linux-kernel - Re: Over-eager swapping

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Date:	Wed, 18 Aug 2010 11:57:14 -0400
From:	Lee Schermerhorn <Lee.Schermerhorn@...com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Chris Webb <chris@...chsys.com>,
	Minchan Kim <minchan.kim@...il.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Andi Kleen <andi@...stfloor.org>,
	Christoph Lameter <cl@...ux-foundation.org>
Subject: Re: Over-eager swapping

On Wed, 2010-08-18 at 23:21 +0800, Wu Fengguang wrote:
> Andi, Christoph and Lee:
> 
> This looks like an "unbalanced NUMA memory usage leading to premature
> swapping" problem.

What is the value of the vm.zone_reclaim_mode sysctl?  If it is !0, the
system will go into zone reclaim before allocating off-node pages.
However, it shouldn't "swap" in this case unless (zone_reclaim_mode & 4)
!= 0.  And even then, zone reclaim should only reclaim file pages, not
anon.  In theory...

Note:  zone_reclaim_mode will be enabled by default [= 1] if the SLIT
contains any distances > 2.0 [20].  Check SLIT values via 'numactl
--hardware'.

Lee

> 
> Thanks,
> Fengguang
> 
> On Wed, Aug 18, 2010 at 10:46:59PM +0800, Chris Webb wrote:
> > Wu Fengguang <fengguang.wu@...el.com> writes:
> > 
> > > Did you enable any NUMA policy? That could start swapping even if
> > > there are lots of free pages in some nodes.
> > 
> > Hi. Thanks for the follow-up. We haven't done any configuration or tuning of
> > NUMA behaviour, but NUMA support is definitely compiled into the kernel:
> > 
> >   # zgrep NUMA /proc/config.gz 
> >   CONFIG_NUMA_IRQ_DESC=y
> >   CONFIG_NUMA=y
> >   CONFIG_K8_NUMA=y
> >   CONFIG_X86_64_ACPI_NUMA=y
> >   # CONFIG_NUMA_EMU is not set
> >   CONFIG_ACPI_NUMA=y
> >   # grep -i numa /var/log/dmesg.boot 
> >   NUMe: Allocated memnodemap from b000 - 1b540
> >   NUMA: Using 20 for the hash shift.
> > 
> > > Are your free pages equally distributed over the nodes? Or limited to
> > > some of the nodes? Try this command:
> > > 
> > >         grep MemFree /sys/devices/system/node/node*/meminfo
> > 
> > My worst-case machines current have swap completely turned off to make them
> > usable for clients, but I have one machine which is about 3GB into swap with
> > 8GB of buffers and 3GB free. This shows
> > 
> >   # grep MemFree /sys/devices/system/node/node*/meminfo
> >   /sys/devices/system/node/node0/meminfo:Node 0 MemFree:          954500 kB
> >   /sys/devices/system/node/node1/meminfo:Node 1 MemFree:         2374528 kB
> > 
> > I could definitely imagine that one of the nodes could have dipped down to
> > zero in the past. I'll try enabling swap on one of our machines with the bad
> > problem late tonight and repeat the experiment. The node meminfo on this box
> > currently looks like
> > 
> >   # grep MemFree /sys/devices/system/node/node*/meminfo
> >   /sys/devices/system/node/node0/meminfo:Node 0 MemFree:           82732 kB
> >   /sys/devices/system/node/node1/meminfo:Node 1 MemFree:         1723896 kB
> > 
> > Best wishes,
> > 
> > Chris.
> 
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