Re: [Qemu-devel] Re: [RFC: 0/2] patch for QEMU HPET periodic timer emulation to alleviate time drift

[Anthony Liguori <anthony@codemonkey.ws>]

On 02/07/2011 07:14 AM, Avi Kivity wrote:
> On 02/07/2011 03:11 PM, Anthony Liguori wrote:
>> On 02/07/2011 06:34 AM, Avi Kivity wrote:
>>> On 02/04/2011 10:56 AM, Jan Kiszka wrote:
>>>> >
>>>> >  This should be a rare event.  If you are missing 50% of your
>>>> >  notifications, not amount of gradual catchup is going to help 
>>>> you out.
>>>>
>>>> But that's the only thing this patch is after: lost ticks at QEMU 
>>>> level.
>>>
>>> Most lost ticks will happen at the vcpu level.  The iothread has low 
>>> utilization and will therefore be scheduled promptly, whereas the 
>>> vcpu thread may have high utilization and will thus be preempted.  
>>> When it is preempted for longer than the timer tick, we will see 
>>> vcpu-level coalescing.  All it takes is 2:1 overcommit to see time 
>>> go half as fast; I don't think you'll ever see that on bare metal.
>>
>> But that's not to say that doing something about lost ticks in QEMU 
>> isn't still useful.
>>
>
> If it doesn't solve the majority of the problems it isn't very useful 
> IMO.  It's a good first step, but not sufficient for real world use 
> with overcommit.

Even if we have a way to detect coalescing, we still need to make sure 
we don't lose ticks in QEMU.  So regardless of whether it solves the 
majority of problems, we need this anyway.

Regards,

Anthony Liguori