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[Joseph Salisbury <joseph.salisbury@xxxxxxxxxxxxx>]

** Changed in: linux (Ubuntu)
   Importance: Undecided => High

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https://bugs.launchpad.net/bugs/1346917

Title:
  Using KSM on NUMA capable machines can cause KVM guest performance and
  stability issues

Status in “linux” package in Ubuntu:
  Fix Released
Status in “linux” source package in Trusty:
  In Progress

Bug description:
  [Impact]

  When using KVM on NUMA machines, both Linux and Windows guests can
  exhibit very poor performance and potential crashes. Disabling KSM is
  a known workaround to fix this issue.

  [Fix]

  The following patch fixes the issue in our testing:
  http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=64a9a34e22896dad430e21a28ad8cb00a756fefc

  This patch is present in v3.14-rc1 and onwards.

  [Test Case]

  General test case:
  1) On a NUMA capable machine, setup the machine as a KVM hypervisor
    - lscpu should show more than 1 NUMA node
  2) Install 4 KVM VMs
  3) Run the following in another terminal to ensure that pages_shared and pages_sharing is increasing
   - watch 'tail /sys/kernel/mm/ksm/*'
  4) In another terminal run a program that continually pings each node and alerts on high latencies

  What we've observed is that in Linux guests, the ping latencies can go
  into the ~2 second range for a few pings, then return back to the <
  1ms range. (This is machine dependent.) In addition, occasionally when
  running this test with Windows guests we observe BSODs during this
  test.

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