Here is a simple patch that may help to improve the
performance of MapReduce when shutting down a thread pool.
Rather than releasing a new version of PS5, we are just
providing this patch--you should make this modification
in your code directly.

In the function destroy: pool -> unit in the file
shared/thread_pool.ml, insert the line

    Queue.clear q;

between the two lines

    Mutex.lock m;
    let n = !nworkers in