Discussion :

[camboui]

Bonjour,

Beaucoup de mes programmes sont mono-thread, mais depuis bien longtemps la runtime library de visual studio n'est plus dispo qu'en multi-thread. Je me suis souvent dit que je devais y perdre en performances à laisser la CRT faire des locks inutiles.
Est-ce une question légitime, ou bien les pertes de performances sont-elles négligeables ?

Je pense en particulier à la gestion du tas (heap).
Je suppose qu'à chaque new et delete la CRT vérouille les appels via un mutex (ou CriticalSection). L'usage de new et delete est implicitement pléthorique sur les containers (std::set, std::vector, std::string, etc). Mais je n'ai aucune idée du coût induit.

Merci.

[Bktero]

Si tu passes ton programme dans un profiler, ça te donne des indications ?

[dalfab]

Il est certain que la nécessité d'un mutex pour accéder à l'allocation augmente le temps d'accès.
L'ordre de grandeur pour prendre/rendre un mutex libre est de :
* 5ns+20ns environ sur un système temps réel
* 50ns+100ns environ sous Windows.
Ça n'est pas négligeable sous Windows à comparer à la recherche/libération d'une entrée heap qui devrait être très variable (au moins 20ns mais pourrait atteindre 10µs).
Il faudrait faire des tests pour valider cela.

La question est plutôt combien y a-t-il d'allocations/dés-allocations par seconde dans un code. C'est évidemment variable, mais devrait être très faible, là aussi il faudrait effectuer des tests pour mesurer l'impact.

[Bousk]

Je ne me soucierais pas trop des lock autour de new et delete. Ces appels sont très liés au système qui devait déjà avoir un système de lock pour permettre le multitâche/processus et donc présent depuis Windows 95 ou 98 ? Le premier Windows multi-tâche/processus/... il me semble.
Sinon on a fait des tests ici, et utiliser les critical section est meilleure que les std::mutex pour les os Microsoft.

[dalfab]

Le fait que les sections critiques sont plus optimales sous Windows est essentiellement un problème de vocabulaire.
La section critique Windows correspond à un mutex Posix
Le mutex Windows correspond à un mutex partagé Posix.
Et donc ici, la librairie doit vraisemblablement utiliser des sections critiques.

[camboui]

Ok, merci à tous
Effectivement le coût des lock/unlock est manifestement faible, et probablement négligeable à coté de la complexité d'un code fonctionnel en production.

J'ai fait un test qui compare différentes méthodes de lock/unlock.
Le code protégé en fait le strict minimum, à savoir incrémenter ou décrémenter. Son impact est faible et permet de comparer les différents lock/unlock.

Code :

Sélectionner tout - Visualiser dans une fenêtre à part

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#include <future>
#include <shared_mutex>
#include "boost/smart_ptr/detail/spinlock.hpp"
 
 
volatile int nvalue = 0;
 
int loop_nolock(bool inc, int limit)
{
	//std::_tcout << "Started " << inc << " " << limit << std::endl;
	for (int i = 0; i < limit; ++i)
	{
		if (inc)
			++nvalue;
		else
			--nvalue;
	}
	return limit;
}
 
 
volatile std::atomic_int avalue = 0;
 
int loop_atomiclock(bool inc, int limit)
{
	//std::_tcout << "Started " << inc << " " << limit << std::endl;
	for (int i = 0; i < limit; ++i)
	{
		if (inc)
			++avalue;
		else
			--avalue;
	}
	return limit;
}
 
 
volatile LONG ivalue = 0;
 
int loop_interlock(bool inc, int limit)
{
	//std::_tcout << "Started " << inc << " " << limit << std::endl;
	for (int i = 0; i < limit; ++i)
	{
		if (inc)
			::InterlockedIncrement(&ivalue);
		else
			::InterlockedDecrement(&ivalue);
	}
	return limit;
}
 
 
std::mutex mlck;
volatile int mvalue = 0;
 
