Re: [Qemu-devel] Rethinking missed tick catchup

[Anthony Liguori]

Gleb Natapov <address@hidden> writes:

> On Thu, Sep 13, 2012 at 09:35:18AM -0500, Anthony Liguori wrote:
>> Gleb Natapov <address@hidden> writes:
>> 
>> > On Thu, Sep 13, 2012 at 09:06:29AM -0500, Anthony Liguori wrote:
>> >> "Daniel P. Berrange" <address@hidden> writes:
>> >> 
>> >> I think it's better for QEMU to talk to qemu-ga.  We can tell when a large
>> >> period of time has passed in QEMU because we'll accumulate a large
>> >> number of missed ticks.
>> >> 
>> > With RTC configured to use vm clock we will not.
>> 
>> Not for host suspend.  For stop and live migration, we stop vm_clock.
>> But QEMU isn't aware of host suspend so vm_clock cannot be stopped.
>> 
> Hmm, true. What about hooking into suspend and doing vmstop during
> suspend. 

Is suspend the only foreseeable way for this problem to happen?  I don't
think it is which is what concerns me about any approach that relies on
"hooking suspend".

Also, I don't think there is a generic way to "hook suspend".

>> >> This could happen because of stop, host suspend, live migration to a
>> >> file, etc.
>> >> 
>> >> It's much easier for us to call into qemu-ga to do the time correction
>> >> whenever this event occurs than to try and have libvirt figure out when
>> >> it's necessary.
>> > And if guest does not have qemu-ga what is better inject interrupts like
>> > crazy for next 2 minutes or leave guest with incorrect time?
>> 
>> Yes, at least that's fixable by the end-user.  QEMU consuming 100% CPU
>> for a prolonged period of time isn't fixable.
>> 
> You mean yes to "leave guest with incorrect time"? QEMU will still
> consume 100% of cpu for some time calling qemu_timer callback millions
> times. timedrift code is not the right level to fix that.

Not if we put a cap on how many interrupts we'll try to catch up.

As I mentioned previously, if we acrue more than X number of missed
ticks, we should simply declare bankruptcy and reset the counter.

When that occurs, *if* qemu-ga is present, we should ask qemu-ga to
reset the guest's clock based on reading the hardware clock via a
'guest-resync-time' command.

If it isn't, time will be off.  Hopefully the guest is running NTP and
can correct itself.  Otherwise, at least the admin can manually fix the
time.

Regards,

Anthony Liguori

>
> --
>                       Gleb.