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Date: Sat, 24 Dec 2011 07:45:03 +1100 From: Dave Chinner <david@...morbit.com> To: nowhere <nowhere@...kenden.ath.cx> Cc: Michal Hocko <mhocko@...e.cz>, linux-kernel@...r.kernel.org, linux-mm@...ck.org Subject: Re: Kswapd in 3.2.0-rc5 is a CPU hog On Fri, Dec 23, 2011 at 03:04:02PM +0400, nowhere wrote: > В Пт., 23/12/2011 в 21:20 +1100, Dave Chinner пишет: > > On Fri, Dec 23, 2011 at 01:01:20PM +0400, nowhere wrote: > > > В Чт., 22/12/2011 в 09:55 +1100, Dave Chinner пишет: > > > > On Wed, Dec 21, 2011 at 10:52:49AM +0100, Michal Hocko wrote: > > > > > [Let's CC linux-mm] > > > > > > > > > > On Wed 21-12-11 07:10:36, Nikolay S. wrote: > > > > > > Hello, > > > > > > > > > > > > I'm using 3.2-rc5 on a machine, which atm does almost nothing except > > > > > > file system operations and network i/o (i.e. file server). And there is > > > > > > a problem with kswapd. > > > > > > > > > > What kind of filesystem do you use? > > > > > > > > > > > > > > > > > I'm playing with dd: > > > > > > dd if=/some/big/file of=/dev/null bs=8M > > > > > > > > > > > > I.e. I'm filling page cache. > > > > > > > > > > > > So when the machine is just rebooted, kswapd during this operation is > > > > > > almost idle, just 5-8 percent according to top. > > > > > > > > > > > > After ~5 days of uptime (5 days, 2:10), the same operation demands ~70% > > > > > > for kswapd: > > > > > > > > > > > > PID USER S %CPU %MEM TIME+ SWAP COMMAND > > > > > > 420 root R 70 0.0 22:09.60 0 kswapd0 > > > > > > 17717 nowhere D 27 0.2 0:01.81 10m dd > > > > > > > > > > > > In fact, kswapd cpu usage on this operation steadily increases over > > > > > > time. > > > > > > > > > > > > Also read performance degrades over time. After reboot: > > > > > > dd if=/some/big/file of=/dev/null bs=8M > > > > > > 1019+1 records in > > > > > > 1019+1 records out > > > > > > 8553494018 bytes (8.6 GB) copied, 16.211 s, 528 MB/s > > > > > > > > > > > > After ~5 days uptime: > > > > > > dd if=/some/big/file of=/dev/null bs=8M > > > > > > 1019+1 records in > > > > > > 1019+1 records out > > > > > > 8553494018 bytes (8.6 GB) copied, 29.0507 s, 294 MB/s > > > > > > > > > > > > Whereas raw disk sequential read performance stays the same: > > > > > > dd if=/some/big/file of=/dev/null bs=8M iflag=direct > > > > > > 1019+1 records in > > > > > > 1019+1 records out > > > > > > 8553494018 bytes (8.6 GB) copied, 14.7286 s, 581 MB/s > > > > > > > > > > > > Also after dropping caches, situation somehow improves, but not to the > > > > > > state of freshly restarted system: > > > > > > PID USER S %CPU %MEM TIME+ SWAP COMMAND > > > > > > 420 root S 39 0.0 23:31.17 0 kswapd0 > > > > > > 19829 nowhere D 24 0.2 0:02.72 7764 dd > > > > > > > > > > > > perf shows: > > > > > > > > > > > > 31.24% kswapd0 [kernel.kallsyms] [k] _raw_spin_lock > > > > > > 26.19% kswapd0 [kernel.kallsyms] [k] shrink_slab > > > > > > 16.28% kswapd0 [kernel.kallsyms] [k] prune_super > > > > > > 6.55% kswapd0 [kernel.kallsyms] [k] grab_super_passive > > > > > > 5.35% kswapd0 [kernel.kallsyms] [k] down_read_trylock > > > > > > 4.03% kswapd0 [kernel.kallsyms] [k] up_read > > > > > > 2.31% kswapd0 [kernel.kallsyms] [k] put_super > > > > > > 1.81% kswapd0 [kernel.kallsyms] [k] drop_super > > > > > > 0.99% kswapd0 [kernel.kallsyms] [k] __put_super > > > > > > 0.25% kswapd0 [kernel.kallsyms] [k] __isolate_lru_page > > > > > > 0.23% kswapd0 [kernel.kallsyms] [k] free_pcppages_bulk > > > > > > 0.19% kswapd0 [r8169] [k] rtl8169_interrupt > > > > > > 0.15% kswapd0 [kernel.kallsyms] [k] twa_interrupt > > > > > > > > > > Quite a lot of time spent shrinking slab (dcache I guess) and a lot of > > > > > spin lock contention. > > > > > > > > That's just scanning superblocks, not apparently doing anything > > > > useful like shrinking dentries or