What do you know about regarding IRQ processing methods + drivers? (line-based vs message-based)

[Coldblackice]

In a thread here: http://forums.guru3d.com/showthread.php?t=378044

…it discusses IRQ’s, particularly the difference between line-based IRQ’s vs. message-based ones. It discusses a tweak for possibly switching devices from using the older line-based IRQ’s (hard-wired based) to the newer and more efficient message-based (using memory locations/access instead of hard-wired).

Supposedly, this can improve a system’s performance, particularly if there are a number of devices that are sharing IRQ’s.

In that thread, a number of people mentioned that install updated Intel drivers + Intel RST automatically switched a number of their controllers/devices to the newer message-based method, without them having to manually tweak this via registry.

Do you know anything about this? Is there potential benefit to be gained from doing this?

Looking at my own current x58 system, I can see that I only have one device which is (apparently) using message-based: my AMD GPU. All of the other controllers, such as I/O, PCIe, USB, network, SATA, audio, etc. – they’re all on line-based IRQ’s, and most of them are stacked with others on one IRQ. For example, IRQ 16 has eight devices stacked on it!

Anyway, I’m wondering what your thoughts are regarding this, and if it’d be helpful if I tweaked these to all try to use message-based, or perhaps if installing RST might possibly help in this. I’m currently on Intel 11.x.1006 (AHCI).

Thanks,

[Coldblackice]

Here’s a paragraph in that thread giving some more insight into this (I thought it’d be helpful for anyone new to this, as I am):

You ask why this isn’t on by default? Because of Microsoft, that’s why! We’re just trying to enable it, which is pretty easy. However, Microsoft is the one who suggests setting MSI to one MSI per physical core (by default, it’s set to 1 per CPU socket). Why? Again, because of Microsoft! Microsoft doesn’t allow drivers to set more than one MSI per socket. Yet, MS suggests setting MSI to one per physical core. So for a company like Intel, it’s relatively easy to set things to one per physical core, but not for companies like AMD or Nvidia.

These days, MS is so hellbent on saving power that they push for one interrupt per socket. When properly tweaked, LatencyMon is likely to be at the top of the list. Don’t ask me why the idea with MSI is to reduce the numbers (this include page fault). You shouldn’t get lots of page faults. If you do, something is wrong somewhere.

So what does the industry think regarding all of this? Microsoft can enable this, and probably does, since the performance gains are very nice. MS doesn’t have a lot of hardware, so it’s easy for them. But for hardware-makers, it’s more difficult. The simplest way is to have a small program asking the user what devices they have and then set it that way. I would love to see something like this.

Bothering setting various MSI to enable and setting the value to 1msi per CPU core .on an fx we re talking 8 core this means 8 MSI . I suspect this might be of help for those with say 2 or more 7990.

You have to remember this – everything in a computer is controlled by interrupt. If I go at your house and try to speak with you, but you don’t answer because you’re too busy, I’ll be at your door waiting for a long time for you to acknowledge me. And all while other people are also trying to communicate with you at the same time.

This issue is a bigger problem these days because people are much more social, they record and stream things at the same time, etc. – all of this puts huge pressure on the interrupt system.

Even when enabled, it means only 1 CPU core will be used for interrupt. So what happens if you have 38 other interrupts? Yep, they all have to wait in line for their turn. But if you have 1 MSI per CPU core, the probability that they’ll all be busy at the same time is very low.

I suspect this will be fully set and properly enabled by Microsoft as the default for Xbox One, especially since it’s meant to be a very "interactive" machine.

[Coldblackice]

Fernando, are you there? Just wondering if you have an opinion regarding this. Thanks.

[Fernando]

@Coldblackice

Fernando, are you there?

Yes, I am here, but not there.

Just wondering if you have an opinion regarding this.

Unfortunately I am not an expert regarding this topic.

[lordkag]

There is no definitive answer, it should be tested individually. I have seen drivers that ignore this setting, others that give a yellow bang, others that BSOD. I believe mbk1969 has written a tool for easily changing to MSI interrupts, among other useful utilities.

[Coldblackice]

@Fernando

Yes, I am here, but not there. Unfortunately I am not an expert regarding this topic.

Very Socratic of you

@lordkag

There is no definitive answer, it should be tested individually. I have seen drivers that ignore this setting, others that give a yellow bang, others that BSOD. I believe mbk1969 has written a tool for easily changing to MSI interrupts, among other useful utilities.

K thanks. I’ll look up mbk1969 and try out his utility(ies) to see how it fares. I appreciate the info.