[concurrency-interest] unit testing concurrency code.

Rick Beton richard.beton at roke.co.uk
Tue Sep 20 05:16:30 EDT 2005

Brian Goetz wrote:

 > Testing concurrent code is an extension of testing regular code.  
 > First, you must have good tests for functionality, and be able to test
 > as many of your classes invariants as possible.  The trick is then
 > trying to generate as many random interleavings of operations as you
 > can, without the test framework introducing timing artifacts that will
 > prevent certain interleavings from being tested.

Surely this overlooks something: there are four kinds of dynamic failure
(deadlock, livelock, starvation and race) that you cannot prove are
absent simply by testing alone.  You can prove they are *present* if
your testing finds them, but you can't prove they are *absent*.

This is quite important if you are writing concurrency classes for other
people to use.  You cannot write tests with enough coverage to handle
every end-user's use cases.  Simply covering a certain subset of dynamic
behaviour and then passing such classes as "tested" may be a bit too
optimistic and misleading.

I'd like to draw your attention to Peter Welch's posting ("CSP, the
pi-calculus and CPA-2005") and the work he and others have done using
CSP as the basis for establishing thread reliability (establishing the
absence of deadlock, livelock, starvation and race).  My own experience
is limited to being a JCSP user, rather than having much theoretical
skill.  There are some straightforward design rules that guarantee
deadlock freedom for example.  This makes JCSP a simple strategy to
apply to practical usage.



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