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From: Willy Tarreau <w@1wt.eu>
To: Toon van der Pas <toon@hout.vanvergehaald.nl>
Cc: Kumar Gala <galak@kernel.crashing.org>,
	Linux Kernel list <linux-kernel@vger.kernel.org>
Subject: Re: tuning/tweaking VM settings for low memory (preventing OOM)
Date: Sat, 13 Jan 2007 15:30:27 +0100	[thread overview]
Message-ID: <20070113143027.GA16868@1wt.eu> (raw)
In-Reply-To: <20070113131601.GA12901@shuttle.vanvergehaald.nl>

Hi Toon,

On Sat, Jan 13, 2007 at 02:16:01PM +0100, Toon van der Pas wrote:
> On Sat, Jan 13, 2007 at 08:22:18AM +0100, Willy Tarreau wrote:
> > > 
> > > Which makes me think that we aren't writing back fast enough.  If I  
> > > mount the drive "sync" the issue clearly goes away.
> > > 
> > > It appears from an strace we are doing ftruncate64(5, 178257920) when  
> > > we OOM.
> > > 
> > > Any ideas on VM parameters to tweak so we throttle this from occurring?
> > 
> > Take a look at /proc/sys/vm/bdflush. There are several useful parameters
> > there (doc is in linux-xxx/Documentation). For instance, the first column
> > is the percentage of memory used by writes before starting to write on
> > disk. When using tcpdump intensively, I lower this one to about 1%.
> > 
> > Willy
> 
> Hi Willy,
> 
> I know you're doing a great job on keeping the 2.4 kernel in shape,
> but do you also have a good advice for people with more recent
> kernels?  (hint: the file /proc/sys/vm/bdflush is missing)

OK OK OK... Next time I will have coffee *before* replying :-)

Check /proc/sys/vm/dirty_ratio and dirty_background_ratio. Both are
percentage of total memory. The first one is for "foreground" writes
(ie the writing process may block) while the second one is for
"background" writes :

$ uname -a
Linux hp 2.6.16-rc2-pa1 #1 Fri Feb 3 23:34:56 MST 2006 parisc unknown
$ cat /proc/sys/vm/dirty_ratio 
40
$ cat /proc/sys/vm/dirty_background_ratio 
10

Again, lowering those values should help writing data to disk sooner.
Also you should take a look at min_free_kbytes (although I've not played
with it yet) :

[from Documentation/sysctl/vm.txt] :
  min_free_kbytes:

  This is used to force the Linux VM to keep a minimum number 
  of kilobytes free.  The VM uses this number to compute a pages_min
  value for each lowmem zone in the system.  Each lowmem zone gets 
  a number of reserved free pages based proportionally on its size.

Docuemntation/filesystems/proc.txt is your friend here too.

Regards,
Willy


  reply	other threads:[~2007-01-13 14:30 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-01-12 21:58 Kumar Gala
2007-01-13  4:54 ` Nick Piggin
2007-01-15 17:13   ` Kumar Gala
2007-01-13  7:22 ` Willy Tarreau
2007-01-13 13:16   ` Toon van der Pas
2007-01-13 14:30     ` Willy Tarreau [this message]
2007-01-13 23:58       ` Toon van der Pas

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