Re: Over-eager swapping

[Wu Fengguang]

Andi, Christoph and Lee:

This looks like an "unbalanced NUMA memory usage leading to premature
swapping" problem.

Thanks,
Fengguang

On Wed, Aug 18, 2010 at 10:46:59PM +0800, Chris Webb wrote:
> Wu Fengguang <fengguang.wu@xxxxxxxxx> 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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