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// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
// Windows Timer Primer
// A good article:
// A good mozilla bug:
// The default windows timer, GetSystemTimeAsFileTime is not very precise.
// It is only good to ~15.5ms.
// QueryPerformanceCounter is the logical choice for a high-precision timer.
// However, it is known to be buggy on some hardware. Specifically, it can
// sometimes "jump". On laptops, QPC can also be very expensive to call.
// It's 3-4x slower than timeGetTime() on desktops, but can be 10x slower
// on laptops. A unittest exists which will show the relative cost of various
// timers on any system.
// The next logical choice is timeGetTime(). timeGetTime has a precision of
// 1ms, but only if you call APIs (timeBeginPeriod()) which affect all other
// applications on the system. By default, precision is only 15.5ms.
// Unfortunately, we don't want to call timeBeginPeriod because we don't
// want to affect other applications. Further, on mobile platforms, use of
// faster multimedia timers can hurt battery life. See the intel
// article about this here:
// To work around all this, we're going to generally use timeGetTime(). We
// will only increase the system-wide timer if we're not running on battery
// power.
#include "base/time/time.h"
#include <windows.h>
#include <mmsystem.h>
#include <stdint.h>
#include "base/bit_cast.h"
#include "base/cpu.h"
#include "base/lazy_instance.h"
#include "base/logging.h"
#include "base/synchronization/lock.h"
#include "base/threading/platform_thread.h"
using base::ThreadTicks;
using base::Time;
using base::TimeDelta;
using base::TimeTicks;
namespace {
// From MSDN, FILETIME "Contains a 64-bit value representing the number of
// 100-nanosecond intervals since January 1, 1601 (UTC)."
int64_t FileTimeToMicroseconds(const FILETIME& ft) {
// Need to bit_cast to fix alignment, then divide by 10 to convert
// 100-nanoseconds to microseconds. This only works on little-endian
// machines.
return bit_cast<int64_t, FILETIME>(ft) / 10;
void MicrosecondsToFileTime(int64_t us, FILETIME* ft) {
DCHECK_GE(us, 0LL) << "Time is less than 0, negative values are not "
"representable in FILETIME";
// Multiply by 10 to convert microseconds to 100-nanoseconds. Bit_cast will
// handle alignment problems. This only works on little-endian machines.
*ft = bit_cast<FILETIME, int64_t>(us * 10);
int64_t CurrentWallclockMicroseconds() {
return FileTimeToMicroseconds(ft);
// Time between resampling the un-granular clock for this API. 60 seconds.
const int kMaxMillisecondsToAvoidDrift = 60 * Time::kMillisecondsPerSecond;
int64_t initial_time = 0;
TimeTicks initial_ticks;
void InitializeClock() {
initial_ticks = TimeTicks::Now();
initial_time = CurrentWallclockMicroseconds();
// The two values that ActivateHighResolutionTimer uses to set the systemwide
// timer interrupt frequency on Windows. It controls how precise timers are
// but also has a big impact on battery life.
const int kMinTimerIntervalHighResMs = 1;
const int kMinTimerIntervalLowResMs = 4;
// Track if kMinTimerIntervalHighResMs or kMinTimerIntervalLowResMs is active.
bool g_high_res_timer_enabled = false;
// How many times the high resolution timer has been called.
uint32_t g_high_res_timer_count = 0;
// The lock to control access to the above two variables.
base::LazyInstance<base::Lock>::Leaky g_high_res_lock =
// Returns the current value of the performance counter.
uint64_t QPCNowRaw() {
LARGE_INTEGER perf_counter_now = {};
// According to the MSDN documentation for QueryPerformanceCounter(), this
// will never fail on systems that run XP or later.
return perf_counter_now.QuadPart;
} // namespace
// Time -----------------------------------------------------------------------
// The internal representation of Time uses FILETIME, whose epoch is 1601-01-01
// 00:00:00 UTC. ((1970-1601)*365+89)*24*60*60*1000*1000, where 89 is the
// number of leap year days between 1601 and 1970: (1970-1601)/4 excluding
// 1700, 1800, and 1900.
// static
const int64_t Time::kTimeTToMicrosecondsOffset = INT64_C(11644473600000000);
// static
Time Time::Now() {
if (initial_time == 0)
// We implement time using the high-resolution timers so that we can get
// timeouts which are smaller than 10-15ms. If we just used
// CurrentWallclockMicroseconds(), we'd have the less-granular timer.
