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Interesting catch-up behaviour with GL multi-threading

Subject: Interesting catch-up behaviour with GL multi-threading
From: Christopher Hunt <email@hidden>
Date: Mon, 28 May 2007 20:29:11 +1000
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Having finally built my application for Intel (what took you, I hear you say), I find that enabling Mac OS X's gl multi-threading capability causes an interesting side effect.

Firstly I don't think my application is particularly CPU bound when it comes to GL anyhow so I'm not sure that there's a performance gain to be had with GL multi-threading. Regardless, I enabled it and my application's CPU based functions are definitely sped up given the multi-threading mode - however I now see a lag between my app's processing and what gets rendered. For example, I have a globe with the earth textured on to it. The earth can be dragged around using the mouse as you'd expect. What I find is that the earth continues to spin a bit after I've stopped dragging. If I disable the multi- threading mode of GL then my rendering is synchronised with my mouse dragging and there is continued spinning.

Interesting?

I'm double-buffering and flushing via my context's flushBuffer method to sync with vertical retrace.

Perhaps the GL multi-threaded mode is not for my app given its present efficiency but I thought I'd share this observation in case there's something I'm missing i.e. perhaps I should using multi- threading GL but somehow synchronise with the GL thread when I'm flushing my context...

Ideas? Comments?

BTW: I'm using a 2.66Ghz Quad Xeon so there's some real parallelism happening. :-)

Cheers,
-C
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