base/time/time_win.cc

// 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:  http://www.ddj.com/windows/184416651
// A good mozilla bug:  http://bugzilla.mozilla.org/show_bug.cgi?id=363258
//
// 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:
// http://softwarecommunity.intel.com/articles/eng/1086.htm
//
// 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"

#pragma comment(lib, "winmm.lib")
#include <windows.h>
#include <mmsystem.h>

#include "base/basictypes.h"
#include "base/cpu.h"
#include "base/lazy_instance.h"
#include "base/logging.h"
#include "base/synchronization/lock.h"

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 FileTimeToMicroseconds(const FILETIME& ft) {
  // Need to bit_cast to fix alignment, then divide by 10 to convert
  // 100-nanoseconds to milliseconds. This only works on little-endian
  // machines.
  return bit_cast<int64, FILETIME>(ft) / 10;
}

void MicrosecondsToFileTime(int64 us, FILETIME* ft) {
  DCHECK_GE(us, 0LL) << "Time is less than 0, negative values are not "
      "representable in FILETIME";

  // Multiply by 10 to convert milliseconds to 100-nanoseconds. Bit_cast will
  // handle alignment problems. This only works on little-endian machines.
  *ft = bit_cast<FILETIME, int64>(us * 10);
}

int64 CurrentWallclockMicroseconds() {
  FILETIME ft;
  ::GetSystemTimeAsFileTime(&ft);
  return FileTimeToMicroseconds(ft);
}

// Time between resampling the un-granular clock for this API.  60 seconds.
const int kMaxMillisecondsToAvoidDrift = 60 * Time::kMillisecondsPerSecond;

int64 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 =
    LAZY_INSTANCE_INITIALIZER;

}  // 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 Time::kTimeTToMicrosecondsOffset = GG_INT64_C(11644473600000000);

// static
Time Time::Now() {
  if (initial_time == 0)
    InitializeClock();

  // 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) {
      InitializeClock();
      continue;
    }

    return Time(elapsed + Time(initial_time));
  }
}

// static
Time Time::NowFromSystemTime() {
  // Force resync.
  InitializeClock();
  return Time(initial_time);
}

// static
Time Time::FromFileTime(FILETIME ft) {
  if (bit_cast<int64, 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>(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)
    return;
  g_high_res_timer_enabled = enable;
  if (!g_high_res_timer_count)
    return;
  // 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) {
    timeEndPeriod(kMinTimerIntervalLowResMs);
    timeBeginPeriod(kMinTimerIntervalHighResMs);
  } else {
    timeEndPeriod(kMinTimerIntervalHighResMs);
    timeBeginPeriod(kMinTimerIntervalLowResMs);
  }
}

// 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(g_high_res_timer_count != max);
    ++g_high_res_timer_count;
    if (g_high_res_timer_count == 1)
      timeBeginPeriod(period);
  } else {
    DCHECK(g_high_res_timer_count != 0);
    --g_high_res_timer_count;
    if (g_high_res_timer_count == 0)
      timeEndPeriod(period);
  }
  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.
  SYSTEMTIME st;
  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);

  FILETIME ft;
  bool success = true;
  // Ensure that it's in UTC.
  if (is_local) {
    SYSTEMTIME utc_st;
    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));
    return;
  }

  // FILETIME in UTC.
  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) {
    SYSTEMTIME utc_st;
    // 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));
    return;
  }

  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 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.
TimeTicks 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 TimeTicks() + 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
// http://blogs.msdn.com/oldnewthing/archive/2005/09/02/459952.aspx, 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 = TimeTicks (*)(void);

TimeTicks InitialNowFunction();
TimeTicks InitialSystemTraceNowFunction();

// See "threading notes" in InitializeNowFunctionPointers() for details on how
// concurrent reads/writes to these globals has been made safe.
NowFunction g_now_function = &InitialNowFunction;
NowFunction g_system_trace_now_function = &InitialSystemTraceNowFunction;
int64 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
  // InitializeNowFunctionPointers(), has happened by this point.
  ATOMIC_THREAD_FENCE(memory_order_acquire);

  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 whole_seconds = qpc_value / g_qpc_ticks_per_second;
  int64 leftover_ticks = qpc_value - (whole_seconds * g_qpc_ticks_per_second);
  return TimeDelta::FromMicroseconds(
      (whole_seconds * Time::kMicrosecondsPerSecond) +
      ((leftover_ticks * Time::kMicrosecondsPerSecond) /
       g_qpc_ticks_per_second));
}

TimeTicks QPCNow() {
  LARGE_INTEGER now;
  QueryPerformanceCounter(&now);
  return TimeTicks() + QPCValueToTimeDelta(now.QuadPart);
}

bool IsBuggyAthlon(const base::CPU& cpu) {
  // On Athlon X2 CPUs (e.g. model 15) QueryPerformanceCounter is unreliable.
  return cpu.vendor_name() == "AuthenticAMD" && cpu.family() == 15;
}

void InitializeNowFunctionPointers() {
  LARGE_INTEGER ticks_per_sec = {0};
  if (!QueryPerformanceFrequency(&ticks_per_sec))
    ticks_per_sec.QuadPart = 0;

  // If Windows cannot provide a QPC implementation, both Now() and
  // NowFromSystemTraceTime() must use the low-resolution clock.
  //
  // If the QPC implementation is expensive and/or unreliable, Now() will use
  // the low-resolution clock, but NowFromSystemTraceTime() will use the QPC (in
  // the hope that it is still useful for tracing purposes). 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, both Now functions can use the high-resolution QPC clock. As of
  // 4 January 2015, ~68% of users fall within this category.
  NowFunction now_function;
  NowFunction system_trace_now_function;
  base::CPU cpu;
  if (ticks_per_sec.QuadPart <= 0) {
    now_function = system_trace_now_function = &RolloverProtectedNow;
  } else if (!cpu.has_non_stop_time_stamp_counter() || IsBuggyAthlon(cpu)) {
    now_function = &RolloverProtectedNow;
    system_trace_now_function = &QPCNow;
  } else {
    now_function = system_trace_now_function = &QPCNow;
  }

  // Threading note 1: In an unlikely race condition, it's possible for two or
  // more threads to enter InitializeNowFunctionPointers() 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;
  ATOMIC_THREAD_FENCE(memory_order_release);
  g_now_function = now_function;
  g_system_trace_now_function = system_trace_now_function;
}

TimeTicks InitialNowFunction() {
  InitializeNowFunctionPointers();
  return g_now_function();
}

TimeTicks InitialSystemTraceNowFunction() {
  InitializeNowFunctionPointers();
  return g_system_trace_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 g_now_function();
}

// static
bool TimeTicks::IsHighResolution() {
  if (g_now_function == &InitialNowFunction)
    InitializeNowFunctionPointers();
  return g_now_function == &QPCNow;
}

// static
TimeTicks TimeTicks::ThreadNow() {
  NOTREACHED();
  return TimeTicks();
}

// static
TimeTicks TimeTicks::NowFromSystemTraceTime() {
  return g_system_trace_now_function();
}

// static
TimeTicks TimeTicks::FromQPCValue(LONGLONG qpc_value) {
  return TimeTicks() + QPCValueToTimeDelta(qpc_value);
}

// TimeDelta ------------------------------------------------------------------

// static
TimeDelta TimeDelta::FromQPCValue(LONGLONG qpc_value) {
  return QPCValueToTimeDelta(qpc_value);
}