Windows 7 计时功能 - 如何正确使用 GetSystemTimeAdjustment？

Windows 7 timing functions - How to use GetSystemTimeAdjustment correctly?

提问by fdlm

I ran some tests using the GetSystemTimeAdjustmentfunction on Windows 7, and got some interesting results which I cannot explain. As fas as I understand, this method should return if the system time is synchronized periodically and if it is, at which interval and with which increment it is updated (see GetSystemTimeAdjustment function on MSDN).

我GetSystemTimeAdjustment在 Windows 7 上使用该函数进行了一些测试，得到了一些我无法解释的有趣结果。据我所知，如果系统时间是定期同步的，则此方法应该返回，如果是，则以哪个时间间隔和哪个增量更新（请参阅 MSDN 上的 GetSystemTimeAdjustment 函数）。

From this I follow that if I query the system time for example using GetSystemTimeAsFileTimerepeatingly I should either get no change (the system clock has not been updated), or a change which is a multiple of the increment retrieved by GetSystemTimeAdjustment. Question one:Is this assumption correct?

从这里我遵循，如果我查询系统时间，例如GetSystemTimeAsFileTime重复使用，我应该没有变化（系统时钟尚未更新），或者变化是由GetSystemTimeAdjustment. 问题一：这个假设正确吗？

Now consider the following testing code:

现在考虑以下测试代码：

#include <windows.h>
#include <iostream>
#include <iomanip>

int main()
{
    FILETIME fileStart;
    GetSystemTimeAsFileTime(&fileStart);
    ULARGE_INTEGER start;
    start.HighPart = fileStart.dwHighDateTime;
    start.LowPart = fileStart.dwLowDateTime;

    for (int i=20; i>0; --i)
    {
        FILETIME timeStamp1;
        ULARGE_INTEGER ts1;

        GetSystemTimeAsFileTime(&timeStamp1);

        ts1.HighPart = timeStamp1.dwHighDateTime;
        ts1.LowPart  = timeStamp1.dwLowDateTime;

        std::cout << "Timestamp: " << std::setprecision(20) << (double)(ts1.QuadPart - start.QuadPart) / 10000000 << std::endl;

    }

    DWORD dwTimeAdjustment = 0, dwTimeIncrement = 0, dwClockTick;
    BOOL fAdjustmentDisabled = TRUE;
    GetSystemTimeAdjustment(&dwTimeAdjustment, &dwTimeIncrement, &fAdjustmentDisabled);

    std::cout << "\nTime Adjustment disabled: " << fAdjustmentDisabled
        << "\nTime Adjustment: " << (double)dwTimeAdjustment/10000000
        << "\nTime Increment: " << (double)dwTimeIncrement/10000000 << std::endl;

}

It takes 20 timestamps in a loop and prints them to the console. In the end it prints the increment with which the system clock is updated. I would expect the differences between the timestamps printed in the loop to be either 0 or multiples of this increment. However, I get results like this:

它循环使用 20 个时间戳并将它们打印到控制台。最后，它打印系统时钟更新的增量。我希望循环中打印的时间戳之间的差异是此增量的 0 或倍数。但是，我得到这样的结果：

Timestamp: 0
Timestamp: 0.0025000000000000001
Timestamp: 0.0074999999999999997
Timestamp: 0.01
Timestamp: 0.012500000000000001
Timestamp: 0.014999999999999999
Timestamp: 0.017500000000000002
Timestamp: 0.022499999999999999
Timestamp: 0.025000000000000001
Timestamp: 0.0275
Timestamp: 0.029999999999999999
Timestamp: 0.032500000000000001
Timestamp: 0.035000000000000003
Timestamp: 0.040000000000000001
Timestamp: 0.042500000000000003
Timestamp: 0.044999999999999998
Timestamp: 0.050000000000000003
Timestamp: 0.052499999999999998
Timestamp: 0.055
Timestamp: 0.057500000000000002

Time Adjustment disabled: 0
Time Adjustment: 0.0156001
Time Increment: 0.0156001

So it appears that the system time is updated using an interval of about 0.0025 seconds and not 0.0156 seconds as return by GetSystemTimeAdjustment.

因此，系统时间似乎是使用大约 0.0025 秒而不是 0.0156 秒的间隔更新的GetSystemTimeAdjustment。

Question two:What is the reason for this?

问题二：这是什么原因？

回答by Arno

The GetSystemTimeAsFileTimeAPI provides access to the system's wall clock in file time format.

该GetSystemTimeAsFileTimeAPI可以访问系统的挂钟文件时间格式。

A 64-bit FILETIME structure receives the system time as FILETIME in 100ns units, which have been expired since Jan 1, 1601. The call to GetSystemTimeAsFileTimetypically requires 10 ns to 15 ns.

