Why did CRITICAL_SECTION performance become worse on Win8

Question

It seems like CRITICAL_SECTION performance became worse on Windows 8 and higher. (see graphs below)

The test is pretty simple: some concurrent threads do 3 million locks each to access a variable exclusively. You can find the C++ program at the bottom of the question. I run the test on Windows Vista, Windows 7, Windows 8, Windows 10 (x64, VMWare, Intel Core i7-2600 3.40GHz).

The results are on the image below. The X-axis is the number of concurrent threads. The Y-axis is the elapsed time in seconds (lower is better).

What we can see:

• SRWLock performance is approximately the same for all platforms
• CriticalSection performance became worse relatively SRWL on Windows 8 and higher

The question is: Can anybody please explain why did CRITICAL_SECTION performance become worse on Win8 and higher?

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Some notes:

• The results on real machines are pretty the same - CS is much worse than both std::mutex, std::recursive_mutex and SRWL on Win8 and higher. However I have no chance to run the test on different OSes with the same CPU.
• std::mutex implementation for Windows Vista is based on CRITICAL_SECTION, but for Win7 and higher std::mutex is based on SWRL. It is correct for both MSVS17 and 15 (To make sure search for primitives.h file at MSVC++ installation and look for stl_critical_section_vista and stl_critical_section_win7 classes) This explains the difference between std::mutex performance on Win Vista and others.
• As it is said in comments, the std::mutex is a wrapper, so the possible explanation for some overhead relatively SRWL may be overhead introduced by the wrapper code.

---

#include <chrono>
#include <iostream>
#include <mutex>
#include <string>
#include <thread>
#include <vector>

#include <Windows.h>

const size_t T = 10;
const size_t N = 3000000;
volatile uint64_t var = 0;

const std::string sep = ";";

namespace WinApi
{
    class CriticalSection
    {
        CRITICAL_SECTION cs;
    public:
        CriticalSection() { InitializeCriticalSection(&cs); }
        ~CriticalSection() { DeleteCriticalSection(&cs); }
        void lock() { EnterCriticalSection(&cs); }
        void unlock() { LeaveCriticalSection(&cs); }
    };

    class SRWLock
    {
        SRWLOCK srw;
    public:
        SRWLock() { InitializeSRWLock(&srw); }
        void lock() { AcquireSRWLockExclusive(&srw); }
        void unlock() { ReleaseSRWLockExclusive(&srw); }
    };
}

template <class M>
void doLock(void *param)
{
    M &m = *static_cast<M*>(param);
    for (size_t n = 0; n < N; ++n)
    {
        m.lock();
        var += std::rand();
        m.unlock();
    }
}

template <class M>
void runTest(size_t threadCount)
{
    M m;
    std::vector<std::thread> thrs(threadCount);

    const auto start = std::chrono::system_clock::now();

    for (auto &t : thrs) t = std::thread(doLock<M>, &m);
    for (auto &t : thrs) t.join();

    const auto end = std::chrono::system_clock::now();

    const std::chrono::duration<double> diff = end - start;
    std::cout << diff.count() << sep;
}

template <class ...Args>
void runTests(size_t threadMax)
{
    {
        int dummy[] = { (std::cout << typeid(Args).name() << sep, 0)... };
        (void)dummy;
    }
    std::cout << std::endl;

    for (size_t n = 1; n <= threadMax; ++n)
    {
        {
            int dummy[] = { (runTest<Args>(n), 0)... };
            (void)dummy;
        }
        std::cout << std::endl;
    }
}

int main()
{
    std::srand(time(NULL));
    runTests<std::mutex, WinApi::CriticalSection, WinApi::SRWLock>(T);
    return 0;
}

The test project was built as Windows Console Application on Microsoft Visual Studio 17 (15.8.2) with the folowing settings:

• Use of MFC: Use MFC in a Static Library
• Windows SDK Version: 10.0.17134.0
• Platform Toolset: Visual Studio 2017 (v141)
• Optimization: O2, Oi, Oy-, GL

Answer 1

See Windows Critical Section - how to disable spinning completely Starting from Windows 8, Microsoft changed implementation (without even a word in documentation) of default behavior of Critical Section (if you use InitializeCriticalSection(&cs), you will get spinning with undocumented dynamic spin adjustment algorithm enabled). See my comment here: https://randomascii.wordpress.com/2012/06/05/in-praise-of-idleness/#comment-57420

For your test, try using InitializeCriticalSectionAndSpinCount(&cs,1) instead of InitializeCriticalSection(&cs). This should make it behave somewhat similar to Windows 7, though there are plenty of other changes in that area.