linux-kernel - Re: OOM killer gripe (was Re: What still uses the block layer?)

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Date:	Mon, 15 Oct 2007 22:45:01 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	david@...g.hm
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Rob Landley <rob@...dley.net>, Theodore Tso <tytso@....edu>,
	James Bottomley <James.Bottomley@...eleye.com>,
	Matthew Wilcox <matthew@....cx>, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org, Jens Axboe <axboe@...e.de>,
	Suparna Bhattacharya <suparna@...ibm.com>,
	Nick Piggin <piggin@...erone.com.au>
Subject: Re: OOM killer gripe (was Re: What still uses the block layer?)

david@...g.hm writes:

>
> on some kernel versions you are correct about needing swap > ram, but on current
> versions you are not. the swap space gets allocated as needed, and re-used as
> needed (I don't know the mechanism of this, but I remember the last time this
> changed from vm=max(ram,swap) to vm=ram+swap)

I don't think I can recall a linux kernel that required swap > ram.
However for serious swapping under linux having swap > ram was very
useful and pretty much a requirement for a workload that involved
swapping heavily (not thrashing).

>> I have not heard of many people swapping and not thrashing lately.
>> I think part of the problem is that we do random access to the swap
>> partition which makes us seek limited.  And since the number of
>> seeks per unit time has been increasing at a linear or slower rate
>> that if we are doing random disk I/O then the amount we can use
>> the disk for is very limited.   I wonder if we could figure out
>> how to push and pull 1M or bigger chunks into and out of swap?
>
> it has been noted by many people that linux is very slow to pull things back
> into ram from swap, significantly slower then simple seed limiting would seem to
> account for.

Yes.  It may be the large amount of random access (my current guess)
or it may be something else.

I'm wonder if I should build an application with a configurable
data set and working set that can be used for swap testing.  I don't
think it would be very hard and it might help sort through some of
the swap performance problems.

Eric



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