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@pmttavara
Last active July 9, 2024 00:48
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Obtain RDTSC frequency on Win32 and Linux
// SPDX-FileCopyrightText: © 2022 Phillip Trudeau-Tavara <pmttavara@protonmail.com>
// SPDX-License-Identifier: 0BSD
// https://hero.handmade.network/forums/code-discussion/t/7485-queryperformancefrequency_returning_10mhz_bug/2
// https://learn.microsoft.com/en-us/virtualization/hyper-v-on-windows/tlfs/timers#partition-reference-tsc-mechanism
#include <stdbool.h>
#include <stdint.h>
#define WIN32_LEAN_AND_MEAN
#define VC_EXTRALEAN
#define NOMINMAX
#include <Windows.h>
#include <intrin.h>
static inline uint64_t get_rdtsc_freq(void) {
// Cache the answer so that multiple calls never take the slow path more than once
static uint64_t tsc_freq = 0;
if (tsc_freq) {
return tsc_freq;
}
// Fast path: Load kernel-mapped memory page
HMODULE ntdll = LoadLibraryA("ntdll.dll");
if (ntdll) {
int (*NtQuerySystemInformation)(int, void *, unsigned int, unsigned int *) =
(int (*)(int, void *, unsigned int, unsigned int *))GetProcAddress(ntdll, "NtQuerySystemInformation");
if (NtQuerySystemInformation) {
volatile uint64_t *hypervisor_shared_page = NULL;
unsigned int size = 0;
// SystemHypervisorSharedPageInformation == 0xc5
int result = (NtQuerySystemInformation)(0xc5, (void *)&hypervisor_shared_page, sizeof(hypervisor_shared_page), &size);
// success
if (size == sizeof(hypervisor_shared_page) && result >= 0) {
// docs say ReferenceTime = ((VirtualTsc * TscScale) >> 64)
// set ReferenceTime = 10000000 = 1 second @ 10MHz, solve for VirtualTsc
// => VirtualTsc = 10000000 / (TscScale >> 64)
tsc_freq = (10000000ull << 32) / (hypervisor_shared_page[1] >> 32);
// If your build configuration supports 128 bit arithmetic, do this:
// tsc_freq = ((unsigned __int128)10000000ull << (unsigned __int128)64ull) / hypervisor_shared_page[1];
}
}
FreeLibrary(ntdll);
}
// Slow path
if (!tsc_freq) {
// Get time before sleep
uint64_t qpc_begin = 0; QueryPerformanceCounter((LARGE_INTEGER *)&qpc_begin);
uint64_t tsc_begin = __rdtsc();
Sleep(2);
// Get time after sleep
uint64_t qpc_end = qpc_begin + 1; QueryPerformanceCounter((LARGE_INTEGER *)&qpc_end);
uint64_t tsc_end = __rdtsc();
// Do the math to extrapolate the RDTSC ticks elapsed in 1 second
uint64_t qpc_freq = 0; QueryPerformanceFrequency((LARGE_INTEGER *)&qpc_freq);
tsc_freq = (tsc_end - tsc_begin) * qpc_freq / (qpc_end - qpc_begin);
}
// Failure case
if (!tsc_freq) {
tsc_freq = 1000000000;
}
return tsc_freq;
}
#include <stdio.h>
int main() {
printf("Calling get_rdtsc_freq() 1,000 times...\n");
uint64_t start = __rdtsc();
uint64_t freq = 0;
for (int i = 0; i < 1000; i++) {
freq = get_rdtsc_freq();
}
uint64_t end = __rdtsc();
printf("...took %f milliseconds.\n", (end - start) * 1000.0 / freq);
printf("RDTSC frequency is %lu (%.2f GHz).\n", (unsigned long)freq, freq * 0.000000001);
}
// SPDX-FileCopyrightText: © 2022 Phillip Trudeau-Tavara <pmttavara@protonmail.com>
// SPDX-License-Identifier: 0BSD
// https://linux.die.net/man/2/perf_event_open
// https://stackoverflow.com/a/57835630
#include <stdbool.h>
#include <stdint.h>
#include <sys/mman.h>
#include <linux/perf_event.h>
#include <time.h>
#include <unistd.h>
#include <x86intrin.h>
static inline uint64_t get_rdtsc_freq(void) {
// Cache the answer so that multiple calls never take the slow path more than once
static uint64_t tsc_freq = 0;
if (tsc_freq) {
return tsc_freq;
}
// Fast path: Load kernel-mapped memory page
struct perf_event_attr pe = {0};
pe.type = PERF_TYPE_HARDWARE;
pe.size = sizeof(struct perf_event_attr);
pe.config = PERF_COUNT_HW_INSTRUCTIONS;
pe.disabled = 1;
pe.exclude_kernel = 1;
pe.exclude_hv = 1;
// __NR_perf_event_open == 298 (on x86_64)
int fd = syscall(298, &pe, 0, -1, -1, 0);
if (fd != -1) {
struct perf_event_mmap_page *pc = (struct perf_event_mmap_page *)mmap(NULL, 4096, PROT_READ, MAP_SHARED, fd, 0);
if (pc) {
// success
if (pc->cap_user_time == 1) {
// docs say nanoseconds = (tsc * time_mult) >> time_shift
// set nanoseconds = 1000000000 = 1 second in nanoseconds, solve for tsc
// => tsc = 1000000000 / (time_mult >> time_shift)
tsc_freq = (1000000000ull << (pc->time_shift / 2)) / (pc->time_mult >> (pc->time_shift - pc->time_shift / 2));
// If your build configuration supports 128 bit arithmetic, do this:
// tsc_freq = ((__uint128_t)1000000000ull << (__uint128_t)pc->time_shift) / pc->time_mult;
}
munmap(pc, 4096);
}
close(fd);
}
// Slow path
if (!tsc_freq) {
// Get time before sleep
uint64_t nsc_begin = 0; { struct timespec t; if (!clock_gettime(CLOCK_MONOTONIC_RAW, &t)) nsc_begin = (uint64_t)t.tv_sec * 1000000000ull + t.tv_nsec; }
uint64_t tsc_begin = __rdtsc();
usleep(10000); // 10ms gives ~4.5 digits of precision - the longer you sleep, the more precise you get
// Get time after sleep
uint64_t nsc_end = nsc_begin + 1; { struct timespec t; if (!clock_gettime(CLOCK_MONOTONIC_RAW, &t)) nsc_end = (uint64_t)t.tv_sec * 1000000000ull + t.tv_nsec; }
uint64_t tsc_end = __rdtsc();
// Do the math to extrapolate the RDTSC ticks elapsed in 1 second
tsc_freq = (tsc_end - tsc_begin) * 1000000000 / (nsc_end - nsc_begin);
}
// Failure case
if (!tsc_freq) {
tsc_freq = 1000000000;
}
return tsc_freq;
}
#include <stdio.h>
int main() {
printf("Calling get_rdtsc_freq() 1,000 times...\n");
uint64_t start = __rdtsc();
uint64_t freq = 0;
for (int i = 0; i < 1000; i++) {
freq = get_rdtsc_freq();
}
uint64_t end = __rdtsc();
printf("...took %f milliseconds.\n", (end - start) * 1000.0 / freq);
printf("RDTSC frequency is %lu (%.2f GHz).\n", (unsigned long)freq, freq * 0.000000001);
}
@bogez57
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bogez57 commented Dec 15, 2022

Awesome. This literally dropped right as I was about to look more into this stuff for my own profiler. It was meant to be

@markodevv
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@bogez57 Same, i was really wondering how to do this. Thanks.

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