inodes attached to each sb. i.e. > > > > the shrinkers are being called an awful lot and basically have > > > > nothing to do. I'd be suspecting a problem higher up in the stack to > > > > do with how shrink_slab is operating or being called. > > > > > > > > I'd suggest gathering event traces for mm_shrink_slab_start/ > > > > mm_shrink_slab_end to try to see how the shrinkers are being > > > > driven... > > > > > > > > Cheers, > > > > > > > > Dave. > > > > > > I have recompiled kernel with tracers, and today the problem is visible > > > again. So here is the trace for mm_shrink_slab_start (it is HUGE): > > > > > > kswapd0 421 [000] 103976.627873: mm_shrink_slab_start: prune_super+0x0 0xffff88011b00d300: objects to shrink 12 gfp_flags GFP_KERNELGFP_NOTRACK pgs_scanned 32 lru_pgs 942483 cache items 1500 delt > > > kswapd0 421 [000] 103976.627882: mm_shrink_slab_start: prune_super+0x0 0xffff88011a20ab00: objects to shrink 267 gfp_flags GFP_KERNELGFP_NOTRACK pgs_scanned 32 lru_pgs 942483 cache items 5300 del > > > > And possibly useless in this form. I need to see the > > mm_shrink_slab_start/mm_shrink_slab_end events interleaved so I can > > see exactly how much work each shrinker call is doing, and the start > > events are truncated so not all the info I need is present. > > > > Perhaps you should install trace-cmd. > > > > $ trace-cmd record -e mm_shrink_slab* > > (wait 30s, then ^C) > > $ trace-cmd report > shrink.trace > > > > And then compress and attach the trace file or put up on the web > > somewhere for me ot download if it's too large for email... > > > > As it is, there's ~940k pages in the LRU, and shrink_slab is being > > called after 32, 95, 8, 8, 32 and 32 pages on the LRU have been > > scanned. That seems like the shrinkers are being called rather too > > often. > > > > The end traces indicate the shrinker caches aren't able to free > > anything. So it looks like the vmscan code has got itself in a > > situation where it is not scanning many pages between shrinker > > callouts, and the shrinkers scan but can't make any progress. Looks > > like a vmscan balancing problem right now, not anything to do with > > the shrinker code. A better trace will confirm that. > > > > FWIW, if you use trace-cmd, it might be worthwhile collecting all the > > vmscan trace events too, as that might help the VM folk understand > > the problem without needing to ask you for more info. > > ./trace-cmd record -e vmscan/* > > Here is the report of trace-cmd while dd'ing > https://80.237.6.56/report-dd.xz Ok, it's not a shrink_slab() problem - it's just being called ~100uS by kswapd. The pattern is: - reclaim 94 (batches of 32,32,30) pages from iinactive list of zone 1, node 0, prio 12 - call shrink_slab - scan all caches - all shrinkers return 0 saying nothing to shrink - 40us gap - reclaim 10-30 pages from inactive list of zone 2, node 0, prio 12 - call shrink_slab - scan all caches - all shrinkers return 0 saying nothing to shrink - 40us gap - isolate 9 pages from LRU zone ?, node ?, none isolated, none freed - isolate 22 pages from LRU zone ?, node ?, none isolated, none freed - call shrink_slab - scan all caches - all shrinkers return 0 saying nothing to shrink 40us gap And it just repeats over and over again. After a while, nid=0,zone=1 drops out of the traces, so reclaim only comes in batches of 10-30 pages from zone 2 between each shrink_slab() call. The trace starts at 111209.881s, with 944776 pages on the LRUs. It finishes at 111216.1 with kswapd going to sleep on node 0 with 930067 pages on the LRU. So 7 seconds to free 15,000 pages (call it 2,000 pages/s) which is awfully slow.... vmscan gurus - time for you to step in now... Cheers, Dave. -- Dave Chinner david@...morbit.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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