A Timely Re-Discovery

Most users have no need to worry about the internals of a computer: point, click, run, play games, and spend money if they want something faster. However one of the important features in a system relates to how they measure time. A modern system relies on a series of both hardware and software timers, both internal and external, in order to maintain a linear relation between requests, commands, execution, and interrupts.

The timers have different users, such as following instructions, maintaining video coherency, tracking real time, or managing the flow of data. Timers can (but not always) use external references to ensure their own consistency – damage, unexpected behavior, and even thermal environments can cause timers to lose their accuracy.

Timers are highly relevant for benchmarking. Most benchmark results are a measure of work performed per unit time, or in a given time. This means that both the numerator and the denominator need to be accurate: the system has to be able to measure what amount of work has been processed, and how long it took to do it in. Ideally there is no uncertainty in either of those values, giving an accurate result.

With the advent of Windows 8, between Intel and Microsoft, the way that the timers were used in the OS were changed. Windows 8 had the mantra that it had to ‘support all devices’, all the way from the high-cost systems down to the embedded platforms. Most of these platforms use what is called an RTC, a ‘real time clock’, to maintain the real-world time – this is typically a hardware circuit found in almost all devices that need to keep track of time and the processing of data. However, compared to previous versions of Windows, Microsoft changed the way it uses timers, such that it was compatible with systems that did not have a hardware-based RTC, such as low-cost and embedded devices. The RTC was an extra cost that could be saved if the software was built to do so.

Ultimately, any benchmark software in play has to probe the OS to determine the current time during the benchmark to then at the end give an accurate result. However the concept of time, without an external verifying source, is an arbitrarily defined constant – without external synchronization, there is no guarantee that ‘one second’ on the system equals ‘one second’ in the real world. For the most part, all of us rely on the reporting from the OS and the hardware that this equality is true, and there are a lot of hardware/software engineers ensuring that this is the case.

However, back in 2013, it was discovered that it was fairly easy to 'distort time' on a Windows 8 machine. After loading into the operating system, any adjustment in the base frequency of the processor, which is usually 100 MHz, can cause the ‘system time’ to desynchronise with ‘real time’. This was a serious issue in the extreme overclocking community, where world records require the best system tuning: when comparing two systems at the same frequency but with different base clock adjustments, up to a 7% difference in results were observed when there should have been a sub-1% difference. This was down to how Windows was managing its timers, and was observed on most modern systems.

For home users, most would suspect that this is not an issue. Most users tend not to adjust the base frequencies of their systems manually. For the most part that is true. However, as shown in some of our motherboard testing over the years, frequency response due to default BIOS settings can provide an observable clock drift around a specified value, something which can be exacerbated by the thermal performance. Having a system with observable clock drift, and subsequent timing drift, is not a good thing. It relies on the accuracy and quality of the motherboard components, as well as the state of the firmware. This issue has formally been classified as ‘RTC Bias’.

The extreme overclocking community, after analysing the issue, found a solution: forcing the High Performance Event Timer, known as HPET, found in the chipset. Some of our readers will have heard of HPET before, however our analysis is more interesting than it first appears.

Why A PC Has Multiple Timers

Aside from the RTC, a modern system makes use of many timers. All modern x86 processors have a Time Stamp Counter (TSC) for example, that counts the number of cycles from a given core, which was seen back in the day as a high-resolution, low-overhead way to get CPU timing information. There is also a Query Performance Counter (QPC), a Windows implementation that relies on the processor performance metrics to get a better resolution version of the TSC, which was developed in the advent of multi-core systems where the TSC was not applicable. There is also a timer function provided by the Advanced Configuration and Power Interface (ACPI), which is typically used for power management (which means turbo related functionality). Legacy timing methodologies, such as the Programmable Interval Timer (PIT), are also in use on modern systems. Along with the High Performance Event Timer, depending on the system in play, these timers will run at different frequencies.

Ryzen 7 2700X with HPET Off
ASUS ROG Crosshair VII Hero

Core i7-8700K with HPET Off
ASRock Z370 Gaming i7

Core i7-6950X with HPET On
ASUS X99-E-10G

Core i7-6700K with HPET Off

Core i5-5200U with HPET Off

Core i7-3960X with HPET Off

The timers will be used for different parts of the system as described above. Generally, the high performance timers are the ones used for work that is more time sensitive, such as video streaming and playback. HPET, for example, was previously referred to by its old name, the Multimedia Timer. HPET is also the preferred timer for a number of monitoring and overclocking tools, which becomes important in a bit.

With the HPET timer being at least 10 MHz as per the specification, any code that requires it is likely to be more in sync with the real-world time (the ‘one-second in the machine’ actually equals ‘one-second in reality’) than using any other timer.

In a standard Windows installation, the operating system has access to all the timers available. The software used above is a custom tool developed to show if a system has any of those four timers (but the system can have more). For the most part, depending on the software instructions in play, the operating system will determine which timer is to be used – from a software perspective, it is fundamentally difficult to determine which timers will be available, so the software is often timer agnostic. There is not much of a way to force an algorithm to use one timer or another without invoking specific hardware or instructions that rely on a given timer, although the timers can be probed in software like the tool above.

HPET is slightly different, in that it can be forced to be the only timer. This is a two stage process:

The first stage is that it needs to be enabled in the BIOS. Depending on the motherboard and the chipset, there may or may not be an option for this. The options are usually for enable/disable, however this is not a simple on/off switch. When disabled, HPET is truly disabled. However, when enabled, this only means that the HPET is added to the pool of potential timers that the OS can use.