int loop_mutexlock(bool inc, int limit)
{
	//std::_tcout << "Started " << inc << " " << limit << std::endl;
	for (int i = 0; i < limit; ++i)
	{
		std::lock_guard<std::mutex> guard(mlck);
		if (inc)
			++mvalue;
		else
			--mvalue;
	}
	return limit;
}
 
 
std::shared_mutex smlck;
volatile int smvalue = 0;
 
int loop_shmutexlock(bool inc, int limit)
{
	//std::_tcout << "Started " << inc << " " << limit << std::endl;
	for (int i = 0; i < limit; ++i)
	{
		std::lock_guard<std::shared_mutex> guard(smlck);
		if (inc)
			++smvalue;
		else
			--smvalue;
	}
	return limit;
}
 
 
//https://msdn.microsoft.com/en-us/library/windows/desktop/aa904937(v=vs.85).aspx
class win_slimRWlock
{
public:
    void lock()
    {
        ::AcquireSRWLockExclusive(&srw);
    }
    void unlock()
    {
        ::ReleaseSRWLockExclusive(&srw);
    }
 private:
    SRWLOCK srw SRWLOCK_INIT;
} slck;
volatile int svalue = 0;
 
int loop_win_slimRWlock(bool inc, int limit)
{
	//std::_tcout << "Started " << inc << " " << limit << std::endl;
	for (int i = 0; i < limit; ++i)
	{
		std::lock_guard<win_slimRWlock> guard(slck);
		if (inc)
			++svalue;
		else
			--svalue;
	}
	return limit;
}
 
 
//https://msdn.microsoft.com/en-us/library/windows/desktop/ms682530(v=vs.85).aspx
class win_CriticalSectionlock
{
public:
    void lock()
    {
        ::EnterCriticalSection(&cs);
    }
    void unlock()
    {
        ::LeaveCriticalSection(&cs);
    }
	win_CriticalSectionlock()
	{
		::InitializeCriticalSection(&cs);//AndSpinCount(&cs, 1000);
	}
	~win_CriticalSectionlock()
	{
		::DeleteCriticalSection(&cs);
	}
private:
    CRITICAL_SECTION cs;
} cslck;
volatile int csvalue = 0;
 
int loop_win_CriticalSectionlock(bool inc, int limit)
{
	//std::_tcout << "Started " << inc << " " << limit << std::endl;
	for (int i = 0; i < limit; ++i)
	{
		std::lock_guard<win_CriticalSectionlock> guard(cslck);
		if (inc)
			++csvalue;
		else
			--csvalue;
	}
	return limit;
}
 
 
//http://preshing.com/20120226/roll-your-own-lightweight-mutex/
class win_Benaphorelock
{
public:
    void lock()
    {
        if (::_InterlockedIncrement(&m_counter) > 1) // x86/64 guarantees acquire semantics
            ::WaitForSingleObject(m_semaphore, INFINITE);
    }
    void unlock()
    {
        if (::_InterlockedDecrement(&m_counter) > 0) // x86/64 guarantees release semantics
            ::ReleaseSemaphore(m_semaphore, 1, NULL);
    }
	win_Benaphorelock()
	{
		m_counter = 0;
        m_semaphore = ::CreateSemaphore(NULL, 0, 1, NULL);
	}
	~win_Benaphorelock()
	{
		::CloseHandle(m_semaphore);
	}
private:
    LONG m_counter;
    HANDLE m_semaphore;
} phlck;
volatile int phvalue = 0;
 
int loop_win_Benaphorelock(bool inc, int limit)
{
	//std::_tcout << "Started " << inc << " " << limit << std::endl;
	for (int i = 0; i < limit; ++i)
	{
		std::lock_guard<win_Benaphorelock> guard(phlck);
		if (inc)
			++phvalue;
		else
			--phvalue;
	}
	return limit;
}
 
