linux-kernel - Re: [PATCH -mm] vmscan: bail out of page reclaim after swap_cluster

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Message-ID: <491D8CEC.5050106@redhat.com>
Date:	Fri, 14 Nov 2008 09:36:28 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH -mm] vmscan: bail out of page reclaim after swap_cluster_max
 pages

Andrew Morton wrote:
> On Thu, 13 Nov 2008 17:12:08 -0500 Rik van Riel <riel@...hat.com> wrote:
> 
>> Sometimes the VM spends the first few priority rounds rotating back
>> referenced pages and submitting IO.  Once we get to a lower priority,
>> sometimes the VM ends up freeing way too many pages.
>>
>> The fix is relatively simple: in shrink_zone() we can check how many
>> pages we have already freed and break out of the loop.
>>
>> However, in order to do this we do need to know how many pages we already
>> freed, so move nr_reclaimed into scan_control.
> 
> There was a reason for not doing this, but I forget what it was.  It might require
> some changelog archeology.  iirc it was to do with balancing scanning rates
> between the various things which we scan.

I've seen worse symptoms without this code, though. Pretty
much all 2.6 kernels show bad behaviour occasionally.

Sometimes the VM gets in such a state where multiple processes
cannot find anything readily evictable, and they all end up
at a lower priority level.

This can cause them to evict more than half of everything from
memory, before breaking out of the pageout loop and swapping
things back in.  On my 2GB desktop, I've seen as much as 1200MB
memory free due to such a swapout storm.  It is possible more is
free at the top of the cycle, but X and gnome-terminal and top
and everything else is stuck, so that's not actually visible :)

I am not convinced that a scanning imbalance is more serious.

Of course, one thing we could do is exempt kswapd from this check.
During light reclaim, kswapd does most of the eviction so scanning
should remain balanced.  Having one process fall down to a lower
priority level is also not a big problem.

As long as the direct reclaim processes do not also fall into the
same trap, the situation should be manageable.

Does that sound reasonable to you?

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