time.hpp File Reference

Time primitives. More...

#include "fastmm/core/config_macros.hpp"
#include "fastmm/core/seqlock.hpp"
#include <x86intrin.h>
#include <compare>
#include <cstdint>

Go to the source code of this file.

Classes
struct	fastmm::Duration
struct	fastmm::Timestamp
struct	fastmm::Cycles
struct	fastmm::TscCalibration
struct	fastmm::ClockReadings
	Clock readings used by TscCalibrator. More...
struct	fastmm::TscAnchor
	One simultaneous reading of the three clocks (the TSC is the midpoint of a tight bracket). More...
struct	fastmm::TscRecalibration
class	fastmm::TscCalibrator
	Long-baseline TSC calibration for the calibrator (control) thread. More...
class	fastmm::TscClock
	Periodic recalibration (see docs/explanation/architecture.md, "Clock calibration"): a calibrator thread measures with calibrate_tsc() and publishes into a Seqlocked<TscCalibration>; every TscClock user keeps its own copy and calls refresh() from its loop. More...
class	fastmm::SimClock
	Deterministic clock for sim/backtest/replay. 1 cycle == 1 ns. More...

Namespaces
namespace	fastmm

Concepts
concept	fastmm::ClockLike

Enumerations
enum class	fastmm::TscKind : std::uint8_t { fastmm::None , fastmm::Constant , fastmm::Invariant }
	What /proc/cpuinfo says about the TSC (cached after the first call): Invariant constant_tsc + nonstop_tsc: wall time and intervals from the TSC. More...
enum class	fastmm::TscRefresh : std::uint8_t {   fastmm::None = 0 , fastmm::Adopted , fastmm::Reanchored , fastmm::Stepped ,   fastmm::Rejected }
	Outcome of TscClock::refresh() / reanchor(). More...

Functions
constexpr Duration	fastmm::nanoseconds (std::int64_t v) noexcept
constexpr Duration	fastmm::microseconds (std::int64_t v) noexcept
constexpr Duration	fastmm::milliseconds (std::int64_t v) noexcept
constexpr Duration	fastmm::seconds (std::int64_t v) noexcept
FASTMM_FORCE_INLINE Cycles	fastmm::rdtscp () noexcept
	Serialising read (rdtscp) is what latency hops want: it waits for prior loads to retire.
FASTMM_FORCE_INLINE Cycles	fastmm::rdtsc () noexcept
Timestamp	fastmm::wall_now () noexcept
	CLOCK_REALTIME via vDSO (~20 ns). Implemented in src/core/time.cpp.
Timestamp	fastmm::steady_now () noexcept
TscKind	fastmm::tsc_kind () noexcept
bool	fastmm::has_invariant_tsc () noexcept
FASTMM_FORCE_INLINE std::uint64_t	fastmm::tsc_interval_ns (const TscCalibration &c, std::uint64_t cycles) noexcept
	A TSC cycle interval in ns; 0 without a rate.
ClockReadings	fastmm::system_clock_readings () noexcept
TscCalibration	fastmm::calibrate_tsc (Duration window=milliseconds(50)) noexcept
	Startup calibration (spins for window): TscCalibrator::start(), so the rate is measured against CLOCK_MONOTONIC_RAW and a host wall-clock step inside the window cannot distort it; the anchor follows CLOCK_REALTIME.
TscCalibration	fastmm::calibrate_tsc (Duration window, const ClockReadings &readings) noexcept
constexpr const char *	fastmm::to_string (TscRefresh r) noexcept
FASTMM_FORCE_INLINE std::int64_t	fastmm::tsc_to_ns (const TscCalibration &c, std::uint64_t tsc) noexcept
	TSC reading -> wall ns under c.

Detailed Description

Time primitives.

Timestamp is ns since the Unix epoch; Cycles is a raw TSC reading.

TscClock converts rdtsc to wall-clock ns with a 32.32 fixed-point multiplier calibrated against CLOCK_REALTIME, refreshed at run time through a Seqlocked<TscCalibration> without letting time go backwards. Without an invariant TSC (constant_tsc and nonstop_tsc) wall time comes from clock_gettime (detected once from /proc/cpuinfo, see src/core/time.cpp); with constant_tsc alone, as on many cloud VMs, the TSC still stamps latency intervals. SimClock is the deterministic replacement for backtests/replay: same interface, time only moves when told.