 
//http://anki3d.org/spinlock/
class custom_spinlock
{
public:
    void lock()
    {
        while(lck.test_and_set(std::memory_order_acquire))
        {}
    }
    void unlock()
    {
        lck.clear(std::memory_order_release);
    }
private:
    std::atomic_flag lck ATOMIC_FLAG_INIT;
} clck;
volatile int cvalue = 0;
 
int loop_customspinlock(bool inc, int limit)
{
	//std::_tcout << "Started " << inc << " " << limit << std::endl;
	for (int i = 0; i < limit; ++i)
	{
		std::lock_guard<custom_spinlock> guard(clck);
		if (inc)
			++cvalue;
		else
			--cvalue;
	}
	return limit;
}
 
 
boost::detail::spinlock blck;
volatile int bvalue = 0;
 
int loop_boostspinlock(bool inc, int limit)
{
	//std::_tcout << "Started " << inc << " " << limit << std::endl;
	for (int i = 0; i < limit; ++i)
	{
		std::lock_guard<boost::detail::spinlock> guard(blck);
		if (inc)
			++bvalue;
		else
			--bvalue;
	}
	return limit;
}
 
 
void foo_main()
{
	//inspiration from http://demin.ws/blog/english/2012/05/05/atomic-spinlock-mutex/
#ifdef _DEBUG
	int const lolimit = 18000000;
	int const hilimit = 20000000;
#else
	int const lolimit = 180000000;
	int const hilimit = 200000000;
#endif
    std::chrono::time_point<std::chrono::high_resolution_clock> start, end;
    std::chrono::duration<double> elapsed_seconds;
 
    std::_tcout << _T("\nloop_nolock\n");
    start = std::chrono::high_resolution_clock::now();
	{
		auto f = std::async(std::launch::async, std::bind(loop_nolock, true, hilimit));
		loop_nolock(false, lolimit);
		f.wait();
		std::_tcout << nvalue << std::endl;
	}
    end = std::chrono::high_resolution_clock::now();
	elapsed_seconds = end-start;
    std::_tcout << _T("loop_nolock elapsed time: ") << elapsed_seconds.count() << _T("s (") << (elapsed_seconds.count()*1e9/(lolimit+hilimit)) << _T("ns)\n");
 
    std::_tcout << _T("\nloop_atomiclock\n");
    start = std::chrono::high_resolution_clock::now();
	{
		auto f = std::async(std::launch::async, std::bind(loop_atomiclock, true, hilimit));
		loop_atomiclock(false, lolimit);
		f.wait();
		std::_tcout << avalue << std::endl;
	}
    end = std::chrono::high_resolution_clock::now();
	elapsed_seconds = end-start;
    std::_tcout << _T("loop_atomiclock elapsed time: ") << elapsed_seconds.count() << _T("s (") << (elapsed_seconds.count()*1e9/(lolimit+hilimit)) << _T("ns)\n");
 
    std::_tcout << _T("\nloop_interlock\n");
    start = std::chrono::high_resolution_clock::now();
	{
		auto f = std::async(std::launch::async, std::bind(loop_interlock, true, hilimit));
		loop_interlock(false, lolimit);
		f.wait();
		std::_tcout << ivalue << std::endl;
	}
    end = std::chrono::high_resolution_clock::now();
	elapsed_seconds = end-start;
    std::_tcout << _T("loop_interlock elapsed time: ") << elapsed_seconds.count() << _T("s (") << (elapsed_seconds.count()*1e9/(lolimit+hilimit)) << _T("ns)\n");
 
    std::_tcout << _T("\nloop_mutexlock\n");
    start = std::chrono::high_resolution_clock::now();
	{
		auto f = std::async(std::launch::async, std::bind(loop_mutexlock, true, hilimit));
		loop_mutexlock(false, lolimit);
		f.wait();
		std::_tcout << mvalue << std::endl;
	}
    end = std::chrono::high_resolution_clock::now();
	elapsed_seconds = end-start;
    std::_tcout << _T("loop_mutexlock elapsed time: ") << elapsed_seconds.count() << _T("s (") << (elapsed_seconds.count()*1e9/(lolimit+hilimit)) << _T("ns)\n");
 
