c++ thread division into microprocessor

I have a question... I need to build an app multi-thread and my question is: if I have a 2cpu processor, is automatically that my 2 threads are separately one by processor? and if I have 4 threads and my pc have 4cpu, are again 1 per processor? and if I have 4 processor and 2 cpus, how is divided??

thanks in adv开发者_StackOverflowance

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This is not really a question which can be answered unless you specify the operating system at a minimum.

C++ itself knows nothing of threads, they are a service provided by the OS to the execution environment, and depend on that OS for its implementation.

As a general observation, I'm pretty certain that Linux schedules threads independently so that multiple threads can be spread across different CPUs and/or cores. I suspect Windows would do the same.

Some OS' will allow you to specify thread affinity, the ability for threads (and sometimes groups of threads) to stick with a single CPU but, again, that's an OS issue rather than a C++ one.

For Windows (as per your comment), you may want to read this introduction. Windows provides a SetProcessAffinityMask() function for controlling affinity of all threads in a given process or SetThreadAffinityMask() for controlling threads independently.

But, usually, you'll find it's best to leave these alone and let the OS sort it out - unless you have a specific need for different behaviour, the OS will almost certainly make the right decisions.

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How threads get allocated to processors is specific to the OS your application is running on. Typically most OS's don't make any guarantees about how your threads are split across the processors, although some do have some low level APIs to allow you to specify thread affinity.

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If your threads are CPU bound, then they will certainly tend to be scheduled on all available CPUs.

If your threads are IO bound, then if you only have one thread per CPU, most of the CPUs will be sitting idle. This is why - when attempting to maximize performace - it is important to measure what is happening and either find a hard coded ratio of threads per CPU, or use the operating systems thread pooling mechanism which has access to enough information to keep exactly as many threads active as there are CPU cores.

You generally dont want to have MORE active threads that CPUs (i.e. threads that arn't blocked waiting for IO to complete) as the act of switching between active threads on a CPU does incur small cost that can add up.