How many threads does it take to make them a bad choice?

Question

I have to write a not-so-large program in C++, using boost::thread.

The problem at hand, is to process a large (maybe thousands or tens of thousands. Hundreds and millon

Answer 1

According to Amdahl's law that was discussed by Herb Sutter in his article:

Some amount of a program's processing is fully "O(N)" parallelizable (call this portion p), and only that portion can scale directly on machines having more and more processor cores. The rest of the program's work is "O(1)" sequential (s). [1,2] Assuming perfect use of all available cores and no parallelization overhead, Amdahl's Law says that the best possible speedup of that program workload on a machine with N cores is given by

In your case I/O operations could take most of the time, as well as synchronization issues. You could count time that will be spend in blocking(?) slow I/O operations and approximately find number of threads that will be suitable for your task.

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Full list of concurrency related articles by Herb Sutter could be found here.