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

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Date:	Thu, 19 Nov 2009 02:09:09 +0100 (CET)
From:	Sven-Haegar Koch <haegar@...net.de>
To:	Dan Merillat <dan.merillat@...il.com>
cc:	Tomasz Chmielewski <mangoo@...g.org>, 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

On Wed, 18 Nov 2009, Dan Merillat wrote:

> On Wed, Nov 18, 2009 at 4:45 PM, Tomasz Chmielewski <mangoo@...g.org> 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.

The machine is currently 300mb in swap, together with >2gb shown by free 
as cached - as I just copied 10gb around nothing I worry about.

For me when it happened it looked like when a big bunch of data is to 
be commited to disk xfs seeks between superblock/journal and data
and syncs/waits between each step - reducing the write speed of the
disk from the normal >20mb/s to less than 2mb/s and also delaying all 
other disk-accesses in the meantime.

c'ya
sven

ps: I know with disabling barriers I sacrifice some data safety, but 
    better a usable and fast system than everything ultra-safe and
    slow - for safety in suche rare case there is a backup.

-- 
The lights are fading out, once more...
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