Testing approach for multi-threaded software

Question

I have a piece of mature geospatial software that has recently had areas rewritten to take better advantage of the multiple processors available in modern PCs. Specifically

Answer 1

Whilst I agree with @rstevens answer in that there's currently no way to unit test threading issues with 100% certainty there are some things that I've found useful.

Firstly whatever tests you have make sure you run them on lots of different spec boxes. I have several build machines, all different, multi-core, single core, fast, slow, etc. The good thing about how diverse they are is that different ones will throw up different threading issues. I've regularly been surprised to add a new build machine to my farm and suddenly have a new threading bug exposed; and I'm talking about a new bug being exposed in code that has run 10000s of times on the other build machines and which shows up 1 in 10 on the new one...

Secondly most of the unit testing that you do on your code needn't involve threading at all. The threading is, generally, orthogonal. So step one is to tease the code apart so that you can test the actual code that does the work without worrying too much about the threaded nature. This usually means creating an interface that the threading code uses to drive the real code. You can then test the real code in isolation.

Thridly you can test where the threaded code interacts with the main body of code. This means writing a mock for the interface that you developed to separate the two blocks of code. By now the threading code is likely much simpler and you can then often place synchronisation objects in the mock that you've made so that you can control the code under test. So, you'd spin up your thread and wait for it to set an event by calling into your mock and then have it block on another event which your test code controls. The test code can then step the threaded code from one point in your interface to the next.

Finally (if you've decoupled things enough that you can do the earlier stuff then this is easy) you can then run larger pieces of the multi-threaded parts of the app under test and make sure you get the results that you expect; you can play with the priority of the threads and maybe even add a couple of test threads that simply eat CPU to stir things up a bit.

Now you run all of these tests many many times on different hardware...

I've also found that running the tests (or the app) under something like DevPartner BoundsChecker can help a lot as it messes with the thread scheduling such that it sometimes shakes out hard to find bugs. I also wrote a deadlock detection tool which checks for lock inversions during program execution but I only use that rarely.

You can see an example of how I test multi-threaded C++ code here: http://www.lenholgate.com/blog/2004/05/practical-testing.html