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[Alan Pevec <1100446@xxxxxxxxxxxxxxxxxx>]

** Changed in: nova/folsom
       Status: Fix Committed => Fix Released

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https://bugs.launchpad.net/bugs/1100446

Title:
  libvirt driver connection validation causes unnecessary process
  execution with libvirt/qemu

Status in OpenStack Compute (Nova):
  Fix Released
Status in OpenStack Compute (nova) folsom series:
  Fix Released

Bug description:
  A  VM transits  from BUILD to ACTIVE status can take 26 second with
  libvirt/qemu.

  This transition is critical in the gate system's performance too.

  https://github.com/openstack/nova/blob/c215b5ec79516111456dfc2a63fa0facf5946ab0/nova/virt/libvirt/driver.py#L365
  This call should replaced to something cheaper,  Like LibVirt Version (or Hostname query .)
  Or by an something even cheaper solution.

  Note:
  The one minute periodical status update also leads to this expensive call. I do not think the architecture changes frequently.
  Consider query it only on service start-up.

  If you just use the getCapabilies only at startup, you can reduce the
  ~26 second to ~13 second!

  If your qemu supports multiple architecture it is much slower, and by
  fixing this issue,  you can have even greater performance.

  You can see the executions done by the libvirtd by this.  
  strace -Ff -p <libvirtd_pid> -e execve

  You will see several hundred/ or thousands(multi arch) of similar
  execve lines:

  29010 +++ exited with 0 +++
  5382  --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=29010, si_status=0, si_utime=1, si_stime=0} ---
  29011 execve("/usr/bin/qemu-system-x86_64", ["/usr/bin/qemu-system-x86_64", "-device", "?", "-device", "pci-assign,?", "-device", "virtio-blk-pci,?", "-device", "virtio-net-pci,?", "-device", "scsi-disk,?"], [/* 2 vars */]) = 0
  29011 +++ exited with 0 +++
  5382  --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=29011, si_status=0, si_utime=1, si_stime=0} ---
  29012 execve("/usr/bin/qemu-system-x86_64", ["/usr/bin/qemu-system-x86_64", "-cpu", "?"], [/* 2 vars */]) = 0
  29012 +++ exited with 0 +++
  5382  --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=29012, si_status=0, si_utime=1, si_stime=0} ---
  29013 execve("/usr/bin/qemu-system-x86_64", ["/usr/bin/qemu-system-x86_64", "-help"], [/* 2 vars */]) = 0
  29013 +++ exited with 0 +++
  5382  --- SIGCHLD {si_signo=SIGCHLD, si_code=CLD_EXITED, si_pid=29013, si_status=0, si_utime=1, si_stime=0} ---
  29014 execve("/usr/bin/qemu-system-x86_64", ["/usr/bin/qemu-system-x86_64", "-device", "?", "-device", "pci-assign,?", "-device", "virtio-blk-pci,?", "-device", "virtio-net-pci,?", "-device", "scsi-disk,?"], [/* 2 vars */]) = 0
  29014 +++ exited with 0 +++

  (you can add a -ttt argument for time measurement )

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