I thought I understood how MbUnit's parallel test execution worked, but the behaviour I'm seeing differs sufficiently much from my expectation that I suspect I'm missing something!

I have a set of UI tests that I wish to run concurrently. All of the tests are in the same assembly, split across three different namespaces. All of the tests are completely independent of one another, so I'd like all of them to be eligible for parallel execution.

To that end, I put the following in the AssemblyInfo.cs:

[assembly: DegreeOfParallelism(8)]

[assembly: Parallelizable(TestScope.All)]

My understanding was that this combination of assembly attributes should cause all of the tests to be considered [Parallelizable], and that the test runner should use 8 threads during execution. My individual tests are marked with the [Test] attribute, and nothing else. None of them are data-driven.

However, what I actually see is at most 5-6 threads being used, meaning that my test runs are taking longer than they should be.

Am I missing something? Do I need to do anything else to ensure that all of my 8 threads are being used by the runner?

N.B. The behaviour is the same irrespective of which runner I use. The GUI, command line and TD.Net runners all behave the same as described above, again leading me to think I've missed something.

EDIT: As pointed out in the comments, I'm running v3.1 of MbUnit (update 2 build 397). The documentation suggests that the assembly level [parallelizable] attribute is available, but it does also seem to reference v3.2 of the framework despite that not yet being available.

EDIT 2: To further clarify, the structure of my assembly is as follows:

assembly
- namespace
   - fixture
      - tests (each carrying only the [Test] attribute)
   - fixture
      - tests (each carrying only the [Test] attribute)
- namespace
   - fixture
      - tests (each carrying only the [Test] attribute)
   - fixture
      - tests (each carrying only the [Test] attribute)
- namespace
   - fixture
      - tests (each carrying only the [Test] attribute)
   - fixture
      - tests (each carrying only the [Test] attribute)