Package libcpucycles: Information

libcpucycles is a microlibrary for counting CPU cycles. Cycle counts are
not as detailed as Falk diagrams but are the most precise timers available
to typical software; they are central tools used in understanding and
improving software performance.

The libcpucycles API is simple: include <cpucycles.h>, call cpucycles()
to receive a long long whenever desired, and link with -lcpucycles.

Internally, libcpucycles understands machine-level cycle counters
for amd64 (both PMC and TSC), arm32, arm64 (both PMC and VCT),
loong64, mips64, ppc32, ppc64, riscv32, riscv64, s390x, sparc64,
and x86. libcpucycles also understands four OS-level mechanisms,
which give varying levels of accuracy: mach_absolute_time, perf_event,
CLOCK_MONOTONIC, and, as a fallback, microsecond-resolution gettimeofday.

When the program first calls cpucycles(), libcpucycles automatically
benchmarks the available mechanisms and selects the mechanism that
does the best job. Subsequent cpucycles() calls are thread-safe and
very fast. An accompanying cpucycles-info program prints a summary of
cycle-counter accuracy.

For comparison, there is a simple-sounding __rdtsc() API provided by
compilers, but this works only on Intel/AMD CPUs and is generally noisier
than PMC. There is a __builtin_readcyclecounter() that works on more CPUs,
but this works only with clang and has the same noise problems. Both
of these mechanisms put the burden on the caller to figure out what can
be done on other CPUs. Various packages include their own more portable
abstraction layers for counting cycles (see, e.g., FFTW's cycle.h, used
to automatically select from among multiple implementations provided by
FFTW), but this creates per-package effort to keep up with the latest
cycle counters. The goal of libcpucycles is to provide state-of-the-art
cycle counting centrally for all packages to use.

Package libcpucycles: Information

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