[osg-users] Rendering performance issues

Bob Balfour bob at BAL4.com
Mon Jul 7 15:37:23 PDT 2008


I'm experiencing some performance peculiarities that I don't quite 
understand. My platform is an HP Blackbird, Vista OS, 4-CPU, 2 NVidia 
8800, OSG2.4.

I have attached 4 timing images:

The first image (vsyncOFF-1GPU.jpg) shows the timing rendering 1 slave 
camera on an extended (2880x900) dual-monitor on a single Nvidia card.  
Note the 211Hz rendering rate with VSync off and the substantial 
Draw/GPU overlap.  [Note that in all of these timing tests none of the 
CPUs appeared more than 50% loaded.]  This is the type of performance I 
was anticipating.

Now all I did was turn on Vsync.  The second image 
(vsyncON-app-long.jpg) now shows a 60Hz rendering rate as expected (with 
a delay after GPU waiting for VSync), but no Draw/GPU overlap, there's 
even a slight gap between the two?  Why is this different now?

Now I flew thru the scene to a point where the Draw became quite small. 
The third image (vsyncON-app-short.jpg)  shows the Cull,Draw,GPU all 
<1.0, but I'm only achieving a 46Hz rendering rate because of the 
considerable gap between Draw and GPU.  Why is the GPU now being delayed 
so much after Draw?? It's almost like the GPU is stuck starting out 
there mid-frame?

Now I turned VSync back OFF, but this time rendered TWO slave cameras 
(each 2880x900, one on each Nvidia card), and the same effect appeared 
(see the forth image vsyncOFF-2GPUs.jpg) where the GPUs are being 
delayed to mid-frame, and there is quite a delay after the GPUs complete 
(which should not be there with VSync OFF?).  Because of these 
substantial delays which I don't understand, performance has degraded 
considerably from my initial test image, which had good overlap and no 
delays.

Is there any explanation for this, and can the delays be eliminated and 
performance improved?

Also, is there a purpose/need for VSync using LCD flatpanel monitors or 
DLP projectors?

Thanks.

Bob.
-- 

Robert E. Balfour, Ph.D.
Exec. V.P. & CTO,  BALFOUR Technologies LLC
960 So. Broadway, Suite 108, Hicksville NY 11801
Phone: (516)513-0030  Fax: (516)513-0027  email: bob at BAL4.com
/"Solutions in four dimensions" with *fourDscape*®/

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