64 位 FILETIME 结构以 100ns 为单位接收系统时间作为 FILETIME，自 1601 年 1 月 1 日起已过期。调用GetSystemTimeAsFileTime通常需要 10 ns 到 15 ns。

In order to investigate the real accuracy of the system time provided by this API, the granularity that comes along with the time values needs to be discussed. In other words: How often is the system time updated? A first estimate is provided by the hidden API call:

为了调查此 API 提供的系统时间的真实准确性，需要讨论与时间值一起出现的粒度。换句话说：系统时间多久更新一次？第一个估计值由隐藏的 API 调用提供：

NTSTATUS NtQueryTimerResolution(OUT PULONG MinimumResolution, 
                                OUT PULONG MaximumResolution, 
                                OUT PULONG ActualResolution);

NtQueryTimerResolutionis exported by the native Windows NT library NTDLL.DLL. The ActualResolution reported by this call represents the update period of the system time in 100 ns units, which does not necessarily match the interrupt period. The value depends on the hardware platform. Common hardware platforms report 156,250 or 100,144 for ActualResolution; older platforms may report even larger numbers; newer systems, particulary when HPET(High Precision Event Timer) or constant/invariant TSCare supported, may return 156,001 for ActualResolution.

NtQueryTimerResolution由本机 Windows NT 库 NTDLL.DLL 导出。本次调用上报的ActualResolution以100ns为单位表示系统时间的更新周期，不一定与中断周期匹配。该值取决于硬件平台。常见的硬件平台为ActualResolution报告 156,250 或100,144；较旧的平台可能会报告更大的数字；较新的系统，尤其是在HPET（高精度事件计时器）或constant/invariant TSC受支持时，可能会为ActualResolution返回156,001。

This is one of the heartbeats controlling the system. The MinimumResolutionand the ActualResolutionare relevant for the multimedia timer configuration.

这是控制系统的心跳之一。该MinimumResolution和ActualResolution是相关的多媒体计时器的配置。

The ActualResolutioncan be set by using the API call

该ActualResolution可以使用API调用来设置

NTSTATUS NtSetTimerResolution(IN ULONG RequestedResolution,
                              IN BOOLEAN Set,
                              OUT PULONG ActualResolution);

or via the multimedia timer interface

或通过多媒体定时器接口

MMRESULT timeBeginPeriod(UINT uPeriod);

with the value of uPeriod derived from the range allowed by

uPeriod 的值来自允许的范围

MMRESULT timeGetDevCaps(LPTIMECAPS ptc, UINT cbtc );

which fills the structure

填充结构

typedef struct {
  UINT wPeriodMin;
  UINT wPeriodMax;
} TIMECAPS;

Typical values are 1 ms for wPeriodMin and 1,000,000 ms for wPeriodMax.

wPeriodMin 的典型值为 1 ms，wPeriodMax 的典型值为 1,000,000 ms。

There is an unfortunate misinterpretation when looking an the min/max values here:

在这里查看最小值/最大值时存在一个不幸的误解：

• wPeriodMindefines the minimum period, which is clear in this context.
• MinimumResolutionreturned by NtQueryTimerResolutionon the other hand specifies a resolution. The lowest obtainable resolution (MinimumResolution) is in the range of up to about 20 ms, while the highest obtainable resolution (MaximumResolution) can be 0.5 ms. However, the 0.5 ms resulution is not accessable through a of timeBeginPeriod.

• wPeriodMin定义了最短时间段，在此上下文中很清楚。
• 另一方面，MinimumResolution返回NtQueryTimerResolution的分辨率指定了分辨率。可获得的最低分辨率 ( MinimumResolution) 可达约 20 毫秒，而可获得的最高分辨率 ( MaximumResolution) 可达到 0.5 毫秒。但是，0.5 毫秒的结果无法通过timeBeginPeriod.

The multimedia timer interface handles periods and NtQueryTimerResolution() handles resolutions (reciprocal value of period).

多媒体计时器接口处理周期，NtQueryTimerResolution() 处理分辨率（周期的倒数）。

Summary:GetSystemTimeAdjustmentis not the function to look at. This function only tells how and if time-changes are done. Depending on the setting of the multimedia timer interface timeBeginPeriod, the progress of time may be done more often and in smaller portions. Use NtQueryTimerResolutionto receive the actual time increment. And be aware that the setting of the multimedia timer API does influence the values. (Example: When the media player is showing a video, the times are getting short.)

总结：GetSystemTimeAdjustment不是看功能。此功能仅说明如何以及是否完成时间更改。根据多媒体计时器界面的设置，timeBeginPeriod时间的进度可能会更频繁，更小部分。使用NtQueryTimerResolution接收的实际时间增量。请注意，多媒体计时器 API 的设置确实会影响这些值。（例如：当媒体播放器播放视频时，时间越来越短。）

I diagnosed windows time matters to a large extent. Some of the results can be found here.

