea4 __alloc_pages_limit +<10/b8> (0.28) pid(256)
    > c012eea4 __alloc_pages_limit +<10/b8> (0.30) pid(256)
    Two __alloc_pages_limit

    wakeup_kswapd(0) (from page_alloc.c:392 )
    > c012e3fa wakeup_kswapd +<12/d4> (0.25) pid(256)
    > c0115613 __wake_up +<13/130> (0.41) pid(256)

    schedule() (from page_alloc.c:396 )
    > c011527b schedule +<13/398> (0.66) pid(256->6)
    > c01077db __switch_to +<13/d0> (0.70) pid(6)

    bdflush is running!!!
    > c01893c6 generic_unplug_device +<e/38> (0.25) pid(6)

    bdflush is ready. (but how likely is it that it will run
    for long enough to get hit by a tick i.e. current->counter--
    unless it is it will continue to be preferred to kswapd, and
    since only one process is yielded... )
    > c011527b schedule +<13/398> (0.50) pid(6->256)
    > c01077db __switch_to +<13/d0> (0.29) pid(256)

    back to client, not the additionally runable kswapd...
    Why not - nothing remaining of timeslice.
    Not that the yield only yields one process. Not all
    in runqueue - IMHO. [is this intended?]

    3:rd __alloc_pages_limit this time direct_reclaim
    tests are fulfilled
    > c012eea4 __alloc_pages_limit +<10/b8> (0.22) pid(256)
    > c012d267 reclaim_page +<13/408> (0.54) pid(256)

    Possible (in -prerelease) untested possibilities.

    * Be tougher when yielding.


    wakeup_kswapd(0);
    if (gfp_mask & __GFP_WAIT) {
    __set_current_state(TASK_RUNNING);
    current->policy |= SCHED_YIELD;
    + current->counter--; /* be faster to let kswapd run */
    or
    + current->counter = 0; /* too fast? [not tested] */
    schedule();
    }

    Might be to tough on the client not doing any actual work... think dbench...

    * Be tougher on bflushd, decrement its counter now and then...
    [naive, not tested]

    * Move wakeup of bflushd to kswapd. Somewhere after 'do_try_to_free_pages(..)'
    has been run. Before going to sleep...
    [a variant tested with mixed results - this is likely a better one]


    /*
    * We go to sleep if either the free page shortage
    * or the inactive page shortage is gone. We do this
    * because:
    * 1) we need no more free pages or
    * 2) the inactive pages need to be flushed to disk,
    * it wouldn't help to eat CPU time now ...
    *
    * We go to sleep for one second, but if it's needed
    * we'll be woken up earlier...
    */
    if (!free_shortage() || !inactive_shortage()) {
    /*
    * If we are about to get low on free pages and cleaning
    * the inactive_dirty pages would fix the situation,
    * wake up bdflush.
    */
    if (free_shortage() && nr_inactive_dirty_pages > free_shortage()
    && nr_inactive_dirty_pages >= freepages.high)
    wakeup_bdflush(0);

    interruptible_sleep_on_timeout(&kswapd_wait, HZ);
    }

    --
    Home page:
    http://www.norran.net/nra02596/
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