[GEM-dev] Problem with glitching on OS X at high frame rates

[Theo Burt]

Hi, me and a friend of mine do a lot of live audio-video performance, 
professionally, all using pd/gem as an environment. We both use Macbook 
Pros (different models), and for the last couple of years we've been 
having a serious problem which we now suspect is something to do with 
GEM/PD, rather than, for example, the graphics cards or drivers.

The problem occurs when running at higher frame rates - it is highly 
preferable for us to run at 60 frames per second, because (1) it syncs 
with the refresh rates of most projectors and seems to produce much 
smoother/more regular movement, and (2) often we are using very precise, 
high frequency flashing, for optical effects, that we need to be able to 
sync exactly with screen refreshes. We are generally running the gem 
window in a second screen (such as a monitor or projector), but it 
happens when running on one screen too.

As soon as we run at 60 frames per second (or near it), glitching starts 
to occur frequently in the audio, and the audio goes out of sync with 
GEM. While the glitching is occurring, processes in PD and GEM jerk 
along, pausing frequently. To resolve it (temporarily), the audio has to 
be reset in PD (by reselecting the audio device). The glitching then 
goes away and everything runs smoothly, but it returns spontaneously 
after a short time. The glitching is also exacerbated by moving windows 
around on the screen. It might be worth noting that the same patch 
running on the very same machine in Windows XP (using bootcamp) does not 
have this problem.

We have different GPUs, mine is a Nvidia 9600m GT, and his was an ATI of 
some sort. We are using very simple, geometric graphics, and are not 
approaching the processing limits of the GPUs at all (if I do push the 
graphics card very hard, I notice that frames are skipped rather than 
glitching occurring). I also note that it happens equally when there is 
low CPU activity, so it's not related to that.

My friend has just purchased a new Macbook Pro, yesterday with a 60% 
more powerful Nvidia GPU, and unfortunately the glitching is still 
occurring. It occurs in OS X Lion, and also was happening in Snow 
Leopard. We've tried disabling power management on the GPU, and this 
doesn't help either.

I've looked through the gem code in an attempt to try and understand 
what is happening, but I am afraid it is beyond me!

Does anyone have any idea at all what the problem could be? It would 
make such a huge difference to us if we could resolve it. Thanks very 
much for any help in advance (and for developing GEM in the first place)

All the best, Theo