Re: Futexes ain't fast

On 8/28/2024 5:09 AM, Bonita Montero wrote:

I tested the operating-system specific mutex (CRITICAL_SECTION Or
pthread_mutext_t) against a futex and a std::mutex. I guessed std::mutex
uses th operating system specific mutex internally, but the times varied
so much across Windows and Linux that I gues that std::mutex used at
least a differently parametrized operating system mutex or maybe even
completely own code.
This are the times and each line has a further contender:

[...]

             while( futex.exchange( true, memory_order_acquire ) )
                 futex.wait( true, memory_order_relaxed );
             futex.exchange( false, memory_order_release );
             futex.notify_one();
         } );

[...]
 A wait bit would help out here... ;^) Afaict, this is a rather "naive" use of futexes. In you use case here, the exchange to unlock can be a simple atomic store with release semantics. Also, try to think about calling notify_* only when you absolutely need to... :^)