[Paracomp-devel] Accessing compositor from multiple threads

[George L. <geo...@ma...>]

Hi,

   Is it safe to make calls to the compositor from multiple threads?

The scenario is to have two threads: one for rendering, one for  
display (if a display node):

Thread 1 (render). All nodes have this thread.
----------------------------------------------
pcSystemInitialize()
pcSessionCreate()
pcContextCreateMaster() (or pcContextCreate())
while(1) {
   wait for render request
   do_opengl_render();
   if ( visible_framelet ) {
     pcFrameBegin( one framelet x,y,w,h );
     pcFrameAddFramelet( cbuf, zbuf );
     pcFrameEnd();
   }
   else {
     pcFrameBegin( no framelets );
     pcFrameEnd();
   }
   if ( master )
     request display (release thread 0)
}

Thread 0 (display). Only master node has this thread.
-----------------------------------------------------
while(1) {
    build scene graph
    request render (release thread 1)
    block until display requested
    glClear();
    pcFrameResultChannel( ... &channelOut );
    glDrawPixels( ..., channelOut.address );
}

The display thread can also use the pcFrameWaitOutputHP() extension to  
loop over framelets.

The problem we are seeing is that the compositor is freeing invalid  
memory blocks in core.cpp: Core::StartFrame() line 290:
   if (*iter2)
     free(*iter2)

This doesn't happen in the paracomp samples but they make all calls  
from the same main thread. I know the above thread scheme is a little  
contrived but we require a render/display thread model. Is it safe to  
make pcFrameResultChannel calls from a thread that isn't doing the  
pcFrameBegin/End() calls?

Thanks in advance
George

-- 
George Leaver
Research Computing Services
University of Manchester, UK.
http://www.rcs.manchester.ac.uk