How to control locking about aparapi HOT 6 CLOSED

You have highlighted a problem that I think we need to solve with Aparapi to 
help with map reduce type issues.

One trick (you may have tried this) is to use the pass # (from 
kernel.execute(range,passes) ) to indicate that the final pass should perform 
the reduce step.  This way the data will be kept on the GPU and not shuffled 
back and forth. 

So with a Kernel like 

k = new Kernel(){
  public void run (){
   if (getPassId()<19){
     // do first 19 (0..18) passes 
   }else{
     // do final reduction pass (19)
   }
  }
} 

k.execute(1024, 20);

The real solution (and I am looking at this now) is some way to allow a Kernel 
to have multiple entrypoints.  I have yet to come to a clean syntax for this, 
so would welcome suggestions.

I thought of that, but would that not cause every core to execute both branches 
for each pass (without storing the results)? Or is there a special case that 
happens when all cores take the same branch?

Sorry Kenneth for some reason I missed your question. 

Because we use global memory (as far as GPU is concerned) we must wait for all 
Kernels to complete.  Conceptually they are all running at the same time and we 
can never expect one Kernel to see the result of another.  Even if we know the 
group order.  So I think this will always require us to relaunch the kernel. 

Relaunch is not *that* expensive, especially if we can avoid moving buffers by 
setting setExplicit(true) and taking buffer transfer control ourself. 

I just added a 'life' demo (Conways game of life) which executes a Kernel 
inside a fairly tight loop and which only pulls the buffer from the GOU when 
Swing wishes to display the data. This might help you in your case as well. 

Unless I have completely missed the point ;) 

Gary

I discovered the "localBarrier()" function that seems to do what I was looking 
for (synchronize).
I did try the other suggestion (multi-pass) and that seemed to have a heavy 
performance penalty, but I abandoned the idea, so not sure what caused it.

Feel free to close this issue.