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

[Jan Kiszka]

On 2011-02-07 14:23, Anthony Liguori wrote:
> 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.

Again: please not in an ad-hoc fashion but as a generic services usable
by _all_ periodic timer sources that want to implement compensation.
This infrastructure should also be designed to once integrate IRQ
coalescing information as well.

The point why I'm insisting on a broader solution is that both sources
for lost ticks (iothread and vcpu) end up in the same output: an
adjustment of the injection frequency of the affected timer device.
There is not "HPET" or "RTC" or "PIT" in this, all this may apply to the
SoC timer of some emulated ARM board as well.

Jan

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