[osg-users] Draw threads serialized by default?

Todd J. Furlong todd at inv3rsion.com
Mon Sep 1 08:22:53 PDT 2008


Robert,

The post of yours that Paul linked to sounds very similar to something 
we saw with VR Juggler & OSG a while back: terrible performance with OSG 
apps that had parallel draw threads.  In our case, VR Juggler manages 
threading, but the same may apply to OSG with osgViewer.

For us, it turned out that we had to set the threads' affinity to lock 
them to a particular CPU/core.  The Linux scheduler moved the threads 
around and thrashed the cache, I believe.  Setting the affinity boosted 
the parallel draw performance back up.

The solution we ended up with is twofold:
1. Add a default behavior that sequentially locks draw threads to CPU 
cores (0,1,2,etc. & repeat)
2. Use an environment variable to override the default behavior 
(VJ_DRAW_THREAD_AFFINITY, a space-delimited list of integers).

The default behavior is good for most users, but we can squeeze out a 
little more performance by tweaking the environment variable.  For a 
system with two draw threads and two dual-core CPUs, the default 
behavior locks the draw threads to CPUs 0 & 1, but we get slightly 
better performance if we set VJ_DRAW_THREAD_AFFINITY="2 3".

Regards,
Todd

Robert Osfield wrote:
> Hi Paul,
> 
> On Sat, Aug 30, 2008 at 10:19 PM, Paul Martz <pmartz at skew-matrix.com> wrote:
>> Hi Robert -- Prior to the 2.2 release, code was added to serialize the draw
>> dispatch. Is there a reason that this behavior defaults to ON? (See
>> DisplaySettings.cpp line 135.) I have somehow incorrectly documented this as
>> defaulting to OFF in the ref man. Now that I see it's ON by default, I half
>> wonder if this is a bug. Wanted to check with you: should I change the
>> documentation, or the code? Which is right?
> 
> The settings has been ON since I introduced the option to serialize
> the draw dispatch.
> 
> Just before the 2.6 release I did testing at my end and still found
> serializing the draw dispatch to be far more effiecent on my
> Linux/NVidia drivers so I left the option on.
> 
> In the original thread when I introduced the optional draw mutex into
> the draw dispatch I did call for testing on the performance impact but
> I didn't get sufficient feedback to make a more informed decision than
> just basing it on my own testing.  I would still appreciate more
> testing, as I'd expect that best default setting to vary on different
> hardware and drivers - I for one would love to see better scalability
> in driver/hardware.
> 
> Robert.
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