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From: "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>
Newsgroups: comp.lang.c++
Subject: Re: Futexes ain't fast
Date: Wed, 28 Aug 2024 11:52:13 -0700
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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... :^)