[mythtv-users] Fixed!: Nvidia Sync to Vblank pegging cpu in X

Jim herold jim.herold at comcast.net
Wed Dec 20 02:11:51 UTC 2006

On 12/17/06, J. Miller <mythtv.org at elvenhome.net> wrote:

> >
> > I've been fighting the scourge of myth causing X to consume 100% of the
> > CPU when Xv Sync to Vblank is turned on in nvidia-settings.  Everything
> > works just fine with it unchecked but that is the only sync setting that
> > gives me a perfect picture with no tearing.  Myth's native OpenGL sync
> > still tears a little.  The problem turned out to be the modeline I was
> > using.  It was a 1368x768 56Hz modeline which apparently triggered the
> > issue.  I switched to a 1360x768 60Hz modeline and now everything works
> > peachy.  X still starts consuming large amounts of CPU occasionally but
> > it quickly drops back down to the 2-4% range.  I'm not 100% sure that it
> > was the refresh but a modeline change did the trick.  Hopefully posting
> > this here will help point others that have the same problem in a new
> > direction.
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> >

> NVIDIA have added an option in (I think) 9626 to poll() instead of 
> for vblank, which fixes the high cpu usage when sync to vblank is switched
> on. It's called "UseEvents".

My Modeline was already 1280x720 @ 60Hz.  I've tried tweaking it a number of ways but never came up with a magic modeline that fixed the issue here.

Never thought to try the sync to vblank setting (thanks also to Steven Adeff for this suggestion).  I knew it was recommended to disable that for XvMC, but never connected that it'd be a good idea for Xv as well.  With the sync to vblank off everything looks great on my setup, and the X cpu usage hovers around 10% when watching 720p content.

I tried the "UseEvents" thing.  Interestingly, it works, but increases the "wa" cpu usage in top, and therefore leaves less cpu idle time by roughly 20% when compared with just disabling the sync to vblank.

Thanks all for the help!

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