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Dear all
I parallelized a part of my application. From my point of view the problem is predestined to parallelize it: Simply N independent tasks.

Not parallelized I observed, that on a quad core my process will consume 25% cpu. My naïve assumption was that if I run e.g. three threads the quad core should consume something around 75%.

But it is unfortunately not like this, my application (after parallelization) still consumes only about 25% of cpu...and more... it takes much more time to execute all N jobs. I also checked the three threads and found that each thread processed about 1/3 of the jobs.

Do I need to create real parallel processes and not only threads?

Environment: Borland C++ Builder V6 ( :( ) .
Note: Jobs use a lot of STL containers for job's data. Maybe the problem?

Any ideas?

Thank you very much in advance.
Regards, Idle63
Updated 6-Oct-13 5:00am

1 solution

Its a common misconception that creating multiple threads will automatically improve the performance of a long running task. As long as you let the OS determine processor affinity there is no guarantee that the threads will run in parallel.

I'm sure that you know that a single core even running multiple threads only runs one thread at a time. Add on top of that the overhead of task switching, stack management, and memory space for the threads, they will actually take more time than just doing the task in a single thread.

A computer can only run as many threads in parallel as there are processor cores, and as long as you set the affinity for each thread to use a different core. The OS chooses the core based on which one is least loaded (I think), so if core 1 is the least busy, it may get all 4 cores threads without specifying affinity.
[no name] 5-Oct-13 11:16am
Ron, thank you very much for your answer

a.) As long as you let the OS determine processor affinity ...
Can I influence this? If yes how? N.B. My main application is not assigned to a specific cpu, means it can use any cpu (this information I got from task manager)

b.) Single core
I'm aware of this

c.) Affinity
Again, how can I influence this?

Thanks, regards, Bruno
Ron Beyer 5-Oct-13 11:23am
You can set the affinity using SetProcessorAffinityMask in Windows. You may have to dig around for a good reference on it and the arguments.
[no name] 5-Oct-13 11:51am
I already do this with Affinity. I mean all cpu's are available for the application...but my app do not use them...maybe also a thing that bcb6 is meanwhile antiquated :(
Thanks again for your time. Regards, Bruno
[no name] 7-Oct-13 11:06am
Once more I was searching the lazy way. After hours of debugging I had to recognize that the problem is -as usual- 40cm in front of the screen…it was me…now everything works fine and 7 threads against single thread is five time faster.

Thanks for your help, regards, idle63

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