The reciprocal of the frequency is called the period or tick interval and is expressed in an appropriate International System of Units SI time unit for example, second, millisecond, microsecond, or nanosecond.
The resolution of the timer is equal to the period. Resolution determines the ability to distinguish between any two time stamps and places a lower bound on the smallest time intervals that can be measured. This is sometimes called the tick resolution. This uncertainty is called a quantization error. For typical time-interval measurements, this effect can often be ignored because the quantizing error is much smaller than the time interval being measured.
However, if the period being measured is small and approaches the resolution of the timer, you will need to consider this quantizing error. The size of the error introduced is that of one clock period. QueryPerformanceFrequency returns the frequency of QPC , and the period and resolution are equal to the reciprocal of this value. The performance counter frequency that QueryPerformanceFrequency returns is determined during system initialization and doesn't change while the system is running.
Cases might exist where QueryPerformanceFrequency doesn't return the actual frequency of the hardware tick generator. For example, in many cases, QueryPerformanceFrequency returns the TSC frequency divided by ; and on Hyper-V, the performance counter frequency is always 10 MHz when the guest virtual machine runs under a hypervisor that implements the hypervisor version 1. QueryPerformanceCounter reads the performance counter and returns the total number of ticks that have occurred since the Windows operating system was started, including the time when the machine was in a sleep state such as standby, hibernate, or connected standby.
These examples show how to calculate the tick interval and resolution and how to convert the tick count into a time value. Example 1. QueryPerformanceFrequency returns the value 3,, on a particular machine. What is the tick interval and resolution of QPC measurements on this machine? The tick interval, or period, is the reciprocal of 3,,, which is 0.
Therefore, each tick represents the passing of nanoseconds. Time intervals smaller than nanoseconds can't be measured on this machine. Example 2. On the same machine as the preceding example, the difference of the values returned from two successive calls to QPC is 5. How much time has elapsed between the two calls?
It takes time to access read the tick counter from software, and this access time can reduce the precision of the of the time measurement. This is because the minimum interval time the smallest time interval that can be measured is the larger of the resolution and the access time. For example, consider a hypothetical hardware timer with a nanosecond resolution and an nanosecond access time. Thus, the precision would be nanoseconds not nanoseconds as shown in this calculation.
If the access time is greater than the resolution, don't try to improve the precision by guessing. In other words, it's an error to assume that the time stamp is taken precisely in the middle, or at the beginning or the end of the call. By contrast, consider the following example in which the QPC access time is only 20 nanoseconds and the hardware clock resolution is nanoseconds.
Here the precision is limited by the clock resolution. In practice, you can find time sources for which the time required to read the counter is larger or smaller than the resolution. In either case, the precision will be the larger of the two. This table provides info on the approximate resolution, access time, and precision of a variety of clocks.
Note that some of the values will vary with different processors, hardware platforms, and processor speeds. Because QPC uses a hardware counter, when you understand some basic characteristics of hardware counters, you gain understanding about the capabilities and limitations of QPC. The most commonly used hardware tick generator is a crystal oscillator. The crystal is a small piece of quartz or other ceramic material that exhibits piezoelectric characteristics that provide an inexpensive frequency reference with excellent stability and accuracy.
This frequency is used to generate the ticks counted by the clock. The accuracy of a timer refers to the degree of conformity to a true or standard value. If the frequency of oscillation is too high, the clock will 'run fast', and measured intervals will appear longer than they really are; and if the frequency is too low, the clock will 'run slow', and measured intervals will appear shorter than they really are. For typical time-interval measurements for short duration for example, response time measurements, network latency measurements, and so on , the accuracy of the hardware oscillator is usually sufficient.
However, for some measurements the oscillator frequency accuracy becomes important, particularly for long time intervals or when you want to compare measurements taken on different machines. The remainder of this section explores the effects of the oscillator accuracy.
The crystals' frequency of oscillation is set during the manufacturing process and is specified by the manufacturer in terms of a specified frequency plus or minus a manufacturing tolerance expressed in 'parts per million' ppm , called the maximum frequency offset. By substituting the phrase parts per million with microseconds per second, we can apply this frequency offset error to time-interval measurements.
Accordingly, when measuring a 1 second interval, it would run fast and measure a 1 second interval as 0. A convenient reference is that a frequency error of ppm causes an error of 8. This table presents the measurement uncertainty due to the accumulated error for longer time intervals. The preceding table shows that for small time intervals the frequency offset error can often be ignored.
