Do you know of some performances test of the different ways to get thread local storage in C++?

Question

I'm doing a library that makes extensive use of a thread local variable. Can you point to some benchmarks that test the performances of the different ways to get thread local variables in C++:

• C++0x thread_local variables
• compiler extension (Gcc __thread, ...)
• boost::threads_specific_ptr
• pthread
• Windows
• ...

Does C++0x thread_local performs much better on the compilers providing it?

Answer 1

These are typically implemented as a simple offset in an array in the thread's private memory space. So, accessing the thread specific variable X, of type T,

T y = X;

roughly translates to,

T y = *(T*)(cur_thread.local_tbl[key_X]);

which is too simple to expect a wide variation in performance between implementations. That said, if you find any such benchmarks, please follow up here.

Answer 2

You can always use time.h. Its your friend when testing performance stuff and nothing else is available.