How to get an ideal number of threads in parallel programs in Java?
I need to get an ideal number of threads in a batch program, which runs in batch framework supporting parallel mode, like parallel step in Spring Batch.
As far as I know, it is not good that there are too many threads to execute steps of a program, it may has negative effect to the performance of the program. Some factors could arise performance degradation(context switching, race condition when using shared resources(locking, sync..) ... (are there any other factors?)).
Of course the best way of getting the ideal number of threads is for me to have actual program tests adjusting the number of threads of the program. But in my situation, it is not that easy to have the actual test because many things are needed for the tests(persons, test scheduling, test data, etc..), which are too difficult for me to prepare now. So, before getting the actual tests, I want to know the way of getting a guessable ideal number of threads of my program, as best as I can. What should I consider to get the ideal number of threads(steps) of my program?? number of CPU cores?? number of processes on a machine on which my program would run?? number of database connection?? Is there a rational way such as a formula in a situation like this?
Answer
The most important consideration is whether your application/calculation is CPU-bound or IO-bound.
- If it's IO-bound (a single thread is spending most of its time waiting for external esources such as database connections, file systems, or other external sources of data) then you can assign (many) more threads than the number of available processors - of course how many depends also on how well the external resource scales though - local file systems, not that much probably.
- If it's (mostly) CPU bound, then slightly over the number of available processors is probably best.