linux-kernel - Re: kswapd busy but not swapping

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Message-Id: <1211361217.6463.61.camel@lappy.programming.kicks-ass.net>
Date:	Wed, 21 May 2008 11:13:37 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Christian Kujau <lists@...dbynature.de>
Cc:	LKML <linux-kernel@...r.kernel.org>, kanoj@...xen.com
Subject: Re: kswapd busy but not swapping

On Wed, 2008-05-21 at 01:04 +0200, Christian Kujau wrote:
> Hi,
> 
> I noticed that the [kswapd0] thread was eating 3-7% cpu time but no 
> swapspace has been used yet. This is with a just booted 2.6.25.4 system,
> currently with some disk i/o going on.
> 
> So I figured that kswapd might not be responsible for "swapping" after 
> all, although the name suggests it. Grep'ing Documentation/ for kswapd did 
> not reveal much. Is the paper from Kanoj[0] still valid for 2.6 kernels? 
> The SGI page referenced in Documentation/kernel-docs.txt is down, but 
> there's a .txt version on [1] which tells me:
> 
>    "When memory runs low, and a process can not find a free page, it wakes
>     up kswapd, the memory stealer"
> 
> So, if I'd run low on memory, kswapd would be active. But I don't see that 
> in free(1):
> 
> # free -m
>               total       used       free     shared    buffers     cached
> Mem:          2023       1662        360          0        820        315
> -/+ buffers/cache:        526       1496
> Swap:         1505          0       1505

It could have been busy reclaiming pagecache.

kswapd is the generic reclaim thread - paging anonymous memory to the
swap is but one form thereof.

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