    std::_tcout << _T("\nloop_shmutexlock\n");
    start = std::chrono::high_resolution_clock::now();
	{
		auto f = std::async(std::launch::async, std::bind(loop_shmutexlock, true, hilimit));
		loop_shmutexlock(false, lolimit);
		f.wait();
		std::_tcout << smvalue << std::endl;
	}
    end = std::chrono::high_resolution_clock::now();
	elapsed_seconds = end-start;
    std::_tcout << _T("loop_shmutexlock elapsed time: ") << elapsed_seconds.count() << _T("s (") << (elapsed_seconds.count()*1e9/(lolimit+hilimit)) << _T("ns)\n");
 
    std::_tcout << _T("\nloop_customspinlock\n");
    start = std::chrono::high_resolution_clock::now();
	{
		auto f = std::async(std::launch::async, std::bind(loop_customspinlock, true, hilimit));
		loop_customspinlock(false, lolimit);
		f.wait();
		std::_tcout << cvalue << std::endl;
	}
    end = std::chrono::high_resolution_clock::now();
	elapsed_seconds = end-start;
    std::_tcout << _T("loop_customspinlock elapsed time: ") << elapsed_seconds.count() << _T("s (") << (elapsed_seconds.count()*1e9/(lolimit+hilimit)) << _T("ns)\n");
 
    std::_tcout << _T("\nloop_boostspinlock\n");
    start = std::chrono::high_resolution_clock::now();
	{
		auto f = std::async(std::launch::async, std::bind(loop_boostspinlock, true, hilimit));
		loop_boostspinlock(false, lolimit);
		f.wait();
		std::_tcout << bvalue << std::endl;
	}
    end = std::chrono::high_resolution_clock::now();
	elapsed_seconds = end-start;
    std::_tcout << _T("loop_boostspinlock elapsed time: ") << elapsed_seconds.count() << _T("s (") << (elapsed_seconds.count()*1e9/(lolimit+hilimit)) << _T("ns)\n");
 
    std::_tcout << _T("\nloop_win_slimRWlock\n");
    start = std::chrono::high_resolution_clock::now();
	{
		auto f = std::async(std::launch::async, std::bind(loop_win_slimRWlock, true, hilimit));
		loop_win_slimRWlock(false, lolimit);
		f.wait();
		std::_tcout << svalue << std::endl;
	}
    end = std::chrono::high_resolution_clock::now();
	elapsed_seconds = end-start;
    std::_tcout << _T("loop_win_slimRWlock elapsed time: ") << elapsed_seconds.count() << _T("s (") << (elapsed_seconds.count()*1e9/(lolimit+hilimit)) << _T("ns)\n");
 
    std::_tcout << _T("\nloop_win_slimRWlock sequential\n");
    start = std::chrono::high_resolution_clock::now();
	{
		svalue = 0;
		loop_win_slimRWlock(true, hilimit);
		loop_win_slimRWlock(false, lolimit);
		std::_tcout << svalue << std::endl;
	}
    end = std::chrono::high_resolution_clock::now();
	elapsed_seconds = end-start;
    std::_tcout << _T("loop_win_slimRWlock sequential elapsed time: ") << elapsed_seconds.count() << _T("s (") << (elapsed_seconds.count()*1e9/(lolimit+hilimit)) << _T("ns)\n");
 
