Why thread priority rarely matters

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Overview

Its is tempting to use the Thread.setPriority() option in Java. However for many applications this is more a comment for the developer than something which will make a measurable difference. esp. with multi-core systems.

Why it usually doesn't matter

If you have plenty of free CPU, every thread which can run will run. The OS has no reason not to run a low priority thread or process when it has free resources.

If your system is close to 100% of CPU on every core, the OS has to make a choice as to how much time each thread or process gets on the CPU and it is likely to give favour to higher priority threads over lower priority threads, (many OSes ignore the hint) and other factors are likely to matter as well.

This priority only extends to raw CPU. Threads compete equally for CPU cache, heap space, CPU to memory bandwidth, file cache, disk IO, network IO and everything else. If any of these resource are in competition, they are all equal.

To set a high priority on Windows you need to be an administrator and on Linux you need to be root to set the priority of a thread. Different Implementations and OSes can ignore this hint.

Summary

If your application is heavily CPU bound, using every core, not using any other system resources significantly like IO or memory and your OS doesn't ignore the hint, the thread priority might make a difference.

If in doubt, I wouldn't bother setting it because someone might think it does something. ;)

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