linux-kernel - Re: [PATCH 0/3] OOM detection rework v4

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Date:	Wed, 16 Dec 2015 15:58:44 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Hillf Danton <hillf.zj@...baba-inc.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	<linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] OOM detection rework v4

On Tue, 15 Dec 2015 19:19:43 +0100 Michal Hocko <mhocko@...nel.org> wrote:

> 
> ...
>
> * base kernel
> $ grep "Killed process" base-oom-run1.log | tail -n1
> [  211.824379] Killed process 3086 (mem_eater) total-vm:85852kB, anon-rss:81996kB, file-rss:332kB, shmem-rss:0kB
> $ grep "Killed process" base-oom-run2.log | tail -n1
> [  157.188326] Killed process 3094 (mem_eater) total-vm:85852kB, anon-rss:81996kB, file-rss:368kB, shmem-rss:0kB
> 
> $ grep "invoked oom-killer" base-oom-run1.log | wc -l
> 78
> $ grep "invoked oom-killer" base-oom-run2.log | wc -l
> 76
> 
> The number of OOM invocations is consistent with my last measurements
> but the runtime is way too different (it took 800+s).

I'm seeing 211 seconds vs 157 seconds?  If so, that's not toooo bad.  I
assume the 800+s is sum-across-multiple-CPUs?  Given that all the CPUs
are pounding away at the same data and the same disk, that doesn't
sound like very interesting info - the overall elapsed time is the
thing to look at in this case.

> One thing that
> could have skewed results was that I was tail -f the serial log on the
> host system to see the progress. I have stopped doing that. The results
> are more consistent now but still too different from the last time.
> This is really weird so I've retested with the last 4.2 mmotm again and
> I am getting consistent ~220s which is really close to the above. If I
> apply the WQ vmstat patch on top I am getting close to 160s so the stale
> vmstat counters made a difference which is to be expected. I have a new
> SSD in my laptop which migh have made a difference but I wouldn't expect
> it to be that large.
> 
> $ grep "DMA32.*all_unreclaimable? no" base-oom-run1.log | wc -l
> 4
> $ grep "DMA32.*all_unreclaimable? no" base-oom-run2.log | wc -l
> 1
> 
> * patched kernel
> $ grep "Killed process" patched-oom-run1.log | tail -n1
> [  341.164930] Killed process 3099 (mem_eater) total-vm:85852kB, anon-rss:82000kB, file-rss:336kB, shmem-rss:0kB
> $ grep "Killed process" patched-oom-run2.log | tail -n1
> [  349.111539] Killed process 3082 (mem_eater) total-vm:85852kB, anon-rss:81996kB, file-rss:4kB, shmem-rss:0kB

Even better.

> $ grep "invoked oom-killer" patched-oom-run1.log | wc -l
> 78
> $ grep "invoked oom-killer" patched-oom-run2.log | wc -l
> 77
> 
> $ grep "DMA32.*all_unreclaimable? no" patched-oom-run1.log | wc -l
> 1
> $ grep "DMA32.*all_unreclaimable? no" patched-oom-run2.log | wc -l
> 0
> 
> So the number of OOM killer invocation is the same but the overall
> runtime of the test was much longer with the patched kernel. This can be
> attributed to more retries in general. The results from the base kernel
> are quite inconsitent and I think that consistency is better here.

It's hard to say how long declaration of oom should take.  Correctness
comes first.  But what is "correct"?  oom isn't a binary condition -
there's a chance that if we keep churning away for another 5 minutes
we'll be able to satisfy this allocation (but probably not the next
one).  There are tradeoffs between promptness-of-declaring-oom and
exhaustiveness-in-avoiding-it.

> 
> 2) 2 writers again with 10s of run and then 10 mem_eaters to consume as much
>    memory as possible without triggering the OOM killer. This required a lot
>    of tuning but I've considered 3 consecutive runs without OOM as a success.

"a lot of tuning" sounds bad.  It means that the tuning settings you
have now for a particular workload on a particular machine will be
wrong for other workloads and machines.  uh-oh.

> ...
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