std::hardwa...erence_size(3)

💅 NAME 💅

Okay bestie, welcome to the std::hardwa...erence_size(3) page from the C++ Standard Libary manual. This is the one about std::hardware_destructive_interference_size and std::hardware_constructive_interference_size, and it's lowkey iconic because it lets you talk about cache lines without guessing. Let's get into it 💖

• std::hardware_destructive_interference_size,std::hardware_constructive_interference_size — std::hardware_destructive_interference_size,std::hardware_constructive_interference_size — yes, both, together, exactly like that 💅

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✨ Synopsis ✨

Defined in header <new> ✨

These two are inline constexpr std::size_t, and they've been around since C++17. The value is /*implementation-defined*/, so the implementation picks it. Not us. Slay.

inline constexpr std::size_t
    hardware_destructive_interference_size =
(1) (since C++17)
/*implementation-defined*/;
inline constexpr std::size_t
    hardware_constructive_interference_size =
(2) (since C++17)
/*implementation-defined*/;

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);

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🧠 Notes 🧠

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

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

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🎀 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. Obsessed with the drama of false sharing tbh 😭

// 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

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👀 See also 👀

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

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Okay that's the whole page, bestie 🫶 This cutie is from http://cppreference.com, dated 2024.06.10, and the footer title was std::hardwa...erence_size(3). Go forth and align your atomics, queen 👑💖 slay responsibly ✨