    std::_tcout << _T("\nloop_win_CriticalSectionlock\n");
    start = std::chrono::high_resolution_clock::now();
	{
		auto f = std::async(std::launch::async, std::bind(loop_win_CriticalSectionlock, true, hilimit));
		loop_win_CriticalSectionlock(false, lolimit);
		f.wait();
		std::_tcout << csvalue << std::endl;
	}
    end = std::chrono::high_resolution_clock::now();
	elapsed_seconds = end-start;
    std::_tcout << _T("loop_win_CriticalSectionlock elapsed time: ") << elapsed_seconds.count() << _T("s (") << (elapsed_seconds.count()*1e9/(lolimit+hilimit)) << _T("ns)\n");
 
	std::_tcout << _T("\nloop_win_CriticalSectionlock sequential\n");
    start = std::chrono::high_resolution_clock::now();
	{
		csvalue = 0;
		loop_win_CriticalSectionlock(true, hilimit);
		loop_win_CriticalSectionlock(false, lolimit);
		std::_tcout << csvalue << std::endl;
	}
    end = std::chrono::high_resolution_clock::now();
	elapsed_seconds = end-start;
    std::_tcout << _T("loop_win_CriticalSectionlock sequential elapsed time: ") << elapsed_seconds.count() << _T("s (") << (elapsed_seconds.count()*1e9/(lolimit+hilimit)) << _T("ns)\n");
 
    std::_tcout << _T("\nloop_win_Benaphorelock\n");
    start = std::chrono::high_resolution_clock::now();
	{
		auto f = std::async(std::launch::async, std::bind(loop_win_Benaphorelock, true, hilimit/100));
		loop_win_Benaphorelock(false, lolimit/100);
		f.wait();
		std::_tcout << phvalue << std::endl;
	}
    end = std::chrono::high_resolution_clock::now();
	elapsed_seconds = end-start;
    std::_tcout << _T("loop_win_Benaphorelock elapsed time: ") << elapsed_seconds.count() << _T("s (") << (elapsed_seconds.count()*1e9/(lolimit/100+hilimit/100)) << _T("ns)\n");
}

Et les résultats:

Code :

Sélectionner tout - Visualiser dans une fenêtre à part

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loop_nolock
162029654
loop_nolock elapsed time: 0.41865s (1.10171ns)

loop_atomiclock
20000000
loop_atomiclock elapsed time: 5.40209s (14.216ns)

loop_interlock
20000000
loop_interlock elapsed time: 5.70157s (15.0041ns)

loop_mutexlock
20000000
loop_mutexlock elapsed time: 9.27985s (24.4206ns)

loop_shmutexlock
20000000
loop_shmutexlock elapsed time: 5.65883s (14.8917ns)

loop_customspinlock
20000000
loop_customspinlock elapsed time: 13.6456s (35.9095ns)

loop_boostspinlock
20000000
loop_boostspinlock elapsed time: 8.2854s (21.8037ns)

loop_win_slimRWlock
20000000
loop_win_slimRWlock elapsed time: 5.63678s (14.8336ns)

loop_win_slimRWlock sequential
20000000
loop_win_slimRWlock sequential elapsed time: 5.73371s (15.0887ns)

loop_win_CriticalSectionlock
20000000
loop_win_CriticalSectionlock elapsed time: 22.7562s (59.8847ns)

loop_win_CriticalSectionlock sequential
20000000
loop_win_CriticalSectionlock sequential elapsed time: 6.95204s (18.2948ns)

loop_win_Benaphorelock
200000
loop_win_Benaphorelock elapsed time: 18.5011s (4868.71ns)

Les gagnants sont std::shared_mutex et les Slim Reader/Writer Locks de windows (sous VS2015 il semble bien que le std::shared_mutex utilisent les Slim Reader/Writer Locks, d'où les performances équivalentes).
Les perdants sont ceux qui prétendent faire mieux que les solutions standards ou natives (le Benaphore est catastrophique...).

En conclusion, j'ose croire que Microsoft utilise ce qu'il a de mieux dans sa runtime library, à savoir les Slim Reader/Writer Locks.
Et sinon, les Critical Section ne s'en sortent pas si mal (et dans mon test, ajouter un spin counter n'aide en rien, que du contraire).