hi, 

in our testplans, we needed a sort of "semaphore" mechanism, e.g.: 
- we have problems if more than 10 parallel logins to our webapp exists
- we have a lot of threads that create, rename and delete channels. one thread 
can delete a channel that another thread tries to rename.

these are only 2 examples, but since we implemented this controller, we use it 
very often.

for this improvement, JMeterUtils.java has methods for acquiring and releasing 
semaphores. StandardJMeterEngine.java was patched to clear all semaphores when 
starting a test (if someone stops the test and there exists a sempaphore that 
was not released). Moreover, JMeterThread.java was patched, that all semaphores 
in a threadgroup are released if the scheduler stops the test (before releasing 
the semaphore with a sampler).

acquiring and releasing a semaphore can be either done by calling the 2 methods 
acquireSemaphore and releaseSemaphore, or by using the 2 samplers that come 
with the patch (Java Request). If you are interested in this Feature, i would 
implement a SemaphoreSampler with a nice GUI :)

The sampler works like this: 
With the "AcquireSemaphore" sampler, you can define the begin of the critical 
region by calling the acquireSemaphore-function. It acquires the given number 
of permits (count-parameter) from this semaphore, blocking until all are 
available. When leaving the critical region, use the ReleaseSemaphore sampler 
with the unique semaphore name as parameter. 
setting the "count" parameter to "1" would cause this sampler to work like 
a "mutex". 

Semaphores are supported since Java 1.5, so the build script has to be updated 
to build JMeter with Java Version 1.5.

greets,
  chris