ExecutorService Future::get very slow

Question

I\'m parallelizing a quite complex program to get it faster. For this I use most of the time the ExecutorService. Until now it worked pretty well, but then I notice

Answer 1

That is to be expected. It isn't "slower" then; it is just doing its job.

From the javadoc for get():

Waits if necessary for the computation to complete, and then retrieves its result.

Long story short: it seems that you do not understand the concepts you are using in your code. The idea of a Future is that it does things at some point in the future.

And by calling get() you express: I don't mind waiting now until the results of that computation "behind" that Future become available.

Thus: you have to step back and look into your code again; to understand how your different "threads of activity" really work; and how/when they come back together.

One idea that comes to mind: right now, you you are creating your Future objects in a loop; and directly after you created the Future, you call get() on it. That completely contradicts the idea of creating multiple Futures. In other words: instead of going:

foreach X
  create future X.i
  wait/get future X.i

you could do something like

foreach X
  create future X.i

foreach X
  wait/get for future X.i

In other words: allow your futures to really do things in parallel; instead of enforcing sequential processing.

If that doesn't help "enough", then as said: you have to look at your overall design, and determine if there are ways to further "pull apart" things. Right now all activity happens "closely" together; and surprise: when you do a lot of work at the same time, that takes time. But as you might guess: such a re-design could be a lot of work; and is close to impossible without knowing more about your problem/code base.

A more sophisticated approach would be that you write code where some each Future has a way of expressing "I am done" - then you would "only" start all Futures; and wait until the last one comes back. But as said; I can't design a full solution for you here.

The other really important take-away here: don't just blindly use some code that "happens" to work. One essence of programming is to understand each and any concept used in your source code. You should have a pretty good idea what your code is doing before running it and finding "oh, that get() makes things slow".