I’ve done quite a bit more research here, including testing just on my native display with a Simple Client where I know for sure that I’m using just my AMD card. I still get the issue.
I’ve managed to fire up OpenGL Profiler and attach it to our app to trace calls. It renders very fast normally, and only when I get a “grinding” type of effect I notice a 10x increase in the amount of time spent in CGLFlushDrawable, i.e. copying from the back buffer to the front. Based on my research, I wonder if this has to do with mixing single- and double-buffering?
From what I understand using glFlushRenderAPPLE is for single-buffering, and I don’t see a usage of fence, which means Unity -> Syphon server -> Syphon client would mean double -> single -> double buffering.
I found on one post:
Calling flushBuffer more than once within the refresh period should actually block your application (lots of time spent in CGLFlushDrawable appearing in a Shark profile).
If the rendering process it “out of phase” (as you call it in a Nov 2009 quartz-dev thread), maybe a flush is effectively being called too many times in a refresh period?