[osg-submissions] Use atomic reference counts

[Mathias Fröhlich]

Robert,

On Thursday 19 June 2008 17:40, Robert Osfield wrote:
> I've have just done a trial integration of OpenThreads::Atomic into
> osg::Referenced using your changes, and performance results are pretty
> encouraging under Linux/Intel quad core - I've seen 2.8 to 3.8%
> improvement in overall framerate on a large town model I have, this
> for the whole frame so means that the actual improvement during
> cull/draw is better than this percentage.
:)
Yep, that atomic stuff is just worth it ...

> I won't commit the changes I've just merged though, as I recall from
> my first pass review that the solution for the observers wasn't ideal,
> so I'll go back and review these details and look for a better
> solution right now.
Well, we can discuss about that. Such a single lock per observer will be save 
also.

But what is there should be thread safe. That is the typical RCU approach we 
can see in lockless algorithms. This is also what will just happen in some 
sense in most mutex implementations.

The key is that the pointer value needs to be changed atomically. If we do not 
have the observer pointer there we allocate a new observer data structure and 
try to write that to the referenced. If an other thread did the same at the 
same time, we will fail on that atomic compare and swap operation on that 
pointer. In this case we just delete the yet unused observer pointer and use 
the other one that is now there. If our thread is successful to write that 
pointer we use our new data for the observers.

The observer insert/remove is then guarded by the mutex in the observer data 
stuff. So this is safe too.

The 'single observer lock' approach will serialize the observer modification 
globaly. While I believe that this is not a big problem in *almost* all 
cases. I think that we should prefere a completely paralell implementation if 
this one is that easy to have.

Greetings and thanks

Mathias

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