Messages in this thread | | | From | Roger Larsson <> | Subject | Re: scheduling problem? | Date | Wed, 3 Jan 2001 03:39:35 +0100 |
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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); }
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