linux-kernel - Re: Odd swapping issue

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Message-Id: <1216907931.7257.333.camel@twins>
Date:	Thu, 24 Jul 2008 15:58:51 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Rafal Wijata (NULL)" <devnull@...ata.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Odd swapping issue

On Thu, 2008-07-24 at 15:45 +0200, Rafal Wijata (NULL) wrote:
> OK, things are little more clear now I suppose.
> 
> I tried (just for fun) echo 8 > /proc/sys/vm/nr_hugepages
> and it happened again, and no hugepages were allocated of course.
> 
> But this brought the following to my mind.
> 
> Since it's nfs server/client, and the nfs is rather heavily utilized(at 
> least sometimes), maybe nfsd tasks were unable to find long enough lowmem 
> chunk. How can we get such ram? By sweeping pages out to swap, isn't it?
> 
> Now the question is, if Linux actually has such feature like blind swapping 
> out if lowmem is too fragmented to find requested size?
> Can anybody confirm or negate?

Well, not blind - we have lumpy reclaim that tries to focus on the right
spot for large order allocations. However there are a myrad of
conditions that can make it impossible to reclaim certain pages,
therefore defeating this strategy.

> Next, I was thinking then I could run 64bit system, but I don't want to 
> reinstall it. Anybody knows if it's possible to have installed 32bit OS, but 
> run 64bit kernel alone(no 64bit glibc or anything else)?

Yes running as I type ;-)

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