cpu_relax.hpp

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#pragma once
 
/// @file userver/utils/cpu_relax.hpp
/// @brief Helper classes to yield in CPU intensive places
 
#include <chrono>
#include <cstddef>
 
#include <userver/tracing/scope_time.hpp>
 
USERVER_NAMESPACE_BEGIN
 
namespace utils {
 
/// Utility to easily pause ScopeTime, e.g. when yielding
class ScopeTimePause {
public:
    explicit ScopeTimePause(tracing::ScopeTime* scope);
 
    /// Pause a ScopeTime, if any
    void Pause();
 
    /// Unpause the ScopeTime, if any
    void Unpause();
 
private:
    tracing::ScopeTime* scope_;
    std::string scope_name_;
};
 
/// Utility to yield every N iterations in a CPU-bound task to give other tasks
/// an opportunity to get CPU time
class CpuRelax {
public:
    /// @param every number of iterations to call yield. 0 = noop
    /// @param scope the tracing::ScopeTime to pause when yielding, if any
    /// @warning The `ScopeTime` must live at least as long as the `CpuRelax`
    explicit CpuRelax(std::size_t every, tracing::ScopeTime* scope);
 
    CpuRelax(const CpuRelax&) = delete;
    CpuRelax(CpuRelax&&) = delete;
 
    /// Call this method every iteration, eventually it will yield
    void Relax();
 
private:
    ScopeTimePause pause_;
    const std::size_t every_iterations_;
    std::size_t iterations_{0};
};
 
/// Utility to yield in a CPU-bound data processing task
/// to give other tasks an opportunity to get CPU time
class StreamingCpuRelax {
public:
    /// @param check_time_after_bytes number of bytes to call yield
    /// @param scope the tracing::ScopeTime to pause when yielding, if any
    /// @warning The `ScopeTime` must live at least as long as the `CpuRelax`
    explicit StreamingCpuRelax(std::uint64_t check_time_after_bytes, tracing::ScopeTime* scope);
 
    /// Checks time and potentially calls `engine::Yield()`
    /// each `check_time_after_bytes` bytes
    void Relax(std::uint64_t bytes_processed);
 
    /// Returns the total amount of bytes processed since the creation
    /// of `StreamingCpuRelax`
    std::uint64_t GetBytesProcessed() const;
 
private:
    ScopeTimePause pause_;
    std::uint64_t check_time_after_bytes_;
    std::uint64_t total_bytes_{0};
    std::uint64_t bytes_since_last_time_check_{0};
    std::chrono::steady_clock::time_point last_yield_time_;
};
 
}  // namespace utils
 
USERVER_NAMESPACE_END