Recursion on ExecutorService with >1 Thread is Slow

[benjchristensen]

Recursion on NewThreadScheduler or ExecutorScheduler with 1 thread is fast. As number of threads increase in the pool it increasingly slows down. My guess is it's adding overhead jumping between threads and/or CPUs and thus having cache misses or something along those lines.

I found this while testing TestRecursionMemoryUsage and trying Schedulers.threadPoolForComputation() versus Schedulers.newThread().

I confirmed that this changes the performance: Executors.newScheduledThreadPool(2) vs Executors.newScheduledThreadPool(1)

The real code uses numCores for that value which is worse.

We need to look at a way of using a thread-pool sized to the number of threads on the machine so we're not launching new threads every time a scheduler is used, but that recursion efficiently uses the same thread.

[benjchristensen]

Removed ExecutorScheduler and replaced with ComputationScheduler that has a pool of event loops: #1048