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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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