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Print warning when microbenchmarking w/ powersave

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RDTSC on Linux has so much jitter when the CPU is in powersave mode
causing things like microbenchmarks to have a 1000% margin of error
This commit is contained in:
Justine Tunney 2022-06-10 20:51:36 -07:00
parent 41c86fe86b
commit c6d8e516b2
5 changed files with 64 additions and 4 deletions
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42
libc/runtime/warnifpowersave.c Normal file
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/*-*- mode:c;indent-tabs-mode:nil;c-basic-offset:2;tab-width:8;coding:utf-8 -*-│
│vi: set net ft=c ts=2 sts=2 sw=2 fenc=utf-8 :vi│
╞══════════════════════════════════════════════════════════════════════════════╡
│ Copyright 2022 Justine Alexandra Roberts Tunney │
│ │
│ Permission to use, copy, modify, and/or distribute this software for │
│ any purpose with or without fee is hereby granted, provided that the │
│ above copyright notice and this permission notice appear in all copies. │
│ │
│ THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL │
│ WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED │
│ WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE │
│ AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL │
│ DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR │
│ PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER │
│ TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR │
│ PERFORMANCE OF THIS SOFTWARE. │
╚─────────────────────────────────────────────────────────────────────────────*/
#include "libc/calls/calls.h"
#include "libc/runtime/runtime.h"
#include "libc/str/str.h"
#include "libc/sysv/consts/o.h"
// RDTSC on Linux has so much jitter when the CPU is in powersave mode.
// Causing things like microbenchmarks to have a 1000% margin of error.
#define FILE "/sys/devices/system/cpu/cpu0/cpufreq/scaling_governor"
#define WARN \
"warning: this operation isn't reliable in powersave mode. please run:\n\t" \
"echo performance | sudo tee " \
"/sys/devices/system/cpu/cpu*/cpufreq/scaling_governor\n"
void __warn_if_powersave(void) {
int fd;
char buf[16] = {0};
if (!fileexists(FILE)) return;
if ((fd = open(FILE, O_RDONLY)) == -1) return;
read(fd, buf, 15);
close(fd);
if (!startswith(buf, "powersave")) return;
write(2, WARN, sizeof(WARN) - 1);
}