// To make this work, we initialize the clock (initial_time) and the
// counter (initial_ctr). To compute the initial time, we can check
// the number of ticks that have elapsed, and compute the delta.
// To avoid any drift, we periodically resync the counters to the system
// clock.
while (true) {
TimeTicks ticks = TimeTicks::Now();
// Calculate the time elapsed since we started our timer
TimeDelta elapsed = ticks - initial_ticks;
// Check if enough time has elapsed that we need to resync the clock.
if (elapsed.InMilliseconds() > kMaxMillisecondsToAvoidDrift) {
return Time(elapsed + Time(initial_time));
// static
Time Time::NowFromSystemTime() {
// Force resync.
return Time(initial_time);
// static
Time Time::FromFileTime(FILETIME ft) {
if (bit_cast<int64_t, FILETIME>(ft) == 0)
return Time();
if (ft.dwHighDateTime == std::numeric_limits<DWORD>::max() &&
ft.dwLowDateTime == std::numeric_limits<DWORD>::max())
return Max();
return Time(FileTimeToMicroseconds(ft));
FILETIME Time::ToFileTime() const {
if (is_null())
return bit_cast<FILETIME, int64_t>(0);
if (is_max()) {
FILETIME result;
result.dwHighDateTime = std::numeric_limits<DWORD>::max();
result.dwLowDateTime = std::numeric_limits<DWORD>::max();
return result;
FILETIME utc_ft;
MicrosecondsToFileTime(us_, &utc_ft);
return utc_ft;
// static
void Time::EnableHighResolutionTimer(bool enable) {
base::AutoLock lock(g_high_res_lock.Get());
if (g_high_res_timer_enabled == enable)
g_high_res_timer_enabled = enable;
if (!g_high_res_timer_count)
// Since g_high_res_timer_count != 0, an ActivateHighResolutionTimer(true)
// was called which called timeBeginPeriod with g_high_res_timer_enabled
// with a value which is the opposite of |enable|. With that information we
// call timeEndPeriod with the same value used in timeBeginPeriod and
// therefore undo the period effect.
if (enable) {
} else {
// static
bool Time::ActivateHighResolutionTimer(bool activating) {
// We only do work on the transition from zero to one or one to zero so we
// can easily undo the effect (if necessary) when EnableHighResolutionTimer is
// called.
const uint32_t max = std::numeric_limits<uint32_t>::max();
base::AutoLock lock(g_high_res_lock.Get());
UINT period = g_high_res_timer_enabled ? kMinTimerIntervalHighResMs
: kMinTimerIntervalLowResMs;
if (activating) {
DCHECK_NE(g_high_res_timer_count, max);
if (g_high_res_timer_count == 1)
} else {
DCHECK_NE(g_high_res_timer_count, 0u);
if (g_high_res_timer_count == 0)
return (period == kMinTimerIntervalHighResMs);
// static
bool Time::IsHighResolutionTimerInUse() {
base::AutoLock lock(g_high_res_lock.Get());
return g_high_res_timer_enabled && g_high_res_timer_count > 0;
// static
Time Time::FromExploded(bool is_local, const Exploded& exploded) {
// Create the system struct representing our exploded time. It will either be
// in local time or UTC.
st.wYear = static_cast<WORD>(exploded.year);
st.wMonth = static_cast<WORD>(exploded.month);
st.wDayOfWeek = static_cast<WORD>(exploded.day_of_week);
st.wDay = static_cast<WORD>(exploded.day_of_month);
st.wHour = static_cast<WORD>(exploded.hour);
st.wMinute = static_cast<WORD>(exploded.minute);
st.wSecond = static_cast<WORD>(exploded.second);
st.wMilliseconds = static_cast<WORD>(exploded.millisecond);
bool success = true;
// Ensure that it's in UTC.
if (is_local) {
success = TzSpecificLocalTimeToSystemTime(NULL, &st, &utc_st) &&
SystemTimeToFileTime(&utc_st, &ft);
} else {
success = !!SystemTimeToFileTime(&st, &ft);
if (!success) {
NOTREACHED() << "Unable to convert time";
return Time(0);
return Time(FileTimeToMicroseconds(ft));
void Time::Explode(bool is_local, Exploded* exploded) const {
if (us_ < 0LL) {
// We are not able to convert it to FILETIME.
