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 muc

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.