Java 7: Fork/Join Framework

Question

Can someone explain what Fork/Join is?

Answer 1

I will answer what is Fork Join parallelism. This is one of the parallel design pattern widely used in many systems to achieve concurrency. I will explain this design pattern using a example.

For example lets say we have program which executes sequence of tasks :

A -> B -> C -> D. Here A,B,C,D are tasks.

• A takes 8 seconds
• B takes 4 seconds
• C takes 6 seconds
• D takes 7 seconds

So it will take total of 8+4+6+7 = 25 seconds for this program execution.

Now you found out that tasks A,B,C are independent and D is depend on A,B,C tasks' results. Now you may get a feeling that instead of waiting for A to finish we can start the execution of B simultaneously. Same for Task C can start the task simultaneously with A and B. What you can do is: You can invoke 3 new threads by your main thread and assign them A,B,C tasks and wait for the results before start the execution of task D. If your machine has multiple cores then these threads can run in parallel.

Now the execution time taken for the program is :

max(time_taken_A,_B,_C) + time_taken_D + threading_overhead_time

which is almost equal to = 8 + 7 + k = 15 + k;

In fork join parallelism we can offload tasks with a new thread only if these tasks are independent. Other wise we will face race conditions. If you have a program where one task is waiting for another task execution but this is not dependent on its results then you can offload these two tasks with new threads using fork join parallelism and you can get performance boost. But always think about the threading over head. If your tasks are very light weighted then using these parallel patterns will decrease your performance because of the thread creation, context switching overheads.