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

How expensive the simplest thread is depends on the OS (you may also need to tune some OS parameters to get past a certain number of threads). At minimum each has its own CPU state (registers/flags incl. floating point) and stack as well as any thread-specific heap storage.

If each individual thread doesn't need too much distinct state, then you can probably get them pretty cheap by using a small stack size.

In the limit, you may end up needing to use a non-OS cooperative threading mechanism, or even multiplex events yourself using tiny "execution context' objects.

Just start with threads and worry about it later :)