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From: Bonita Montero <Bonita.Montero@gmail.com>
Newsgroups: comp.lang.c++
Subject: Re: notifying from inside or outside
Date: Fri, 9 May 2025 21:23:17 +0200
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Am 09.05.2025 um 20:32 schrieb Chris M. Thomasson:
> On 5/9/2025 6:13 AM, Bonita Montero wrote:
>> The Windows-times were summed-up times where each iteration included
>> the former iterations. Now it's corrected:
>>
>> 10000 rounds
>> inside:
>>          one:
>>                  2.04687 seconds
>>          all:
>>                  4 seconds
>> outside:
>>          one:
>>                  1.03125 seconds
>>          all:
>>                  1.14062 seconds
> [...]
> 
> I need to find some time to properly study your code, but does this 
> indicate that windows condvars do "better" when signaled/broadcasted 
> from the outside?

With Linux notifying from inside is faster, no matter if you broadcast
or notify n times. With Windows strangely notifying from outside is
faster in general.

That's the code:

#if defined(_WIN32)
	#include <Windows.h>
#endif
#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>
#include <atomic>
#include <semaphore>
#include <vector>
#include <string_view>
#if defined(__unix__)
	#include <sys/resource.h>
#endif

using namespace std;

struct params
{
	params( unsigned argc, char **argv );
	bool outside, add, all;
};

int main( int argc, char **argv )
{
	constexpr size_t N = 10'000;
	cout << N << " rounds" << endl;
	int hc = thread::hardware_concurrency(), nClients = hc - 1;
	int64_t tLast = 0;
	for( unsigned outside = 0; outside <= 1; ++outside )
	{
		cout << (outside ? "outside:" : "inside:") << endl;
		for( unsigned all = 0; all <= 1; ++all )
		{
			cout << (all ? "\tall:" : "\tone:") << endl;
			mutex mtx;
			int signalled = 0;
			condition_variable cv;
			atomic_int ai( 0 );
			binary_semaphore bs( false );
			vector<jthread> threads;
			atomic_int64_t nVoluntary( 0 );
			atomic_bool stop( false );
			for( int c = nClients; c; --c )
				threads.emplace_back( [&]
					{
						for( size_t r = N; r; --r )
						{
							unique_lock lock( mtx );
							cv.wait( lock, [&] { return (bool)signalled; } );
							--signalled;
							lock.unlock();
							if( ai.fetch_sub( 1, memory_order_relaxed ) == 1 )
								bs.release( 1 );
						}
#if defined(__unix__)
						rusage ru;
						getrusage( RUSAGE_THREAD, &ru );
						nVoluntary.fetch_add( ru.ru_nvcsw, memory_order_relaxed );
#endif
					} );
			for( size_t r = N; r; --r )
			{
				auto notify = [&]
				{
					if( all )
						cv.notify_all();
					else
						for( int c = nClients; c; cv.notify_one(), --c );
				};
				unique_lock lock( mtx );
				signalled = nClients;
				if( !outside )
					notify();
				ai.store( nClients, memory_order_relaxed );
				lock.unlock();
				if( outside )
					notify();
				bs.acquire();
			}
			stop.store( true, memory_order_relaxed );
			threads.resize( 0 );
#if defined(_WIN32)
			FILETIME ftDummy, ftKernel, ftUser;
			GetProcessTimes( GetCurrentProcess(), &ftDummy, &ftDummy, &ftKernel, 
&ftUser );
			auto ftToU64 = []( FILETIME const &ft ) { return 
(uint64_t)ft.dwHighDateTime << 32 | ft.dwLowDateTime; };
			int64_t t = ftToU64( ftKernel ) + ftToU64( ftUser );
			cout << "\t\t" << (double)(t - tLast) / 1.0e7 << " seconds" << endl;
			tLast = t;
#elif defined(__unix__)
			rusage ru;
			getrusage( RUSAGE_SELF, &ru );
			auto tvToU64 = []( timeval const &tv ) { return (uint64_t)tv.tv_sec * 
1'000'000u + tv.tv_usec; };
			int64_t t = tvToU64( ru.ru_utime ) + tvToU64( ru.ru_stime );
			cout << "\t\t" << (double)nVoluntary.load( memory_order_relaxed ) / 
nClients << " context switches per thread" << endl;
			cout << "\t\t" << (double)(t - tLast) / 1.0e6 << " seconds" << endl;
			tLast = t;
#endif
		}
	}
}