parallele-programmierung/u08-1/README.md

Problem 8.1: Compare ForkJoinPool to ThreadPoolExecutor

According to the lecture, the two types of Java thread pools are optimized for different application scenarios:

• ForkJoinPool for task that are mutually dependent, created by other tasks, short, rarely blocking (i.e. computation-intensive)
• ThreadPoolExecutor for tasks that are mutually independent, submitted "from the outside", long, sometimes blocking

Can you give justifications for these five differences (many vs. few tasks, dependent vs. independent tasks, created by the application vs. created by other tasks, short vs. long tasks, rarely vs. frequently blocking tasks) when considering the different queuing behavior and also the join() operation of the ForkJoinTask? Formulate one sentence for each of the five differences, e.g. "ForkJoinPool is better suited to independent tasks because ...".

Answer:

ForkJoinPool is better suited for mutually dependent tasks because the join() allows for efficient execution of child tasks without blocking the thread

ThreadPoolExecutor is better suited for tasks which can block because blocking threads can make ForkJoinPools much more inefficient.

ThreadPoolExecutor is better suited for long running tasks, because join() executes another task while waiting for the current one to finish, therefore possibly waiting longer than strictly necessary.

ForkJoinPool is better suited for short running tasks because it uses seperate stacks for each thread, therefore avoiding synchronization overhead when taking new tasks.

Tasks usually create other tasks if they are dependent on that task, so since ForkJoinPool is better suited for mutually dependent tasks, it is also better suited for tasks that create other tasks.