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List:       linux-mm
Subject:    Re: Kernel 2.6.8.1: swap storm of death - nr_requests > 1024 on swap partition
From:       Jens Axboe <axboe () suse ! de>
Date:       2004-08-29 16:53:45
Message-ID: 20040829165345.GC11219 () suse ! de
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On Sat, Aug 28 2004, Andrew Morton wrote:
> 
> (Added linux-mm)
> 
> Karl Vogel <karl.vogel@pandora.be> wrote:
> >
> > Andrew Morton wrote:
> > > Karl Vogel <karl.vogel@seagha.com> wrote:
> > > 
> > >>Further testing shows that all the schedulers exhibit this exact same
> > >> problem when run with a nr_requests size of 8192 on the drive hosting 
> > >> the swap partition.
> > >>
> > >> I tried noop, deadline, as and CFQ with:
> > >>
> > >> 	echo 8192 >/sys/block/hda/queue/nr_requests
> > > 
> > > 
> > > That allows up to 2GB of memory to be under writeout at the same time.  The
> > > VM cannot touch any of that memory.
> > 
> > Well I used that value as it is the default for CFQ.. and it was with 
> > CFQ that I had the problems. The patch Jens offered to track down the 
> > problem, commented out this 'q->nr_requests = 8192' in CFQ and it 
> > helped. Therefor I tried the other schedulers with this value to see if 
> > it made a difference.
> > 
> > So if I understand you correctly, CFQ shouldn't be using 8192 on 512Mb 
> > systems?!
> 
> Yup.  It's asking for trouble to allow that much memory to be unreclaimably
> pinned.

It's not pinned, it's in-progress. I think it's really bad behaviour to
_allow_ so much to be in-progress, if you can't handle it. It's silly to
expect the io scheduler to know this and limit it, belongs at a
different level (the vm, where you have such knowledge).

> Of course, you could have the same problem with just 128 requests per
> queue, and lots of queues.  I solved all these problems in the dirty memory
> writeback paths.  But I forgot about swapout!

Precisely. Or 128 requests on a 16MB system. More proof that this is a
vm problem.

-- 
Jens Axboe

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