Re: [Qemu-stable] [Qemu-devel] [PATCH 1/2] audio: honor QEMU_AUDIO_TIMER_PERIOD instead of waking up every *nano* second

[Hans de Goede]

Hi,

On 9 Oct 2013, at 20:42, Hans de Goede wrote:
> Now that we no longer have MIN_REARM_TIMER_NS a bug in the audio subsys has
> clearly shown it self by trying to make a timer fire every nano second.
>
> Note we have a similar problem in 1.6, 1.5 and older but there
> MIN_REARM_TIMER_NS limits the wakeups caused by audio being active to
> 4000 times / second. This still causes a host cpu load of 50 % for simply
> playing audio, where as with this patch git master is at 13%, so we should
> backport this to 1.5 and 1.6 too.

I'm still not sure when this actually started happening, but looking at
RHEL-6 qemu sources to see if that has the issue too, I've learned how
this problem was introduced, the audio_timer callback used to do this:

    qemu_mod_timer (s->ts, qemu_get_clock (vm_clock) + conf.period.ticks);

instead of calling audio_reset_timer(), so in the past there were 2 mod_timer
calls, one from audio_reset_timer(), which scheduled the callback to run
ASAP, and one from the audio_timer callback honering conf.period.hertz.

Then at some point the qemu_mod_timer call in audio_timer was replaced
with calling audio_reset_timer() and we got the problem my patch fixes.

Regards,

Hans