Messages in this thread |  | | | Date | Wed, 23 Feb 2011 18:27:34 +0100 | | From | Andrea Arcangeli <> | | Subject | Re: [Alsa-user] new source of MIDI playback slow-down identified - 5a03b051ed87e72b959f32a86054e1142ac4cf55 thp: use compaction in kswapd for GFP_ATOMIC order > 0 |
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On Wed, Feb 23, 2011 at 05:10:47PM +0000, Mel Gorman wrote: > On Wed, Feb 23, 2011 at 05:24:32PM +0100, Andrea Arcangeli wrote: > > On Wed, Feb 23, 2011 at 04:17:44AM +1030, Arthur Marsh wrote: > > > OK, these patches applied together against upstream didn't cause a crash > > > but I did observe: > > > significant slowdowns of MIDI playback (moreso than in previous cases, > > > and with less than 20 Meg of swap file in use); > > > > > > kswapd0 sharing equal top place in CPU usage at times (e.g. 20 percent). > > > > > > If I should try only one of the patches or something else entirely, > > > please let me know. > > > > Yes, with irq off, schedule won't run and need_resched won't get set. > > > > Stepping back a little, how did you determine that isolate_migrate was the > major problem? In my initial tests using the irqsoff tracer (sampled for > the duration fo the test every few seconds and resetting the max latency > each time), compaction_alloc() was a far worse source of problems and > isolate_migratepage didn't even register. It might be that I'm not testing > on large enough machines though.
I think you're right compaction_alloc is a bigger problem. Your patch to isolate_freepages is a must have and in the right direction.
However I think having large areas set as PageBuddy may be common too, the irq latency source in isolated_migratepages I think needs fixing too. We must be guaranteed to release irqs after max N pages (where N is SWAP_CLUSTER_MAX in my last two patches).
> In another mail, I posted a patch that dealt with compaction_alloc after > finding that IRQs were being disabled for millisecond lengths of time. > That length of time for IRQs being disabled could account for the performance > loss on the network load. Can test the network load with it applied?
kswapd was also running at 100% on all CPUs in that test.
The z1 that doesn't fix the latency source in compaction but that removes compaction from kswapd (a light/hackish version of compaction-no-kswapd-3 that I just posted) fixes the problem completely for the network load too.
So clearly it's not only a problem we can fix in compaction, the irq latency will improve for sure, but we still get an overload from kswapd which is not ok I think.
What I am planning to test on the network load is high-wmark+compaction_alloc_lowlat+compaction-kswapd-3 vs high-wmark+compaction_alloc_lowlat+compaction-no-kswapd-2.
Is this ok? If you want I can test also high-wmark+compaction_alloc_lowlat without compaction-kswapd-3/compaction-no-kswapd-2 but I think the irq-latency source in isolate_migratepages in presence of large PageBuddy regions (after any large application started at boot quits) isn't ok. Also I think having kswapd at 100% cpu load isn't ok. So I doubt we should stop at compaction_alloc_lowlat.
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