我诊断出 Windows 时间在很大程度上很重要。一些结果可以在这里找到。

Note: Time Adjustment: 0.0156001clearly identifies windows VISTA or higher with HPETand/or constant/invariant TSCon your system.

注意：时间调整：0.0156001清楚地识别系统上HPET和/或constant/invariant TSC上的Windows VISTA 或更高版本。

Implementation:If you want to catch the time transition:

实现：如果你想捕捉时间过渡：

FILETIME FileTime,LastFileTime;
long long DueTime,LastTime;
long FileTimeTransitionPeriod; 

GetSystemTimeAsFileTime(&FileTime);
for (int i = 0; i < 20; i++) {
  LastFileTime.dwLowDateTime = FileTime.dwLowDateTime;
  while (FileTime.dwLowDateTime == LastFileTime.dwLowDateTime) GetSystemTimeAsFileTime(&FileTime); 
  // enough to just look at the low part to catch the transition
  CopyMemory(&DueTime,&FileTime,sizeof(FILETIME));
  CopyMemory(&LastTime,&LastFileTime,sizeof(FILETIME));
  FileTimeTransitionPeriod = (long)(DueTime-LastTime);
  fprintf(stdout,"transition period: % 7.4lf ms)\n",(double)(FileTimeTransitionPeriod)/10000);
}   

// WARNING: This code consumes 100% of the cpu for 20 file time increments.
// At the standard file time increment of 15.625 ms this corresponds to 312.5ms!

But:When the filetime transition is very short (e.g. set by timeBeginPeriod(wPeriodMin)) any output like fprintfor std::coutmight destroy the result because it delays the loop. In such cases I'd recommend to store the 20 results in a data structure and do the output afterwards.

但是：当FILETIME过渡很短（例如通过集timeBeginPeriod(wPeriodMin)）像任何输出fprintf或者std::cout是因为它延缓了循环可能会破坏的结果。在这种情况下，我建议将 20 个结果存储在一个数据结构中，然后再进行输出。

And:The filetime transition may not always be the same. It may well be that the file time increment does not match the update period. See the link above to get more details and examples for this bahavior.

并且：文件时间转换可能并不总是相同的。很可能是文件时间增量与更新周期不匹配。请参阅上面的链接以获取有关此行为的更多详细信息和示例。

Edit:Use caution when calling timeBeginPeriod, as frequent calls can significantly affect the system clockMSDN. This behavior applies up to Windows version 7.

编辑：调用 timeBeginPeriod 时要小心，因为频繁调用会显着影响系统时钟MSDN。此行为适用于 Windows 版本 7。

Calls to timeBeginPeriod/timeEndPeriodor NtSetTimerResolutionmay change the system time by as much as ActualResolution. Doing it very often results in considerable changes of the system time. However, when the calls are made at or near the transition of the system time, deviations are much less. Polling for a system time transition/increment ahead of calls to the above function is advised for demanding applications like NTP clients. Synchronizing to an NTP server is difficult when unwanted jumps in the systemtime progess occurs.

调用timeBeginPeriod/timeEndPeriod或NtSetTimerResolution可能将系统时间更改为ActualResolution。经常这样做会导致系统时间发生相当大的变化。但是，当在系统时间转换时或附近进行调用时，偏差要小得多。对于要求苛刻的应用程序（如 NTP 客户端），建议在调用上述函数之前轮询系统时间转换/增量。当系统时间进程中发生不需要的跳跃时，同步到 NTP 服务器很困难。

回答by Hans Passant

You are actually profiling how long one pass through the for() loop takes. I get some more variability but 5 milliseconds is about right, console output is not very fast. Arbitrarily add some more std::cout statements to slow it down.

您实际上是在分析一次通过 for() 循环所需的时间。我得到了更多的可变性，但 5 毫秒是正确的，控制台输出不是很快。任意添加一些更多的 std::cout 语句来减慢它的速度。

回答by Conrad Frix

GetSystemTimeAsFileTime's resolution is dependent on the system. If seen it claimed that its between 10ms and 55ms. Commentators on the MSDN documentput it at 15ms and "sub millisecond". What it actually is seems unclear but I've never seen its resolution claimed as equal to the 100 ns precision of the timestamp.

GetSystemTimeAsFileTime 的分辨率取决于系统。如果看到它声称它在 10ms 和 55ms 之间。MSDN 文档的评论员将其定为 15 毫秒和“亚毫秒”。它实际上似乎不清楚，但我从未见过它的分辨率声称等于时间戳的 100 ns 精度。

This means there's always going to be some variance and its also the reason people use QueryPerformanceFrequencyinstead.

这意味着总会有一些差异，这也是人们使用QueryPerformanceFrequency的原因。