ZeroMemory(exploded, sizeof(*exploded));
FILETIME utc_ft;
MicrosecondsToFileTime(us_, &utc_ft);
// FILETIME in local time if necessary.
bool success = true;
// FILETIME in SYSTEMTIME (exploded).
SYSTEMTIME st = {0};
if (is_local) {
// We don't use FileTimeToLocalFileTime here, since it uses the current
// settings for the time zone and daylight saving time. Therefore, if it is
// daylight saving time, it will take daylight saving time into account,
// even if the time you are converting is in standard time.
success = FileTimeToSystemTime(&utc_ft, &utc_st) &&
SystemTimeToTzSpecificLocalTime(NULL, &utc_st, &st);
} else {
success = !!FileTimeToSystemTime(&utc_ft, &st);
if (!success) {
NOTREACHED() << "Unable to convert time, don't know why";
ZeroMemory(exploded, sizeof(*exploded));
exploded->year = st.wYear;
exploded->month = st.wMonth;
exploded->day_of_week = st.wDayOfWeek;
exploded->day_of_month = st.wDay;
exploded->hour = st.wHour;
exploded->minute = st.wMinute;
exploded->second = st.wSecond;
exploded->millisecond = st.wMilliseconds;
// TimeTicks ------------------------------------------------------------------
namespace {
// We define a wrapper to adapt between the __stdcall and __cdecl call of the
// mock function, and to avoid a static constructor. Assigning an import to a
// function pointer directly would require setup code to fetch from the IAT.
DWORD timeGetTimeWrapper() {
return timeGetTime();
DWORD (*g_tick_function)(void) = &timeGetTimeWrapper;
// Accumulation of time lost due to rollover (in milliseconds).
int64_t g_rollover_ms = 0;
// The last timeGetTime value we saw, to detect rollover.
DWORD g_last_seen_now = 0;
// Lock protecting rollover_ms and last_seen_now.
// Note: this is a global object, and we usually avoid these. However, the time
// code is low-level, and we don't want to use Singletons here (it would be too
// easy to use a Singleton without even knowing it, and that may lead to many
// gotchas). Its impact on startup time should be negligible due to low-level
// nature of time code.
base::Lock g_rollover_lock;
// We use timeGetTime() to implement TimeTicks::Now(). This can be problematic
// because it returns the number of milliseconds since Windows has started,
// which will roll over the 32-bit value every ~49 days. We try to track
// rollover ourselves, which works if TimeTicks::Now() is called at least every
// 49 days.
TimeDelta RolloverProtectedNow() {
base::AutoLock locked(g_rollover_lock);
// We should hold the lock while calling tick_function to make sure that
// we keep last_seen_now stay correctly in sync.
DWORD now = g_tick_function();
if (now < g_last_seen_now)
g_rollover_ms += 0x100000000I64; // ~49.7 days.
g_last_seen_now = now;
return TimeDelta::FromMilliseconds(now + g_rollover_ms);
// Discussion of tick counter options on Windows:
// (1) CPU cycle counter. (Retrieved via RDTSC)
// The CPU counter provides the highest resolution time stamp and is the least
// expensive to retrieve. However, on older CPUs, two issues can affect its
// reliability: First it is maintained per processor and not synchronized
// between processors. Also, the counters will change frequency due to thermal
// and power changes, and stop in some states.
// (2) QueryPerformanceCounter (QPC). The QPC counter provides a high-
// resolution (<1 microsecond) time stamp. On most hardware running today, it
// auto-detects and uses the constant-rate RDTSC counter to provide extremely
// efficient and reliable time stamps.
// On older CPUs where RDTSC is unreliable, it falls back to using more
// expensive (20X to 40X more costly) alternate clocks, such as HPET or the ACPI
// PM timer, and can involve system calls; and all this is up to the HAL (with
// some help from ACPI). According to
//, in the
// worst case, it gets the counter from the rollover interrupt on the
// programmable interrupt timer. In best cases, the HAL may conclude that the
// RDTSC counter runs at a constant frequency, then it uses that instead. On
// multiprocessor machines, it will try to verify the values returned from
// RDTSC on each processor are consistent with each other, and apply a handful
// of workarounds for known buggy hardware. In other words, QPC is supposed to
// give consistent results on a multiprocessor computer, but for older CPUs it
// can be unreliable due bugs in BIOS or HAL.
