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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: Sat, 26 Apr 2025 14:41:31 -0700
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On 4/26/2025 12:25 AM, Bonita Montero wrote:
> Am 26.04.2025 um 08:26 schrieb Bonita Montero:
>> Am 25.04.2025 um 22:01 schrieb Chris M. Thomasson:
>>
>>  > If you say so... Too busy right now. Perhaps sometime later on 
>> tonight.
>>
>>
>> If there would be a thundering herd problem with glibc's condvar it
>> would happen very often with my code since I awake 31 threads at once
>> with my machine.
> 
> 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.