striped_rate_counter.hpp

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#pragma once
 
/// @file userver/utils/statistics/striped_rate_counter.hpp
/// @brief @copybrief utils::statistics::StripedRateCounter
 
#include <atomic>
#include <cstdint>
 
#include <userver/concurrent/striped_counter.hpp>
#include <userver/utils/statistics/fwd.hpp>
#include <userver/utils/statistics/rate.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace utils::statistics {
 
/// @brief Atomic counter of type Rate with relaxed memory ordering, increased
/// memory consumption and decreased contention.
///
/// This class is represented as Rate metric when serializing to statistics.
///
/// Differences from utils::statistics::RateCounter:
//
/// 1. StripedRateCounter takes `8 * N_CORES` memory, while RateCounter is just
///    a single atomic.
/// 2. StripedRateCounter uses split counters on x86_64, which results
///    in perfect performance even under heavy concurrent usage.
///
/// Use StripedRateCounter instead of RateCounter sparingly, in places where a
/// lot of threads are supposed to be hammering on the same metrics.
class StripedRateCounter final {
public:
    using ValueType = Rate;
 
    StripedRateCounter() = default;
 
    explicit StripedRateCounter(Rate desired) { Add(desired); }
 
    explicit StripedRateCounter(Rate::ValueType desired) : StripedRateCounter(Rate{desired}) {}
 
    StripedRateCounter(const StripedRateCounter& other) { val_.Add(other.LoadImpl()); }
 
    StripedRateCounter& operator=(const StripedRateCounter& other) noexcept {
        if (this == &other) return *this;
 
        offset_ = other.LoadImpl() - val_.Read();
        return *this;
    }
 
    StripedRateCounter& operator=(Rate desired) noexcept {
        Store(desired);
        return *this;
    }
 
    void Store(Rate desired) noexcept { offset_ = static_cast<std::uintptr_t>(desired.value) - val_.Read(); }
 
    Rate Load() const noexcept { return Rate{static_cast<std::uint64_t>(LoadImpl())}; }
 
    void Add(Rate arg) noexcept { val_.Add(static_cast<std::uintptr_t>(arg.value)); }
 
    StripedRateCounter& operator++() noexcept {
        val_.Add(1);
        return *this;
    }
 
    StripedRateCounter& operator+=(Rate arg) noexcept {
        Add(arg);
        return *this;
    }
 
    StripedRateCounter& operator+=(const StripedRateCounter& arg) noexcept {
        val_.Add(arg.LoadImpl());
        return *this;
    }
 
private:
    // Implementation note: any operation that assigns a value (instead of adding)
    // would induce race condition if implemented using LoadImpl(). Use offset_
    // directly instead for those.
    std::uintptr_t LoadImpl() const noexcept { return val_.Read() + offset_.load(std::memory_order_relaxed); }
 
    USERVER_NAMESPACE::concurrent::StripedCounter val_;
    std::atomic<std::uintptr_t> offset_{0};
};
 
void DumpMetric(Writer& writer, const StripedRateCounter& value);
 
void ResetMetric(StripedRateCounter& value);
 
}  // namespace utils::statistics
 
USERVER_NAMESPACE_END