Re: [Qemu-devel] Re: [PATCH 0/4] Rework alarm timer infrastrucure - take2

Dor Laor
2007-08-19T14:27:40Z

>>> Yes, good thinking, but this should only be done if it actually
>impacts
>>> something. Reducing overhead from 0.1% to 0.05% is not worthwhile
if
>it
>>> introduces extra complexity.
>>>
>>
>>
>> If the overhead is that small, why are we touching this code in the
>first
>> place?
>>
>
>Accuracy is much more important from my point of view. Also, the
>reduction in the number of signals delivered when the guest uses 100Hz
>is significant.
>

Actually the main motivation for dyn-tick in qemu was to enable smooth
fix of
time drift problem in the guests. On kernels without dyn-tick enabled
sigalarm
didn't reach qemu on accurate times, thus caused convergence of several
pit irqs
into one.

Currently we use a time-drift-fix written by Uri Lublin which counts the
number
of irq acked by the guest and tries to inject another one immidietly
until the drift
is fixed.

This worked fine but caused jumps for multimedia applications.
We wanted a way to smooth it by increasing the rate of pit timer in case
of drift
to a frequency higher than 1000HZ until the drift is fixed.

Dan K. tries to test application behviour with it.
-- Dor.