linux-kernel - Over-eager swapping

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Date:	Mon, 2 Aug 2010 13:47:35 +0100
From:	Chris Webb <chris@...chsys.com>
To:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Over-eager swapping

We run a number of relatively large x86-64 hosts with twenty or so qemu-kvm
virtual machines on each of them, and I'm have some trouble with over-eager
swapping on some (but not all) of the machines. This is resulting in
customer reports of very poor response latency from the virtual machines
which have been swapped out, despite the hosts apparently having large
amounts of free memory, and running fine if swap is turned off.

All of the hosts are running a 2.6.32.7 kernel and have ksm enabled with
32GB of RAM and 2x quad-core processors. There is a cluster of Xeon E5420
machines which apparently doesn't exhibit the problem, and a cluster of
2352/2378 Opteron (NUMA) machines, some of which do. The kernel config of
the affected machines is at

  http://cdw.me.uk/tmp/config-2.6.32.7

This differs very little from the config on the unaffected Xeon machines,
essentially just

  -CONFIG_MCORE2=y
  +CONFIG_MK8=y
  -CONFIG_X86_P6_NOP=y

On a typical affected machine, the virtual machines and other processes
would apparently leave around 5.5GB of RAM available for buffers, but the
system seems to want to swap out 3GB of anonymous pages to give itself more
like 9GB of buffers:

  # cat /proc/meminfo 
  MemTotal:       33083420 kB
  MemFree:          693164 kB
  Buffers:         8834380 kB
  Cached:            11212 kB
  SwapCached:      1443524 kB
  Active:         21656844 kB
  Inactive:        8119352 kB
  Active(anon):   17203092 kB
  Inactive(anon):  3729032 kB
  Active(file):    4453752 kB
  Inactive(file):  4390320 kB
  Unevictable:        5472 kB
  Mlocked:            5472 kB
  SwapTotal:      25165816 kB
  SwapFree:       21854572 kB
  Dirty:              4300 kB
  Writeback:             4 kB
  AnonPages:      20780368 kB
  Mapped:             6056 kB
  Shmem:                56 kB
  Slab:             961512 kB
  SReclaimable:     438276 kB
  SUnreclaim:       523236 kB
  KernelStack:       10152 kB
  PageTables:        67176 kB
  NFS_Unstable:          0 kB
  Bounce:                0 kB
  WritebackTmp:          0 kB
  CommitLimit:    41707524 kB
  Committed_AS:   39870868 kB
  VmallocTotal:   34359738367 kB
  VmallocUsed:      150880 kB
  VmallocChunk:   34342404996 kB
  HardwareCorrupted:     0 kB
  HugePages_Total:       0
  HugePages_Free:        0
  HugePages_Rsvd:        0
  HugePages_Surp:        0
  Hugepagesize:       2048 kB
  DirectMap4k:        5824 kB
  DirectMap2M:     3205120 kB
  DirectMap1G:    30408704 kB

We see this despite the machine having vm.swappiness set to 0 in an attempt
to skew the reclaim as far as possible in favour of releasing page cache
instead of swapping anonymous pages.

After running swapoff -a, the machine is immediately much healthier. Even
while the swap is still being reduced, load goes down and response times in
virtual machines are much improved. Once the swap is completely gone, there
are still several gigabytes of RAM left free which are used for buffers, and
the virtual machines are no longer laggy because they are no longer swapped
out. Running swapon -a again, the affected machine waits for about a minute
with zero swap in use, before the amount of swap in use very rapidly
increases to around 2GB and then continues to increase more steadily to 3GB.

We could run with these machines without swap (in the worst cases we're
already doing so), but I'd prefer to have a reserve of swap available in
case of genuine emergency. If it's a choice between swapping out a guest or
oom-killing it, I'd prefer to swap... but I really don't want to swap out
running virtual machines in order to have eight gigabytes of page cache
instead of five!

Is this a problem with the page reclaim priorities, or am I just tuning
these hosts incorrectly? Is there more detailed info than /proc/meminfo
available which might shed more light on what's going wrong here?

Best wishes,

Chris.
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