Drazen Urch
7bd1550195
* Checkpoint * cpu cycle ffi * Rename * mixnode feature * Bundle libcpucycles
37 lines
2.0 KiB
Markdown
37 lines
2.0 KiB
Markdown
libcpucycles is a public-domain microlibrary for counting CPU cycles.
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Cycle counts are not as detailed as
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[Falk diagrams](https://gamozolabs.github.io/metrology/2019/08/19/sushi_roll.html)
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but are the most precise timers available to typical software; they are
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central tools used in understanding and improving software performance.
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The libcpucycles [API](api.html) is simple: include `<cpucycles.h>`, call
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`cpucycles()` to receive a `long long` whenever desired, and link with
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`-lcpucycles`.
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[Internally](counters.html), libcpucycles understands machine-level
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cycle counters for amd64 (both PMC and TSC), arm32, arm64 (both PMC and
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VCT), mips64, ppc32, ppc64, riscv32, riscv64, s390x, sparc64, and x86.
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libcpucycles also understands four OS-level mechanisms, which give
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varying levels of accuracy: `mach_absolute_time`, `perf_event`,
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`CLOCK_MONOTONIC`, and, as a fallback, microsecond-resolution
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`gettimeofday`.
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When the program first calls `cpucycles()`, libcpucycles automatically
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benchmarks the available mechanisms and [selects](selection.html) the
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mechanism that does the best job. Subsequent `cpucycles()` calls are
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thread-safe and very fast. An accompanying `cpucycles-info` program
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prints a summary of cycle-counter accuracy.
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For comparison, there is a simple-sounding `__rdtsc()` API provided by
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compilers, but this works only on Intel/AMD CPUs and is generally noisier
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than PMC. There is a `__builtin_readcyclecounter()` that works on more
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CPUs, but this works only with `clang` and has the same noise problems.
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Both of these mechanisms put the burden on the caller to figure out what
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can be done on other CPUs. Various packages include their own more
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portable abstraction layers for counting cycles (see, e.g., FFTW's
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[`cycle.h`](https://github.com/FFTW/fftw3/blob/master/kernel/cycle.h),
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used to automatically select from among multiple implementations
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provided by FFTW), but this creates per-package effort to keep up with
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the latest cycle counters. The goal of libcpucycles is to provide
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state-of-the-art cycle counting centrally for all packages to use.
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