DN> blocked on the resource
RC> and how many will be blocked on other things.  In one of my
RC> programs which typically has about 10 threads I use critical
RC> sections because I believe that it would waste CPU time if all
RC> threads used semaphores when accessing certain data structures.  All
RC> I have to do is to use DosEnterCritSect() whenever I'm doing
RC> something that requires exclusive access to the data and I don't
RC> need to worry about semaphores in the other threads.

 DN> The overheads of calling DosEnterCritSec() and DosExitCritSec() will
 DN> be slightly higher than DosRequestMutexSem() and DosReleaseMutexSem()
 DN> because of all the dispatcher checking involved to determine currently
 DN> ready threads. A mutex is a simple lock word with a queue.

   That may be true IF I was to use the same number of system calls.

 DN> Since you need to make one API call prior to using a resource and
 DN> another after you're done with it, you end up making the same number
 DN> of API calls, but you've chosen the more expensive API. Your active
 DN> thread will be slower than if you used mutexes and it will block other
 DN> threads unnecessarily.

   Yes.  However when using semaphores you have to request them before
using the resource and release them afterwards.  If a resource is being
read much more often than it's being written then the use of critical
sections in the code to write to the resource can dramatically decrease the
number of system calls.  If the program performs 1/10th the number of
system calls and each system call takes twice as long then it will perform
those operations 5 times faster.


  Russell Coker


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