java - ForkJoinPool seems to waste a thread

Question

I'm comparing two variations on a test program. Both are operating with a 4-thread ForkJoinPool on a machine with four cores.

In 'mode 1', I use the pool very much like an executor service. I toss a pile of tasks into ExecutorService.invokeAll. I get better performance than from an ordinary fixed thread executor service (even though there are calls to Lucene, that do some I/O, in there).

There is no divide-and-conquer here. Literally, I do

ExecutorService es = new ForkJoinPool(4);
es.invokeAll(collection_of_Callables);

In 'mode 2', I submit a single task to the pool, and in that task call ForkJoinTask.invokeAll to submit the subtasks. So, I have an object that inherits from RecursiveAction, and it is submitted to the pool. In the compute method of that class, I call the invokeAll on a collection of objects from a different class that also inherits from RecursiveAction. For testing purposes, I submit only one-at-a-time of the first objects. What I naively expected to see what all four threads busy, as the thread calling invokeAll would grab one of the subtasks for itself instead of just sitting and blocking. I can think of some reasons why it might not work that way.

Watching in VisualVM, in mode 2, one thread is pretty nearly always waiting. What I expect to see is the thread calling invokeAll immediately going to work on one of the invoked tasks rather than just sitting still. This is certainly better than the deadlocks that would result from trying this scheme with an ordinary thread pool, but still, what up? Is it holding one thread back in case something else gets submitted? And, if so, why not the same problem in mode 1?

So far I've been running this using the jsr166 jar added to java 1.6's boot class path.

Answer 1

ForkJoinTask.invokeAll is forking all tasks, but the first in the list. The first task it runs itself. Then it joins other tasks. It's thread is not released in any way to the pool. So you what you see, it it's thread blocking on other tasks to be complete.