Re: QEMU VNC Audio Patch

["Daniel P. Berrange" <dan berrange com>]

On Fri, Dec 09, 2011 at 10:30:28AM -0500, Steven Carr wrote:
> On Friday 09 December 2011 04:10:49 you wrote:
> 
> > What has been your experiance with the overall quality / performance
> > of the audio coming out of VNC ?
> >
> 
> I started using a modified TightVNC Java Client from 
> http://www.paulsd.com/2009/VNC/   The client kept crashing and had significant 
> audio glitches.   And on top of things, it was a bit awkward to use.   I 
> hacked pulseaudio into the src.rpm for the SL61 gtk-vnc library so that I 
> could add audio output to the virt-manager without having to touch that code 
> as well (KISS principle).   Using this modified rpm, I am able to watch youtube 
> videos, full screen in the VM (1600x1200) and have occasional audio issues.   
> Maybe 1 detectable hiccup a minute.
> 
> Now, my host is a 12-core i7 3.3GHz (6681 BogoMips) on a P6T6 WS Revolution 
> Motherboard with 24GB of ram, running Scientific Linux 6.1 x86-64.   The audio 
> device is an 82801JI HD Audio Controller.   The video device is a six output 
> AMD Radeon 6900 Series using fglrx drivers.   It is a pretty beefy system and 
> I would not consider it typical.
> 
> The Guest OS is WinXP in a QEMU/KVM Virtual Machine (the default VM system on 
> SL61), setup to use the emulated AC97 audio device model (I am not sure if 
> AC97 is a better virtual device).   In the WinXP system, MicroSoft has 
> graciously provided Beethoven's Symphony No. 9 for me to test against.   I 
> play that with Windows Media Player.   qemu-kvm uses 22% of one of my cores 
> and pulseaudio uses 2.6%.   No detectable audio skipping or jumping unless I 
> am running the windows media player at full screen on one of my six displays 
> and displaying some sort of visualization.   I was able to watch non-full 
> screen youtube videos with the occasional audio drop out.

Ok, that is very interesting / useful. I was testing with a Fedora 14
guest, and I hears glitches every couple of seconds, particularly if
any other data is arriving on the vnc socket eg due to screen refresh.

It would not surprise me if there was a significant difference in
behaviour between Windows & Fedora for sound output. So I should
probably try out a WIndows guest too and see how things go.

> > I've tested by playing a FLAC file in the guest, and the audio is
> > slightly crackly. In addition the CPU usage of gvncviewer and
> > pulseaudio on the host is absurdly high - ~40-40%. By comparison,
> > rhymbox playing the same FLAC file in the host, consumes ~0% CPU
> > and causes ~0% cpu usage from pulseaudio. I've no looked into what's
> > going on here & suspect it might be just an artifact of the small
> > buffers QEMU sends, but wondered if you experianced any similar
> > behaviour in your testing ?
> 
>    The previous patch was better in the skipping aspect, where the audio data 
> was dispatched to pulseaudio directly from the event loop, and not redirected 
> back via the GObject interface.   I am not entirely sure, why.   When I 
> slapped in some logging I would see batches of messages come in, then batches 
> being sent to pulse audio.   Are the Audio messages being batched and then 
> processed in some sort of idle loop?  (co-routines?)

That's a useful  data point. The co-routine will basically consume as
much data as possible from the VNC source before returning to the main
loop - eg until it gets EAGAIN on the file descriptor. If there are 
large framebuffer updates coming in, this could be introducing significant
delays between receiving the audio & sending it onto pulseaudio. So it is
possible that we should artifically return to the main loop, even if we
have not got EAGAIN, just to make sure we don't delay audio.

I also noticed that the Fedora guest at least was sending audio in very
small packets - as little as 1 sample in size, but at most 10 samples.
This is what was causing the crazy CPU usage. I tried altering the code
so that it receives 100ms of audio before sending it onto pulseaudio,
and that dropped the CPU usage to acceptable levels. I need to investigate
the audio glitching though, before I commit that.

>    In my case, I am not trying to play video games or watch movies in my VM.   
> I am really looking for 1/3 second audio alerts from pidgin so I don't have to 
> keep checking to see if any of my co-workers has sent me a message.   But I 
> can see people trying to run some sort of full audio/video multimedia 
> application on a Pentium 3 and complaining about it.

Yeah, even your minimal use case is a good step forward. If people want
to watch full screen video inside a VM they really ought to just use
SPICE isntead, since that does some very clever stuff to optimize video
playback. Thanks for your  patches. I make ask you to test again once
I've done some more coding to check it still works well for you.

Regards,
Daniel
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