The second stage is in the operating system. In order to force HPET as the only timer to be used for the OS, it has to be explicitly mentioned in the system Boot Configuration Data (BCD). In standard operation, HPET is not in the BCD, so it remains in the pool of timers for the OS to use. However, for software to guarantee that the HPET is the only timer running, the software will typically request to make a change and make an accompanying system reboot to ensure the software works as planned. Ever wondered why some overclocking software requests a reboot *before* starting the overclock? One of the reasons is sometimes to force HPET to be enabled.

This leads to four potential configuration implementations:

  1. BIOS enabled, OS default: HPET is in list of potential timers
  2. BIOS enabled, OS forced: HPET is used in all situations
  3. BIOS disabled, OS default: HPET is not available
  4. BIOS disabled, OS forced: HPET is not available

Again, for extreme overclockers relying on benchmark results to be equal on Windows 8/10, HPET has to be forced to ensure benchmark consistency. Without it, the results are invalid.

The Effect of a High Performance Timer

With a high performance timer, the system is able to accurately determine clock speeds for monitoring software, or video streaming processing to ensure everything hits in the right order for audio and video. It can also come into play when gaming, especially when overclocking, ensuring data and frames are delivered in an orderly fashion, and has been shown to reduce stutter on overclocked systems. And perhaps most importantly, it avoids any timing issues caused by clock drift.

However, there are issues fundamental to the HPET design which means that it is not always the best timer to use. HPET is a continually upward counting timer, which relies on register recall or comparison metrics rather than a ‘set at x and count-down’ type of timer. The speed of the timer can, at times, cause a comparison to fail, depending on the time to write the compared value to the register and that time already passing. Using HPET for very granular timing requires a lot of register reads/writes, adding to the system load and power draw, and in a workload that requires explicit linearity, can actually introduce additional latency. Usually one of the biggest benefits to disabling HPET on some systems is the reduction in DPC Latency, for example.

An Audit after our Ryzen 2000-series Review AMD and Intel Have Different HPET Guidance
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  • ReverendCatch - Wednesday, April 25, 2018 - link

    I feel like what this ultimately means is Intel has issues with HPET, and that the results everyone else are getting are the problematic ones, not you guys. By forcing a more precise timer, intel's... I dunno... "advantage" as it were is eliminated.

    Seriously, AMD is 1% or less variance despite the timer used. Intel is upward of 30% or more. To me, that is a giant red flag.
  • nevcairiel - Wednesday, April 25, 2018 - link

    Except that normal systems are not going to force HPET, so the more real-world realistic tests/results should really be used.
  • ReverendCatch - Wednesday, April 25, 2018 - link

    The question I suppose I have is, are the results even real or legit at that point. Why does the intel suffer tremendously when using an accurate timer, and pulls ahead when not?

    How does that not sound fishy to you?
  • tmediaphotography - Wednesday, April 25, 2018 - link

    From my reading of the article, it seems that Intel takes a larger hit because they use a more accurate HPET timer (24Mhz on the 8700K), and thus it is more taxing on the system. The calls are very much under the umbrella of things more negatively affected by Spectre, and as the i7 8700K system had an HPET rate at closing on 2x as much as the R7 2700K, it stands to reason the i7 is going to benefit much more from it being turned off.

    tl;dr, the more accurate timer is much more needy on the system, and the system under spectre/meltdown takes an even larger hit at the IO calls to it.
  • Billy Tallis - Wednesday, April 25, 2018 - link

    The HPET can run at a higher frequency without generating more CPU overhead, because it's really just a counter. Making that counter's value grow more quickly doesn't mean the CPU gets more interrupts per second.
  • patrickjp93 - Wednesday, April 25, 2018 - link

    Because it makes perfect sense. Intel's losing more clock cycles since it is at vastly higher clock speeds, and it has Meltdown to contend with on top of Spectre. HPET from my cursory reading is 4 system calls compared to just 2 for TSC+lapic. The performance hit of that should then surprise no one.

    With AVX-512, Intel has a lot of very high throughput instructions that AMD doesn't. If your software uses them, Intel pulls ahead vs. the best equivalent you could write for Epyc. That's not fishy. You're just taking the more optimal path to solving your problem. When Cascade Lake X and Cannon/Ice Lake arrive, this will all be fixed at the hardware level and the overhead will disappear.
  • Cooe - Wednesday, April 25, 2018 - link

    Except that isn't actually true in practice for a wide variety of actual AVX-512 enabled workloads. Running those insanely wide registers drastically increases power draw & thermal ouput and as a result clock-speeds take a nose-dive. In certain SIMD workloads capable of AVX acceleration, this clock-dropoff is so large that EPYC outperforms Skylake-X's AVX-512 support using much much narrower AVX2 instructions/registers simply because it can maintain vastly higher clock-speeds during the load.

    Heck AnandTech even verified this with their own testing way back when. https://www.anandtech.com/show/12084/epyc-benchmar...
  • patrickjp93 - Wednesday, April 25, 2018 - link

    If you're using the widest registers, yes, but there were also a lot of 128 and 256-bit extensions added that were missing from the AVX/2 stack. And Intel will bring the power draw down and the clocks up over time.
  • Dolda2000 - Wednesday, April 25, 2018 - link

    The HPET, despite its name, is not more accurate. The TSC timer is accurate to CPU clock-cycle precision, which is usually more than two orders of magnitude better than the HPET.
  • Billy Tallis - Wednesday, April 25, 2018 - link

    The difference between accuracy and precision is probably important here. TSC is definitely far more precise, but overclocking can make it much less accurate.

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