AMD and Intel Have Different HPET Guidance

A standard modern machine, with a default BIOS and a fresh Windows operating system, will sit on the first situation in the table listed above: the BIOS has HPET enabled, however it is not explicitly forced in the operating system. If a user sets up their machine with no overclocking or monitoring software, which is the majority case, then this is the implementation you would expect for a desktop.

AMD

We reached out to AMD and Intel about their guidance on HPET, because in the past it has both been unclear as well as it has been changed. We also reached out to motherboard manufacturers for their input.

For those that remember the Ryzen 7 1000-series launch, about a year ago from now, one point that was lightly mentioned among the media was that in AMD’s press decks, it was recommended that for best performance, HPET should be disabled in the BIOS. Specifically it was stated that:

Make sure the system has Windows High Precision Event Timer (HPET) disabled. HPET can often be disabled in the BIOS. [T]his can improve performance by 5-8%.

The reasons at the time were unclear as to why, but it was a minor part in the big story of the Zen launch so it was not discussed in detail. However, by the Ryzen 5 1000-series launch, that suggestion was no longer part of the reviewer guide. By the time we hit the Ryzen-2000 series launched last week, the option to adjust HPET in the BIOS was not even in the motherboards we were testing. We cycled back to AMD about this, and they gave the following:

The short of it is that we resolved the issues that caused a performance difference between on/off. Now that there is no need to disable HPET, there is no need for a toggle [in the BIOS].

Interestingly enough, with our ASUS X470 motherboard, we did eventually find the setting for HPET – it was not in any of the drop down menus, but it could be found using their rather nice ‘search’ function. I probed ASUS about whether the option was enabled in the BIOS by default, given that these options were not immediately visible, and was told:

It's enabled and never disabled, since the OS will ignore it by default. But if you enable it, then the OS will use it – it’s always enabled, that way if its needed it is there, as there would be no point in pulling it otherwise.

So from an AMD/ASUS perspective, the BIOS is now going to always be enabled, and it needs to be forced in the OS to be used, however the previous guidance about disabling it in the BIOS has now gone, as AMD expects performance parity.

It is worth noting that AMD’s tool, Ryzen Master, requires a system restart when the user first loads it up. This is because Ryzen Master, the overclocking and monitoring tool, requires HPET to be forced in order to do what it needs to do. In fact, back at the Ryzen 7 launch in 2017, we were told:

AMD Ryzen Master’s accurate measurements present require HPET. Therefore it is important to disable HPET if you already installed and used Ryzen Master prior to game benchmarking.

Ultimately if any AMD user has Ryzen Master installed and has been run at any point, HPET is enabled, even if the software is not running or uninstalled. The only way to stop it being forced in the OS is with a command to chance the value in the BCD, as noted above.

For the Ryzen 2000-series launch last week, Ryzen Master still requires HPET to be enabled to run as intended. So with the new guidance that HPET should have minimal effect on benchmarks, the previous guidance no longer applies.

Ryzen Master is not the only piece of software that requires HPET to be forced in order to do what it needs to do. For any of our readers that have used overclocking software and tools before, or even monitoring tools such as fan speed adjusters – if those tools have requested a restart before being used properly, there is a good chance that in that reboot the command has been run to enable HPET. Unfortunately it is not easy to generate a list, as commands and methods may change from version to version, but it can apply to CPU and GPU overclocking.

Intel

The response we had from Intel was a little cryptic:

[The engineers recommend that] as far as benchmarking is concerned, it should not matter whether or not HPET is enabled or not. There may be some applications that may not function as advertised if HPET is disabled, so to be safe, keep it enabled, across all platforms. Whatever you decide, be consistent across platforms.

A cold reading of this reply would seem to suggest that Intel is recommended HPET to be forced and enabled, however my gut told me that Intel might have confused ‘on’ in the BIOS with ‘forced’ through the OS, and I have asked them to confirm.

Looking back at our coverage of Intel platforms overall, HPET has not been mentioned to any sizeable degree. I had two emails back in 2013 from a single motherboard manufacturer stating that disabling HPET in the BIOS can minimise DPC latency on their motherboard, however no comment was made about general performance. I cannot find anything explicitly from Intel though.

A Timely Re-Discovery Forcing HPET On, Plus Spectre and Meltdown Patches
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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.
    Reply
  • 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. Reply
  • 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?
    Reply
  • 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.
    Reply
  • 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. Reply
  • 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.
    Reply
  • 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...
    Reply
  • 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. Reply
  • 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. Reply
  • 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. Reply

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