[Qemu-devel] [Bug 1505041] Re: Live snapshot revert times increases linearly with snapshot age

[Francois Gouget]

The timer settings I showed were from a Windows XP VM but a Windows 10
VM I created recently (with qemu 2.1 and libvirt 1.2.9) does have the
hypervclock timer and defaults to tickpolicy='catchup' for rtc but it is
missing the hyperv features you mentioned. I'll investigate those.

I'm wary of tickpolicy='catchup' though. My understanding is that it's
needed for the guest's clock to remain accurate over time. However our
VMs stay up at most 30 minutes before being reverted so I don't think
the clock will have drifted by any amount we care about. Really it can
be off by a few days without impacting the tests.

One nasty effect of tickpolicy='catchup' is that when we revert to our
live snapshot the clock runs 10 times as fast as it tries to catch up to
the current date. That would definitely impact our tests, particularly
around timing sound play times or checking serial baud rates. We
normally manually reset the time from within the guest so maybe that
would stop it from running fast. My other concern is that I'm not sure
trying to compensate for missing ticks is better for our tests than
maintaining the illusion that the VM is running at a normal speed even
if it's in reality running at half real time speed. I feel that for our
tests agreement between the audio, serial card and various timer devices
is more important than staying in sync with the outside world.

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https://bugs.launchpad.net/bugs/1505041

Title:
  Live snapshot revert times increases linearly with snapshot age

Status in QEMU:
  New

Bug description:
  The WineTestBot (https://testbot.winehq.org/) uses QEmu live snapshots
  to ensure the Wine tests are always run in a pristine Windows
  environment. However the revert times keep increasing linearly with
  the age of the snapshot, going from tens of seconds to thousands.
  While the revert takes place the qemu process takes 100% of a core and
  there is no disk activity. Obviously waiting over 20 minutes before
  being able to run a 10 second test is not viable.

  Only some VMs are impacted. Based on libvirt's XML files the common
  point appears to be the presence of the following <timer> tags:

      <clock offset='localtime'>
        <timer name='rtc' tickpolicy='delay'/>
        <timer name='pit' tickpolicy='delay'/>
        <timer name='hpet' present='no'/>
      </clock>

  Where the unaffected VMs have the following clock definition instead:

      <clock offset='localtime'/>

  Yet shutting down the affected VMs, changing the clock definition,
  creating a live snapshot and trying to revert to it 6 months later
  results in slow revert times (>400 seconds).

  Changing the tickpolicy to catchup for rtc and/or pit has no effect on
  the revert time (and unsurprisingly causes the clock to run fast in
  the guest).

  
  To reproduce this problem do the following:
  * Create a Windows VM (either 32 or 64 bits). This is known to happen with at 
least Windows 2000, XP, 2003, 2008 and 10.
  * That VM will have the <timer> tags shown above, with the possible addition 
of an hypervclock timer.
  * Shut down the VM.
  * date -s "2014/04/01"
  * Start the VM.
  * Take a live snapshot.
  * Shut down the VM.
  * date -s "<your current date>"
  * Revert to the live snapshot.

  If the revert takes more than 2 minutes then there is a problem.

  
  A workaround is to set track='guest' on the rtc timer. This makes the revert 
fast and may even be the correct solution. But why is it not the default or 
better documented?
   * It setting track='wall' or omitting track, then the revert is slow and the 
clock in the guest is not updated.
   * It setting track='guest' the revert is fast and the clock in the guest is 
not updated.

  
  I found three past mentions of this issue but as far as I can tell none of 
them got anywhere:

  * [Qemu-discuss] massive slowdown for reverts after given amount of time on 
any newer versions
     https://lists.gnu.org/archive/html/qemu-discuss/2013-02/msg00000.html

  * The above post references another one from 2011 wrt qemu 0.14:
     https://lists.gnu.org/archive/html/qemu-devel/2011-03/msg02645.html

  * Comment #9 of Launchpad bug 1174654 matches this slow revert issue. However
     the bug was really about another issue so this was not followed on.
     https://bugs.launchpad.net/qemu/+bug/1174654/comments/9

  
  I'm currently running into this issue with QEmu 2.1 but it looks like this 
bug has been there all along.
  1:2.1+dfsg-12+deb8u2 qemu-kvm
  1:2.1+dfsg-12+deb8u2 qemu-system-common
  1:2.1+dfsg-12+deb8u2 qemu-system-x86

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