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post #1 of 23 (permalink) Old 12-05-2012, 04:49 AM - Thread Starter
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I have an Asus P8Z77-V motherboard and in the bios there is a setting for High Precision Event Timer.

I noticed my DPC latency decreased when i disabled that feature but i have no idea what HPET is used for? is it safe to have it off?

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post #2 of 23 (permalink) Old 12-05-2012, 11:02 AM
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It's possible that disabling HPET simply made DPCs harder to measure accurately, and wasn't doing much to them. What sort of measured decrease are you seeing?

HPET is used for synchronizing a wide variety of things (most importantly thread scheduling in some programs), and should normally be left enabled if you have an OS that supports it. It probably won't noticeably hurt to disable it, but there really shouldn't be a need to.

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post #3 of 23 (permalink) Old 12-05-2012, 11:15 AM
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post #4 of 23 (permalink) Old 12-06-2012, 01:24 AM - Thread Starter
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I found the below searching on google and it looks like I get a more stable latency with it enabled in bios and in windows. By default it was just enabled in bios but disabled in windows.

Quote:
Originally Posted by Neowinian³ 
By default Windows 7 uses different timers in the CPU to calculate stuff. HPET is the newest and best of these timers, but because of default combination of timers it takes longer time for CPU to keep up all the timers and sync between them. Forcing Windows to use HPET only improves performance and leads to greater FPS.

Steps to enable this tweak:

1. Enable HPET in BIOS. If you have HPET option in BIOS then your hardware can support HPET.

2. Enable HPET in Windows by giving this command in admin credential CMD:
bcdedit /set useplatformclock true

3. Reboot
Quote:
Originally Posted by Neowinian³ 
Enabling HPET in BIOS is just half way of enabling HPET, it needs to be enabled in OS too, and in a way that it's the only timer used.

By default windows uses combination of TSC+ACPI timers, not matter if HPET is enabled in BIOS.

TSC+LAPICs Low performance (slow timers + syncing) = 2.76MHz
LAPICs low performance (slow timer - no syncing) = 3.5Mhz
TSC+HPET medium performance (slow and fast timer + syncing) = 3.8Mhz
HPET high performance (fast timer - no syncing) = 14.3MHz

Run the WinTimerTester 1.1 to see your QueryPerformanceFrequency
Then try with HPET, you'll be amazed.

HPET + platformclock=true will give you best timer resolution, frame rate and lowest DPC latency.

You can test timer ratio and QueryPerformanceFrequency with WinTimerTester 1.1 [url]http://www.mediafire...xzo9n84d8lze9nb[/url]
The higher the QueryPerformanceFrequency is the better is performance. You only get high frequency with HPET. The other timers will give you significantly less frequency. Also note that if your ratio is not 1.0000 you are off set (or you have wrongly OC'ed), enable HPET and you should be without sync problems.

If you ever want to go back to default timers admin cmd:
bcdedit /deletevalue useplatformclock

Varying depending on setup, one should get increase up to +30 FPS and from the between.Online games is a good example of boost from HPET.
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post #5 of 23 (permalink) Old 12-06-2012, 01:43 AM - Thread Starter
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Quote:
Originally Posted by Blameless View Post

It's possible that disabling HPET simply made DPCs harder to measure accurately, and wasn't doing much to them. What sort of measured decrease are you seeing?
HPET is used for synchronizing a wide variety of things (most importantly thread scheduling in some programs), and should normally be left enabled if you have an OS that supports it. It probably won't noticeably hurt to disable it, but there really shouldn't be a need to.

I use:

http://www.resplendence.com/latencymon
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post #6 of 23 (permalink) Old 12-13-2012, 12:32 PM
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I've been doing some more experimenting with HPET timer settings vs. DPC latency on my systems.

In general:

- With HPET enabled in both BIOS and OS, idle DPC latency is in the 20-30us range, and generally does not move much under load.

- With HPET enabled in BIOS, but not in Windows, idle DPC latency is in the 5-8us range on my ASUS board (p9X79 WS), and 0-2us on my Gigabyte boards (X58A-UD5 and X79S-UP5-WIFI). Load goes to 20-35us on the ASUS, and stays almost the same as idle on the Gigabyte with the exceptions of rare spikes into the 30-40us.

- With HPET disabled in both BIOS and Windows, idle DPC latency is 0-2us on both brands, load is similar to the above.

Now the real fun starts when I run apps that poll the systems super I/O chip and EPU/VRM controllers:

With HPET fully enabled, programs like HWmonitor, CPU-Z, and just about any other monitoring software produce very frequent spikes into the 300-700us range.

With it HPET fully disabled, spikes only occur once every few minutes, and are of much lower magnitude, about 200-300us.

With regard to differences between the boards, it seems as though the ASUS monitoring chips (Nuvoton et al) produce much more latency while HPET is enabled than the iTE chip on the Gigabyte boards. However, when HPET is completely disabled, the balance shifts back in ASUSes favor.

I never had to disable HPET on my Gigabtye boards, but it seems like a wise thing to do with ASUS. Also, if HPET is necessary for some application, it's critical that any polling of temp/rpm sensors or the like be paused when doing latency sensitive tasks.

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post #7 of 23 (permalink) Old 12-13-2012, 03:30 PM - Thread Starter
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What are you using to measure your reading?

Thanks
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post #8 of 23 (permalink) Old 12-13-2012, 07:39 PM
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DPC Latency Checker from http://www.thesycon.de/eng/latency_check.shtml

Not as detailed as the resplendence checker, but a bit lighter weight and does the trick.

...rightful liberty is unobstructed action according to our will within limits drawn around us by the equal rights of others. I do not add 'within the limits of the law,' because law is often but the tyrant's will, and always so when it violates the right of an individual. -- Thomas Jefferson
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post #9 of 23 (permalink) Old 12-14-2012, 03:54 AM - Thread Starter
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Thanks, from your tests what settings would you recommend?
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post #10 of 23 (permalink) Old 12-14-2012, 10:02 AM
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Depends on your board/configuration. Ideally you should test the settings yourself, but if I were to make a general recommendation, I would go with HPET enabled if you have an iTE super I/O chip, and disabled if you have a Nuvoton chip.

OS may matter as well.Since Windows 8 doesn't support the LAPIC you should probably enable HPET.

...rightful liberty is unobstructed action according to our will within limits drawn around us by the equal rights of others. I do not add 'within the limits of the law,' because law is often but the tyrant's will, and always so when it violates the right of an individual. -- Thomas Jefferson
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