However for long time intervals, even a small frequency offset can result in a substantial measurement uncertainty. Although crystals with much tighter frequency offset tolerances are available, they are more expensive and thus are not used in most computers. To reduce the adverse effects of this frequency offset error, recent versions of Windows, particularly Windows 8, use multiple hardware timers to detect the frequency offset and compensate for it to the extent possible.
This calibration process is performed when Windows is started. As the following examples show, the frequency offset error of a hardware clock influences the achievable accuracy, and the resolution of the clock can be less important. This means that the actual frequency would be 1,, Hz. If we measured a time interval of 24 hours, our measurement would be 4. Suppose the processor TSC clock is controlled by a crystal oscillator and has specified frequency of 3 GHz.
In spite of the impressive resolution, a time-interval measurement of 24 hours will still be 4. This shows that a high resolution TSC clock doesn't necessarily provide more accurate measurements than a lower resolution clock. Example 3. Consider using two different computers to measure the same 24 hour time interval. The first thing that you need to find out is if HPET is supported by a computer system. Since there are that many different versions out there it is hard to say where you will find the setting on your system.
Enabling or disabling the timer in BIOS is only one part of the change that you have to make though. You need to run the following commands on the command line in Windows to enable or disable the exclusive use of the HPET timer.
Note : We recommend that you create a backup of the operating system before you make these changes. There are a couple of tests that you can run to see if the performance is better when the timer is enabled or disabled on your system. While you can check that by playing games or other applications as well, it is usually a good idea to run the following two programs as they provide hard data. WinTimerTest is a lightweight portable program that displays timer related information to you.
You should get a value of around You can download it with a click on the following link: Windows Timer Tester. DPC Latency Checker is the second program that tests how the computer handles real-time data streams. I suggest you run both programs before you make any change to your system, and then again after you have made changes. Some notice slow downs and others that micro-stutters go away after disabling the timer on their system. So, it is definitely a good idea to test all possible settings to see if one makes a difference for you if you noticed issues in first hand or improve the performance of the system.
Martin, you missed the point. Look at the total command lines you posted. They are identical. Both say delete. What sort of improvement could you expect to get? Is it noticeable or simply notional much of the time? John, that is understandable. Some users reported more fps and better response times in games, while others claimed that turning it off removed micro-stutters in games.
If you are experiencing issues while playing games or doing real-time activities, then it may make sense to try the tweak. If everything is golden and all you can hope for are a couple fps more which you may not even notice then it is not really something that I would start to test as the gain is not worth it. It may also help to maximize the NT timer resolution, which can help lower your DPC latency and make certain programs much more responsive.
There are two programs I know of that can do this:. PliotronX said:. JoeRambo Golden Member. Jun 13, 1, 1, Whether HPET gives any benefits or disadvantages is really dependent on the kind of hardware you have. JoeRambo said:. And disabling HPET is very stupid idea in general. Since Windows 7, the operating system runs tests on the underlying hardware to see which hardware is best used for timekeeping.
This is the architectural behavior moving forward. TSC reads are much more efficient and do not incur the overhead associated with a ring transition or access to a platform resource. HPET is programmable timer with plenty of functions. For example on modern operating systems with so called tickless mode, OS will look at it's timer, job etc queues and will set HPET to fire exactly when it is needed for example 66ms later, when some of your app demands wakeup in some system call and not periodically every 10 or 15 ms needlessly burning CPU cycles on doing nothing.
Invariant TSC is cool feature, engineering marvel from Intel in its day. But having same "clock" tick value across all CPU cores in the system is good for time keeping and measurement and sadly can't magically bring back CPU from sleep.
Interesting, didn't know about TSC and the Core line. I'll play with it when I find some time VirtualLarry No Lifer. Aug 25, 52, 7, Since disabling HPET, I haven't noticed anything out of the ordinary other than faster loading, greater responsiveness and smoother gaming. VirtualLarry said:. Surely, it has benefits, or it wouldn't have been created.
Note "wall clock". Invariant TSC is great for keeping track of time, but it's not an event timer source, capable of firing off events interrupts at certain times. HPET is. And your comment about sleep states shows how ignorant you are about these things. He was talking about processor sleep states eg. Or, it could be placebo. I fully admit I don't completely understand this subject matter, but then neither do you nor anyone else in this thread.
Anyway, sleep states don't appear to be affected at all. It is not about effecting sleep states. Consider the following imaginary scenario - your system has ONE cpu and it has no more work to do, but knows it needs to wake in 1ms to execute some periodic work.
Without timer generating interrupt after 1ms it can't reliably wakeup. So that's how timer event source differs from timer source. But doesn't that ring a bell already? That's where snake oil induced improvement started in original post. In my personal opinion it is wise to leave it as is just in case Windows 8.
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