[zeromq-dev] Parallel sort

Oliver Smith oliver at kfs.org
Mon Aug 2 04:22:59 CEST 2010

So, I decided to try and figure out how to implement a parallel sort 
with ZeroMQ message passing.

At this point, I have more fundamental issues than choosing a good 
parallel sort algorithm so -- for now, forget that is the final goal.

Issue 1: The worker threads be created before any work is dispatched, so 
they can be re-used.
- Multiple threads attempting to bind() or connect() ZMQ_REP on an 
inproc:// socket fail with address in use or connection refused. (D'oh)

Issue 2: Ability to "join" pairs of work for divide and conquer.
- Yeah... I may just have been looking at the screen too long today, but 
I can't figure an elegant (and scalable) way to "join" two completed 

Approach 1 (I'm trying to do this in C++, but explaining in Python)

     def divideAndConquer1(work):
         if len(work) <= minsize:
             return directWork(work)
         splitAt = len(work) / 2
         dispatch(divideAndConquer1, work[:splitAt]) # Dispatch right 
         divideAndConquer1(work[:splitAt])    # Process left sub-unit myself

         ... join ...

         return work

Approach 2

     def divideAndConquer2(work):
         if len(work) <= minsize:
             return directWork(work)
         splitAt = len(work) / 2
         dispatch(divideAndConquer2, work[:splitAt])                # 
Dispatch both workloads...
         dispatch(divideAndConquer2, work[splitAt:])

         ... join ...

         return work

I also experimented with a third approach, where the top-level 
dispatching routine starts from the bottom up, i.e. dispatching all the 
"minsize" workloads for directWork, waiting for them all to complete, 
and then combining those work-units into pairs and dispatching these 
super-units to the level-above processing routine.

But I couldn't figure out how to have a single sender dispatch to a 
multiplexed in-socket capable of sending back a reply; I could use a 
pair of down/up stream sockets like I do in AsyncWorker, but that would 
mean the function could only be used from the master thread (i.e it 
would be thread-unsafe).

Appreciate any feedback or advice on any of the above methods, or if 
someone has actually implemented a pipelined parallel sort (bitonic or 
otherwise) that they don't mind sharing....

- Oliver

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