Question

I’m unit testing a class for thread safety. I’m calling its methods from different threads, to make sure all of them receive the same result (in a nutshell). This is how I decide how many threads to use:

int threads = Runtime.getRuntime().availableProcessors() * 4;

Is it a good practice? How many should I use? The more the better?

Answer 1

I generally run 3 sets of tests:

• one thread to check correct behaviour in single threaded environment
• number of processors + 1 (or whatever the typical application setting is)
• many threads (say 1000) to increase context switching and contention

And for the multi threaded tests, I try to maximize interleaving, typically by synchronizing the beginning of the tasks with a CountDownLatch, so that all threads start their tasks more or less at the same time.

I also try to avoid additional synchronization during the tests (for example by using a thread safe structure to store some results) as it might “re-sycnchronize” the tested code artificially – to be assessed on a case by case basis.

JCiP, chapter 12, is a good source of inspiration for efficient multithreading testing (i.e. gives many hints to provoke concurrency bugs).

And finally, as noted by @PeterLawrey, you can’t guarantee that your code is thread safe with testing, you can only try to increase your chance of finding concurrency bugs.