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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 21:20:22 -0700
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On 8/28/2024 9:17 PM, Chris M. Thomasson wrote:
> On 8/28/2024 8:51 PM, Bonita Montero wrote:
>> Am 28.08.2024 um 20:52 schrieb Chris M. Thomasson:
>>> 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 );
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Can be atomic store with memory_order_release in this context of your code.

>>>>              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... :^)
>>
>> Show me your code ...
>>
> 
> I did a while back for a futex mutex, win32 impl iirc... Do I have to 
> find the older code?