# std::hardware_destructive_interference_size, std::hardware_constructive_interference_size

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< cpp‎ | thread

Defined in header `<new>` |  |
inline constexpr std::size_t hardware_destructive_interference_size = /*implementation-defined*/; | (1) | (since C++17)
inline constexpr std::size_t hardware_constructive_interference_size = /*implementation-defined*/; | (2) | (since C++17)

1) Minimum offset between two objects to avoid false sharing. Guaranteed to be at least alignof( std::max_align_t )

    struct keep_apart
    {
        alignas(std::hardware_destructive_interference_size) std::atomic<int> cat;
        alignas(std::hardware_destructive_interference_size) std::atomic<int> dog;
    };

2) Maximum size of contiguous memory to promote true sharing. Guaranteed to be at least alignof( std::max_align_t )

    struct together
    {
        std::atomic<int> dog;
        int puppy;
    };

    struct kennel
    {
        // Other data members...

        alignas(sizeof(together)) together pack;

        // Other data members...
    };

    static_assert(sizeof(together) <= std::hardware_constructive_interference_size);

### Notes

These constants provide a portable way to access the L1 data cache line size.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_hardware_interference_size` | `201703L` | (C++17) | constexpr std::hardware_constructive_interference_size and constexpr std::hardware_destructive_interference_size

### Example

The program uses two threads that atomically write to the data members of the given global objects. The first object fits in one cache line, which results in "hardware interference". The second object keeps its data members on separate cache lines, so possible "cache synchronization" after thread writes is avoided.

Run this code

    #include <atomic>
    #include <chrono>
    #include <cstddef>
    #include <iomanip>
    #include <iostream>
    #include <mutex>
    #include <new>
    #include <thread>

    #ifdef __cpp_lib_hardware_interference_size
        using std::hardware_constructive_interference_size;
        using std::hardware_destructive_interference_size;
    #else
        // 64 bytes on x86-64 │ L1_CACHE_BYTES │ L1_CACHE_SHIFT │ __cacheline_aligned │ ...
        constexpr std::size_t hardware_constructive_interference_size = 64;
        constexpr std::size_t hardware_destructive_interference_size = 64;
    #endif

    std::mutex cout_mutex;

    constexpr int max_write_iterations{10'000'000}; // the benchmark time tuning

    struct alignas(hardware_constructive_interference_size)
    OneCacheLiner // occupies one cache line
    {
        std::atomic_uint64_t x{};
        std::atomic_uint64_t y{};
    }
    oneCacheLiner;

    struct TwoCacheLiner // occupies two cache lines
    {
        alignas(hardware_destructive_interference_size) std::atomic_uint64_t x{};
        alignas(hardware_destructive_interference_size) std::atomic_uint64_t y{};
    }
    twoCacheLiner;

    inline auto now() noexcept { return std::chrono::high_resolution_clock::now(); }

    template<bool xy>
    void oneCacheLinerThread()
    {
        const auto start{now()};

        for (uint64_t count{}; count != max_write_iterations; ++count)
            if constexpr (xy)
                oneCacheLiner.x.fetch_add(1, std::memory_order_relaxed);
            else
                oneCacheLiner.y.fetch_add(1, std::memory_order_relaxed);

        const std::chrono::duration<double, std::milli> elapsed{now() - start};
        std::lock_guard lk{cout_mutex};
        std::cout << "oneCacheLinerThread() spent " << elapsed.count() << " ms\n";
        if constexpr (xy)
            oneCacheLiner.x = elapsed.count();
        else
            oneCacheLiner.y = elapsed.count();
    }

    template<bool xy>
    void twoCacheLinerThread()
    {
        const auto start{now()};

        for (uint64_t count{}; count != max_write_iterations; ++count)
            if constexpr (xy)
                twoCacheLiner.x.fetch_add(1, std::memory_order_relaxed);
            else
                twoCacheLiner.y.fetch_add(1, std::memory_order_relaxed);

        const std::chrono::duration<double, std::milli> elapsed{now() - start};
        std::lock_guard lk{cout_mutex};
        std::cout << "twoCacheLinerThread() spent " << elapsed.count() << " ms\n";
        if constexpr (xy)
            twoCacheLiner.x = elapsed.count();
        else
            twoCacheLiner.y = elapsed.count();
    }

    int main()
    {
        std::cout << "__cpp_lib_hardware_interference_size "
    #   ifdef __cpp_lib_hardware_interference_size
            "= " << __cpp_lib_hardware_interference_size << '\n';
    #   else
            "is not defined, use " << hardware_destructive_interference_size
                                   << " as fallback\n";
    #   endif

        std::cout << "hardware_destructive_interference_size == "
                  << hardware_destructive_interference_size << '\n'
                  << "hardware_constructive_interference_size == "
                  << hardware_constructive_interference_size << "\n\n"
                  << std::fixed << std::setprecision(2)
                  << "sizeof( OneCacheLiner ) == " << sizeof(OneCacheLiner) << '\n'
                  << "sizeof( TwoCacheLiner ) == " << sizeof(TwoCacheLiner) << "\n\n";

        constexpr int max_runs{4};

        int oneCacheLiner_average{0};
        for (auto i{0}; i != max_runs; ++i)
        {
            std::thread th1{oneCacheLinerThread<0>};
            std::thread th2{oneCacheLinerThread<1>};
            th1.join();
            th2.join();
            oneCacheLiner_average += oneCacheLiner.x + oneCacheLiner.y;
        }
        std::cout << "Average T1 time: "
                  << (oneCacheLiner_average / max_runs / 2) << " ms\n\n";

        int twoCacheLiner_average{0};
        for (auto i{0}; i != max_runs; ++i)
        {
            std::thread th1{twoCacheLinerThread<0>};
            std::thread th2{twoCacheLinerThread<1>};
            th1.join();
            th2.join();
            twoCacheLiner_average += twoCacheLiner.x + twoCacheLiner.y;
        }
        std::cout << "Average T2 time: "
                  << (twoCacheLiner_average / max_runs / 2) << " ms\n\n"
                  << "Ratio T1/T2:~ "
                  << 1.0 * oneCacheLiner_average / twoCacheLiner_average << '\n';
    }

Possible output:

    __cpp_lib_hardware_interference_size = 201703
    hardware_destructive_interference_size == 64
    hardware_constructive_interference_size == 64

    sizeof( OneCacheLiner ) == 64
    sizeof( TwoCacheLiner ) == 128

    oneCacheLinerThread() spent 517.83 ms
    oneCacheLinerThread() spent 533.43 ms
    oneCacheLinerThread() spent 527.36 ms
    oneCacheLinerThread() spent 555.69 ms
    oneCacheLinerThread() spent 574.74 ms
    oneCacheLinerThread() spent 591.66 ms
    oneCacheLinerThread() spent 555.63 ms
    oneCacheLinerThread() spent 555.76 ms
    Average T1 time: 550 ms

    twoCacheLinerThread() spent 89.79 ms
    twoCacheLinerThread() spent 89.94 ms
    twoCacheLinerThread() spent 89.46 ms
    twoCacheLinerThread() spent 90.28 ms
    twoCacheLinerThread() spent 89.73 ms
    twoCacheLinerThread() spent 91.11 ms
    twoCacheLinerThread() spent 89.17 ms
    twoCacheLinerThread() spent 90.09 ms
    Average T2 time: 89 ms

    Ratio T1/T2:~ 6.16

### See also

hardware_concurrency [static] | returns the number of concurrent threads supported by the implementation (public static member function of `std::thread`)
hardware_concurrency [static] | returns the number of concurrent threads supported by the implementation (public static member function of `std::jthread`)

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