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From: "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>
Newsgroups: comp.lang.c++
Subject: Re: Solving thundering Herd with glibc...
Date: Tue, 29 Apr 2025 12:26:41 -0700
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On 4/26/2025 8:18 PM, Bonita Montero wrote:
> Am 26.04.2025 um 23:41 schrieb Chris M. Thomasson:
> 
>> On 4/26/2025 12:25 AM, Bonita Montero wrote:
> 
>>>
>>> I just tried to awaken all 31 threads from outside holding the mutex,
>>> but not from inside:
>>>
>>>      for( size_t r = N; r; --r )
>>>      {
>>>          unique_lock lock( mtx );
>>>          signalled = nClients;
>>>          ai.store( nClients, memory_order_relaxed );
>>>          lock.unlock();
>>>          if( argc < 2 )
>>>              cv.notify_all();
>>>          else
>>>              for( int c = nClients; c; cv.notify_one(), --c );
>>>          bs.acquire();
>>>      }
>>>
>>> The result: 7.500 context switches per thread, not 3.000.
>>>
>>>      10000 rounds,
>>>      7498.06 context switches pe thread
>>>
>>> So never signal a condvar to multiple threads from outside !
> 
>> So, do that. It's your software. Do what you like. This is a very old 
>> debate. Take your contrived test and just, roll with it. Whatever.
> 
> There's nothing to debate, I've measured it: If you awake a single
> thread it doesn't matter if you awake from inside or outside, if you
> awake multiple threads awakening them from inside is multiple times
> faster.
> 

Yawn.