Print warning when microbenchmarking w/ powersave

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

libc/runtime/runtime.h

@@ -115,6 +115,7 @@ void __morph_end(void);
 unsigned char *GetFirstInstruction(void);
 unsigned char *GetInstructionLengths(void);
 void __print_maps(void);
+void __warn_if_powersave(void);
 
 COSMOPOLITAN_C_END_
 #endif /* !(__ASSEMBLER__ + __LINKER__ + 0) */

libc/runtime/warnifpowersave.c

@@ -0,0 +1,42 @@
+/*-*- 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);
+}

libc/testlib/ezbenchreport.c

@@ -19,6 +19,7 @@
 #include "libc/fmt/fmt.h"
 #include "libc/math.h"
 #include "libc/runtime/gc.internal.h"
+#include "libc/runtime/runtime.h"
 #include "libc/stdio/stdio.h"
 #include "libc/testlib/testlib.h"
 #include "libc/time/time.h"
@@ -28,10 +29,11 @@ STATIC_YOINK("strnwidth");
 
 void __testlib_ezbenchreport(const char *form, uint64_t c1, uint64_t c2) {
   uint64_t ns1, ns2;
+  __warn_if_powersave();
   ns1 = rintl(ConvertTicksToNanos(c1));
   ns2 = rintl(ConvertTicksToNanos(c2));
   (fprintf)(stderr,
-            VEIL("r", " *     %-19s l: %,9lu𝑐 %,9lu𝑛𝑠   m: %,9lu𝑐 %,9lu𝑛𝑠\n"),
+            VEIL("r", " *     %-19s l: %,9luc %,9luns   m: %,9luc %,9luns\n"),
             form, c1, ns1, c2, ns2);
 }
 
@@ -39,6 +41,7 @@ void __testlib_ezbenchreport_n(const char *form, char z, size_t n, uint64_t c) {
   char msg[128];
   uint64_t bps;
   long double cn, lat;
+  __warn_if_powersave();
   (snprintf)(msg, sizeof(msg), "%s %c=%d", form, z, n);
   cn = ConvertTicksToNanos(c);
   if (!n) {

test/libc/calls/clock_gettime_test.c

@@ -21,6 +21,7 @@
 #include "libc/calls/struct/timespec.h"
 #include "libc/calls/struct/timeval.h"
 #include "libc/calls/syscall_support-sysv.internal.h"
+#include "libc/dce.h"
 #include "libc/nexgen32e/rdtsc.h"
 #include "libc/runtime/runtime.h"
 #include "libc/sysv/consts/auxv.h"
@@ -49,7 +50,19 @@ BENCH(clock_gettime, bench) {
   EZBENCH2("nowl", donothing, nowl());
   EZBENCH2("rdtsc", donothing, rdtsc());
   EZBENCH2("gettimeofday", donothing, gettimeofday(&tv, 0));
-  EZBENCH2("clock_gettime", donothing, clock_gettime(0, &ts));
-  EZBENCH2("__clock_gettime", donothing, __clock_gettime(0, &ts));
-  EZBENCH2("sys_clock_gettime", donothing, sys_clock_gettime(0, &ts));
+  EZBENCH2("clock_gettime 0", donothing, clock_gettime(0, &ts));
+  EZBENCH2("clock_gettime 1", donothing, clock_gettime(1, &ts));
+  EZBENCH2("clock_gettime 4", donothing, clock_gettime(4, &ts));
+  EZBENCH2("__clock_gettime 0", donothing, __clock_gettime(0, &ts));
+  EZBENCH2("__clock_gettime 1", donothing, __clock_gettime(1, &ts));
+  EZBENCH2("__clock_gettime 4", donothing, __clock_gettime(4, &ts));
+  if (IsWindows()) {
+    EZBENCH2("sys_clock_gettime 0", donothing, sys_clock_gettime_nt(0, &ts));
+    EZBENCH2("sys_clock_gettime 1", donothing, sys_clock_gettime_nt(1, &ts));
+    EZBENCH2("sys_clock_gettime 4", donothing, sys_clock_gettime_nt(4, &ts));
+  } else {
+    EZBENCH2("sys_clock_gettime 0", donothing, sys_clock_gettime(0, &ts));
+    EZBENCH2("sys_clock_gettime 1", donothing, sys_clock_gettime(1, &ts));
+    EZBENCH2("sys_clock_gettime 4", donothing, sys_clock_gettime(4, &ts));
+  }
 }

tool/net/lfuncs.c

@@ -642,6 +642,7 @@ int LuaBenchmark(lua_State *L) {
   luaL_checktype(L, 1, LUA_TFUNCTION);
   count = luaL_optinteger(L, 2, 100);
   maxattempts = luaL_optinteger(L, 3, 10);
+  __warn_if_powersave();
   lua_gc(L, LUA_GCSTOP);
 
   for (attempts = 0;;) {