Re: [Plib-users] Threading

[Steve B. <sjb...@ai...>]

"Curtis L. Olson" wrote:

> > Those things are tricky and not 100% guaranteed - but for what you describe,
> > there shouldn't be a problem.
> 
> We have just begun to explore the idea of updating the scene graph
> tree in a seperate thread from the main render thread.  Clearly you
> must never do anything in the update thread that could ever trigger an
> opengl command (such as loading/registering a texture for example.)
> 
> However, other than that we have been surprised that it has worked
> pretty well and is very robust.

Bear in mind that the PROBABILITY of a failure when you have a
threading error can be very small - so it's crucial that you *know*
that you are solidly "safe-by-design" because testing won't show up
the problem in any reasonable amount of time and debuggers are
largely useless.

Particularly:

  * Don't add or remove entries from ssgBranch nodes
    during ssgCullAndDraw or the LOS/HAT routines.

For scenery paging (which I'm guessing is what Curt is doing here),
you can build up a scenery tile in your parallel thread (being careful
about the texture load thing) - BUT connect that into the scene
graph when the tile is fully loaded - and do it in the rendering
thread - so you *know* that it's definitely not doing anything
while it's making the connection.

I generally build a pair of FIFO's between the rendering and paging
threads, passing the addresses of completed terrain tiles in one
direction and in the other direction, send the addresses of tiles
that have already been disconnected from the scene graph (by the
renderer) and which need to be physically deleted by the pager
to save time.

> Do know that threads are not necessarily nirvana, and unless you
> really know what you are doing and exactly what the threading will
> accomplish for you in advance, I'd recommend staying away from them as
> long as you can.

Yes. Without question.

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