linux-kernel - Re: Linux 2.6.31 - very swap-happy with plenty of free RAM

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Date:	Thu, 19 Nov 2009 07:43:02 +0100
From:	Tomasz Chmielewski <mangoo@...g.org>
To:	Dave Chinner <david@...morbit.com>
CC:	Sven-Haegar Koch <haegar@...net.de>,
	Dan Merillat <dan.merillat@...il.com>, preining@...ic.at,
	riel@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: Linux 2.6.31 - very swap-happy with plenty of free RAM

Dave Chinner wrote:

>>>> I have a pretty powerful desktop machine with 8 GB RAM, fast disks with
>>>> RAID-1 etc.
>>>> Every 10 minutes or so, machine is really unresponsive, load jumps to 10 or
>>>> 20. Mouse pointer jumps, it's impossible to change between windows etc.
>>> This is the type of hefty workstation many of the core developers
>>> have, running similar workloads, so I'm somewhat surprised that the
>>> default VM settings have this kind of issue.
>>>
>>>> Do a "swapoff -a", and everything is snappy and responsive as it should,
>>>> there are no more lags.
>>> Yes, that's my exact finding.
>>>
>>> Is this weird IO storm happening for anyone else with plenty of memory
>>> (for their taskload?)
>> For me it happend on my laptop. 3gb RAM, and a 1gb VMWare Windows-XP 
>> instance running, plus the usual like firefox, thunderbird, kde4.
>>
>> Without running vmware it did not happen. And since I have now disabled 
>> barriers on the xfs /home partition (on luks crypto lvm) it also does 
>> not happen anymore.
> 
> Yeah, it looks like dm-crypt recently started supporting barriers in
> commit 647c7db14ef9cacc4ccb3683e206b61f0de6dc2b. Hence XFS will have
> detected barriers work at mount time mount and so is now issuing
> them.
> 
> Similarly, raid1 (mirror) has recently gained barrier support so
> the same issue can be seen there.

What is also interesting, that a normal software RAID-1 sync (i.e. from 
a degraded state) does not seem to make any visible effect on system 
responsiveness.

Uncompress a big tar file, or VM writes out lots of data - system 
becomes really unresponsive.


-- 
Tomasz Chmielewski
http://wpkg.org
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