Suspending semaphores

Hi,

I'm pretty new to threading in webapplications.
I'd like to have multiple named system semaphores; like a semaphore that allows 2 concurrent threads for I/O (heavy cpu usage) and a semaphore that allows 100 concurrent threads for calculations.
Is it possible to suspend all the threads in the I/O semaphore when I call the calculations semaphore? Can it be done on the level of the semaphore instead of the level of the thread?

thnx for all responses,

 Sen
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