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Library
Module
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Class type
High-performance timing.
This module provides the fast function now () which is our best effort high-performance cycle counter for a given platform. For x86 systems this retrieves the CPU's internal time stamp counter using the RDTSC instruction. For systems that do not have a RDTSC instruction, we fallback to using clock_gettime(CLOCK_MONOTONIC).
Here is a benchmark of execution time in nanos and allocations in words:
Type t is an Int63.t and consequently has no allocation overhead (on 64-bit machines), unlike Time.now () which returns a boxed float.
Functions are also provided to estimate the relationship of CPU time-stamp-counter frequency to real time, thereby allowing one to convert from t to Time.t. There are some caveats to this that are worth noting:
The conversion to Time.t depends on an estimate of the time-stamp-counter frequency. This frequency may be volatile on some systems, thereby reducing the utility of this conversion. See the Calibrator module below for details.
The captured t can only be converted to a Time.t if one also has a recently calibrated Calibrator.t from the same machine.
Put another way, it would not make sense to send a sexp of t from one box to another and then convert it to a Time.t, because t counts the number of cycles since reset. So the measure only makes sense in the context of a single machine.
Note that a cursory search for information about time stamp counter usage may give a false impression of its unreliability. Early processor implementations of TSC could be skewed by clock frequency changes (C-states) and by small differences between the startup time of each processor on a multi-processor machine. Modern hardware can usually be assumed to have an "invariant" tsc, and Linux has support to synchronize the initial counters at boot time when multiple processors are present.
A calibrator contains a snapshot of machine-specific information that is used to convert between TSC values and clock time. This information needs to be calibrated periodically such that it stays updated w.r.t. changes in the CPU's time-stamp-counter frequency, which can vary depending on load, heat, etc. (Also see the comment in the .ml file.)