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devel / comp.lang.c++ / Run this under Windows and Linux

SubjectAuthor
o Run this under Windows and LinuxBonita Montero

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Run this under Windows and Linux

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From: Bonita.M...@gmail.com (Bonita Montero)
Newsgroups: comp.lang.c++
Subject: Run this under Windows and Linux
Date: Mon, 30 Oct 2023 08:14:35 +0100
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 by: Bonita Montero - Mon, 30 Oct 2023 07:14 UTC

MSVC's C++20 semaphore class spins to acquire the semaphore;
g++'s / libstdc++'s doesn't spin. I wote a little test program
to rest how long it takes to flip to another thread and back
with a binary_semaphore (always a counting_semaphore<1>).
Here it is:

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

using namespace std;

int main()
{ binary_semaphore
semA( true ),
semB( false );
auto getTimes = []() -> pair<int64_t, int64_t>
{
#if defined(_WIN32)
FILETIME ftDummy, ftKernel, ftUser;
GetThreadTimes( GetCurrentThread(), &ftDummy, &ftDummy, &ftKernel,
&ftUser );
auto compose = []( FILETIME &ft ) { return
((uint64_t)ft.dwHighDateTime << 32 | ft.dwLowDateTime) * 100; };
return { compose( ftKernel ), compose( ftUser ) };
#elif defined(__unix__)
rusage ru;
getrusage( RUSAGE_THREAD, &ru );
auto compose = []( timeval &tv ) -> int64_t { return
(uint64_t)tv.tv_sec * 1'000'000'000u + (uint32_t)tv.tv_usec * 1'000u; };
return { compose( ru.ru_stime ), compose( ru.ru_utime ) };
#endif
};
atomic_int64_t
aFlips( 0 ),
aKernel( 0 ),
aUser( 0 );
atomic_bool stop( false );
auto thr = [&]( binary_semaphore &semMe, binary_semaphore &semYou )
{
auto tBegin = getTimes();
uint64_t f = 0;
for( ; !stop.load( memory_order_relaxed ); ++f )
semMe.acquire(),
semYou.release();
auto tEnd = getTimes();
aFlips += f;
aKernel += tEnd.first - tBegin.first;
aUser += tEnd.second + tBegin.second;
};
vector<jthread> threads;
threads.emplace_back( thr, ref( semA ), ref( semB ) );
threads.emplace_back( thr, ref( semB ), ref( semA ) );
this_thread::sleep_for( 1s );
stop = true;
threads.resize( 0 );
double
kernel = (double)aKernel,
user = (double)aUser,
total = kernel + user,
flips = (double)aFlips;
cout << user / total * 100.0 << "%" << endl;
cout << total / flips << "ns" << endl;
}

Results unter Windows 11 on a AMD Ryzen 7950X (Zen4):
100%
89.3523ns
So 100% of the CPU time is spent in user space and a flip back
and forth takes 90ns.
Results unter Ubuntu 20.04 on a AMD Threadripper 3990X (Zen2):
6.56209%
476.213ns
So under Linux most of the time is spent in kernel space and I think
about 0.5us, thats about 2000 clock cycles with full boost, isn't bad
for an explicit kernel call.
I think this userland spinning isn't really necessary because you
normally a usual mutex and a condition_variable fits for everything
you need. An explicit semaphore is rather needed if you build your
own synchronization primitves on top of the semaphore. And if you do
that you usually have your own spinning with that and you don't need
further spinning if you want go into kernel mode. But this additional
spinning actually doesn't really hurt since the kernel call itself
costs a lot more.


devel / comp.lang.c++ / Run this under Windows and Linux

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