regedit: priority control (IRQ)

  1. Posts : 541
    Windows 10 Pro 64 bit 19042.1415

    regedit: priority control (IRQ)

    Can someone confirm setting priority control under KHLM\SYSTEM\CurrentControlSet\Control\PriorityControl for irq in regedit works ?
    I read it is outdated residue from WindowsNT, but setting priority for cmos and than for gpu reduced input lag, so it seems it works.
    There is also possibility set priority for devices/drivers in SYSTEM\CurrentControlSet\ENUM\pci etc. under affinity policy - by setting DevicePriority 3 for high e.g.
    I talked to latencymon support and they told me affinity policy can be ingored on hardware level like chipset etc.
    But nothing about priority control.
    I wonder if there is actually benefit to setting priority for irq in windows, because i tested it and it seems to me input lag is lower, so i would say it works. I don't have 1000fps camera, but i am extremelly sensitive to input lag and can tell even 6 ms difference. I tested myself with inputlag ab test on blurbusters forums.
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  2. Posts : 3,979
    Windows 7 HP 64 - Windows 10 Pro - Lubuntu

    Since Plug and Play (PnP) devices and Windows NT, I think that you're the first user in years that mention irq (Interrupt Request).
    I also would say that you won't find an answer in here.
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  3. Posts : 1,249
    Windows 10 Pro

    This is the classic IRQ8Priority registry setting, an idea that just will not die.

    Microsoft provides no documentation for this. It doesn't make any sense, certainly not with modern hardware. There is some evidence it existed and did something in NT 3.x, released more than 20 years ago, but if so it was soon dropped.

    If this setting actually worked why is is not set by default? In recent years it seems Microsoft has been obsessed with performance and has spent millions to develop things that provide only modest performance improvements.

    I am convinced any performance improvements you might perceive are due to the placebo effect, which incidentally can be very strong. Even experts who are wary of it have been fooled.
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  4. Posts : 541
    Windows 10 Pro 64 bit 19042.1415
    Thread Starter

    Hmm i don't think it is placebo, i can tell difference, i tested myself on inputlag and i can tell even 6ms, i am extremely sensitive to it. I also have program to test handling interrupts from mouse polling. I will do some test in future yet.
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  5.   My Computer

  6. Posts : 541
    Windows 10 Pro 64 bit 19042.1415
    Thread Starter

    It is not placebo. Prioritizing USB: i have in latencymon 56us max execution time in windows, after maxing polling rate for 1 minute - from multiple tests. With this chnage i get half. And i can notice it on mouse movement. I am extremely sensitive person, i can feel even 6 ms of input lag - tested with input lag ab test from blurbusters! Had like 22-23 from 25 right!

    But it is 100% working, i read it works even in windows 7 and 8! Also i changed IRQ priority for system time and mouse was more consistent. I am using 500hz/0.5ms resolution time, which feels inconsistent, if last mouse packet will miss resolution timer window, it will occur max 499us from time DPC is handled by OS. So there is like variation 499~ us. But than if you don't max your polling rate, or start moving a mouse, one packet can miss timer window and you will get one previous, which occured much later and than you get one packet sooner, when it occurs before next window.

    Tho changing interrupt priority is probably not a good idea in many cases, as it can increase execution time of DPC from other drivers and isn't measure of input lag. Windows/cpu knows what it is doing probably. Tho even some drivers of devices (sata, nvme controller), put themselves to high interrupt priority, other devices are on undefined. Msi_util2 can change this via registry in other location tho, than this tweak!

    It is hard to explain. For people who want to know more: google USB controller, DPC latency and resolution timer.

    Which makes mouse inconsistent especially on 500hz/0.5ms timer. I was using 1ms, because it felt more consistent. Point is, after changing IRQ priority for system timer, mouse movement become more consistent and i could do circles better. So it is working!!!

    DPC latency of USB driver, will become very important for 8k polling mice, but timer resolution only 0.5ms will be huge bottleneck.

    To add:
    I had inspiration from one book what to test, because there is quite number of permutations. However, i found out it is not better than default to prioritize anything. Windows/cpu probably knows, what it is doing. I tried many times and mouse always felt worse, even had higher overall dpc latency, while i got lower for usb - when i prioritized usb. Also interrupt affinity is not worth it (and could destroy your pc if you select disk i heard - tho using it for sound cared, gpu, usb and nic is probably fine, had no problems on 2 computers...) Putting one device to other core - feels worse than having everything on 1 core. I tested some tweaks on 2 computers, never had any better results, by changing affinity and interrupt priority.

    Honestly it depends, prioritizing system timer - gave me more round mouse movement, but higher input lag. Also prioritizing usb felt more responsive, but laggy at the same time! Not better than default overall.
    Last edited by empleat; 09 Nov 2020 at 10:00.
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