// (3) System time. The system time provides a low-resolution (from ~1 to ~15.6
// milliseconds) time stamp but is comparatively less expensive to retrieve and
// more reliable. Time::EnableHighResolutionTimer() and
// Time::ActivateHighResolutionTimer() can be called to alter the resolution of
// this timer; and also other Windows applications can alter it, affecting this
// one.
using NowFunction = TimeDelta (*)(void);
TimeDelta InitialNowFunction();
// See "threading notes" in InitializeNowFunctionPointer() for details on how
// concurrent reads/writes to these globals has been made safe.
NowFunction g_now_function = &InitialNowFunction;
int64_t g_qpc_ticks_per_second = 0;
// As of January 2015, use of <atomic> is forbidden in Chromium code. This is
// what std::atomic_thread_fence does on Windows on all Intel architectures when
// the memory_order argument is anything but std::memory_order_seq_cst:
#define ATOMIC_THREAD_FENCE(memory_order) _ReadWriteBarrier();
TimeDelta QPCValueToTimeDelta(LONGLONG qpc_value) {
// Ensure that the assignment to |g_qpc_ticks_per_second|, made in
// InitializeNowFunctionPointer(), has happened by this point.
DCHECK_GT(g_qpc_ticks_per_second, 0);
// If the QPC Value is below the overflow threshold, we proceed with
// simple multiply and divide.
if (qpc_value < Time::kQPCOverflowThreshold) {
return TimeDelta::FromMicroseconds(
qpc_value * Time::kMicrosecondsPerSecond / g_qpc_ticks_per_second);
// Otherwise, calculate microseconds in a round about manner to avoid
// overflow and precision issues.
int64_t whole_seconds = qpc_value / g_qpc_ticks_per_second;
int64_t leftover_ticks = qpc_value - (whole_seconds * g_qpc_ticks_per_second);
return TimeDelta::FromMicroseconds(
(whole_seconds * Time::kMicrosecondsPerSecond) +
((leftover_ticks * Time::kMicrosecondsPerSecond) /
TimeDelta QPCNow() {
return QPCValueToTimeDelta(QPCNowRaw());
bool IsBuggyAthlon(const base::CPU& cpu) {
// On Athlon X2 CPUs (e.g. model 15) QueryPerformanceCounter is unreliable.
return cpu.vendor_name() == "AuthenticAMD" && == 15;
void InitializeNowFunctionPointer() {
LARGE_INTEGER ticks_per_sec = {};
if (!QueryPerformanceFrequency(&ticks_per_sec))
ticks_per_sec.QuadPart = 0;
// If Windows cannot provide a QPC implementation, TimeTicks::Now() must use
// the low-resolution clock.
// If the QPC implementation is expensive and/or unreliable, TimeTicks::Now()
// will still use the low-resolution clock. A CPU lacking a non-stop time
// counter will cause Windows to provide an alternate QPC implementation that
// works, but is expensive to use. Certain Athlon CPUs are known to make the
// QPC implementation unreliable.
// Otherwise, Now uses the high-resolution QPC clock. As of 21 August 2015,
// ~72% of users fall within this category.
NowFunction now_function;
base::CPU cpu;
if (ticks_per_sec.QuadPart <= 0 ||
!cpu.has_non_stop_time_stamp_counter() || IsBuggyAthlon(cpu)) {
now_function = &RolloverProtectedNow;
} else {
now_function = &QPCNow;
// Threading note 1: In an unlikely race condition, it's possible for two or
// more threads to enter InitializeNowFunctionPointer() in parallel. This is
// not a problem since all threads should end up writing out the same values
// to the global variables.
// Threading note 2: A release fence is placed here to ensure, from the
// perspective of other threads using the function pointers, that the
// assignment to |g_qpc_ticks_per_second| happens before the function pointers
// are changed.
g_qpc_ticks_per_second = ticks_per_sec.QuadPart;
g_now_function = now_function;
TimeDelta InitialNowFunction() {
return g_now_function();
} // namespace
// static
TimeTicks::TickFunctionType TimeTicks::SetMockTickFunction(
TickFunctionType ticker) {
base::AutoLock locked(g_rollover_lock);
TickFunctionType old = g_tick_function;
g_tick_function = ticker;
g_rollover_ms = 0;
g_last_seen_now = 0;
return old;
// static
TimeTicks TimeTicks::Now() {
return TimeTicks() + g_now_function();
// static
bool TimeTicks::IsHighResolution() {
if (g_now_function == &InitialNowFunction)
return g_now_function == &QPCNow;
// static
TimeTicks::Clock TimeTicks::GetClock() {
return IsHighResolution() ?
// static
ThreadTicks ThreadTicks::Now() {
return ThreadTicks::GetForThread(PlatformThread::CurrentHandle());
// static
ThreadTicks ThreadTicks::GetForThread(
const base::PlatformThreadHandle& thread_handle) {
// Get the number of TSC ticks used by the current thread.
ULONG64 thread_cycle_time = 0;
::QueryThreadCycleTime(thread_handle.platform_handle(), &thread_cycle_time);
// Get the frequency of the TSC.
double tsc_ticks_per_second = TSCTicksPerSecond();
if (tsc_ticks_per_second == 0)
return ThreadTicks();
// Return the CPU time of the current thread.
double thread_time_seconds = thread_cycle_time / tsc_ticks_per_second;
return ThreadTicks(
static_cast<int64_t>(thread_time_seconds * Time::kMicrosecondsPerSecond));
// static
bool ThreadTicks::IsSupportedWin() {
static bool is_supported = base::CPU().has_non_stop_time_stamp_counter() &&
return is_supported;
// static
void ThreadTicks::WaitUntilInitializedWin() {
while (TSCTicksPerSecond() == 0)
double ThreadTicks::TSCTicksPerSecond() {
// The value returned by QueryPerformanceFrequency() cannot be used as the TSC
// frequency, because there is no guarantee that the TSC frequency is equal to
// the performance counter frequency.
// The TSC frequency is cached in a static variable because it takes some time
// to compute it.
static double tsc_ticks_per_second = 0;
if (tsc_ticks_per_second != 0)
return tsc_ticks_per_second;
// Increase the thread priority to reduces the chances of having a context
// switch during a reading of the TSC and the performance counter.
int previous_priority = ::GetThreadPriority(::GetCurrentThread());
::SetThreadPriority(::GetCurrentThread(), THREAD_PRIORITY_HIGHEST);
// The first time that this function is called, make an initial reading of the
// TSC and the performance counter.
static const uint64_t tsc_initial = __rdtsc();
static const uint64_t perf_counter_initial = QPCNowRaw();
// Make a another reading of the TSC and the performance counter every time
// that this function is called.
uint64_t tsc_now = __rdtsc();
uint64_t perf_counter_now = QPCNowRaw();
// Reset the thread priority.
::SetThreadPriority(::GetCurrentThread(), previous_priority);
// Make sure that at least 50 ms elapsed between the 2 readings. The first
// time that this function is called, we don't expect this to be the case.
// Note: The longer the elapsed time between the 2 readings is, the more
// accurate the computed TSC frequency will be. The 50 ms value was
// chosen because local benchmarks show that it allows us to get a
// stddev of less than 1 tick/us between multiple runs.
// Note: According to the MSDN documentation for QueryPerformanceFrequency(),
// this will never fail on systems that run XP or later.
LARGE_INTEGER perf_counter_frequency = {};
DCHECK_GE(perf_counter_now, perf_counter_initial);
uint64_t perf_counter_ticks = perf_counter_now - perf_counter_initial;
double elapsed_time_seconds =
perf_counter_ticks / static_cast<double>(perf_counter_frequency.QuadPart);
const double kMinimumEvaluationPeriodSeconds = 0.05;
if (elapsed_time_seconds < kMinimumEvaluationPeriodSeconds)
return 0;
// Compute the frequency of the TSC.
DCHECK_GE(tsc_now, tsc_initial);
uint64_t tsc_ticks = tsc_now - tsc_initial;
tsc_ticks_per_second = tsc_ticks / elapsed_time_seconds;
return tsc_ticks_per_second;
// static
TimeTicks TimeTicks::FromQPCValue(LONGLONG qpc_value) {
return TimeTicks() + QPCValueToTimeDelta(qpc_value);
// TimeDelta ------------------------------------------------------------------
// static
TimeDelta TimeDelta::FromQPCValue(LONGLONG qpc_value) {
return QPCValueToTimeDelta(qpc_value);