Message-Signaled Interrupts (MSI) vs INT and PCI-E GPUs

[daerragh]

Hi. I've got a few questions about PCI-Express interrupts, because there are many misconceptions on the web regarding them. Specifically, I'm interested about PCI-E GPUs and interrupts.
Here's a short article about advantages of MSI interrupts - https://blog.sasken.com/message-signaled-interrupts-in-pcipcie-advantages-over-intx

What others say about the matter:
1. MSI is ALWAYS used on PCI-E because PCI-E simply doesn't support any other interrupt method - true or false? (this is probably false because there are apps that can force MSI on a PCI-E GPU - why do they exist if PCI-E supposedly only uses MSI?)
2. This contradicts the first question: AMD GPUs use MSI by default, while nVidia GPUs use INT (INT being the older interrupt signaling method) - true or false?

Other questions:
3. How is it best to determine if MSI is active for a PCI-E GPU?
4. If it's not active - how is it best to activate it: using MSI_util_v2.exe (or other app) OR manually in Windows Registry (and how)?
5. Are there any trustworthy benchmarks of PCI-E GPUs using MSI vs INT?
6. Is there a good way to check by myself the performance of a GPU using MSI vs INT?

 

[Feyd]

1. It's up to device driver and not every PCI-E device supports MSI mode so that's definitively false.
2. I can only speak from my experience but on Sapphire HD7850 AMD uses MSI by default while on my MSI GTX1070 Nvidia uses INT mode by default (same as my Creative Sound BlasterX AE-5 INT mode default too). Can't say if that applies to all models and driver versions but on current drivers it does. Yes INT is legacy mode for PCI-E devices and it's not supposed to be used at all if i understand PCI-E specification correctly.

3. Either by checking Windows registry or by some app like you already mentioned MSI_util_v2.exe (my preferred choice but that's up to you).
4. As I said MSI utility or Windows registry i would recommend you to use utility and to be safe make Windows restore point before you use it so you can revert back. But only enable it for devices that supports MSI or MSIX mode as utility reports. If you want to change registry then here you will find how you can do that.
5. Not sure if it can even make measurable difference in FPS as this change is usually done to lower system ISR and DPC latency. I didn't do any test to measure that but i can test both modes on my GTX1070 if you want.
6. As I said not sure since i didn't try that before but for latency change you can try LatencyMon which i use to test this change and for me it made nice difference in latency but that can be only exception and not apply to anyone else so can't recommend that only based on my experience.

 

[daerragh]

Thank you for the answer If you are willing to test the performance it would be really helpful. The info on the net about this matter is scarce.

OK. I did a test on the latest Win 10, nVidia 441.87 DCH driver using the latest 3DMark and its PCI-Express test. Running i7-4790K and GTX 1060 6GB on a H81 mainboard chipset (so only PCI-E 2.0 is active).
Results:
INT: 6.68GB/s
MSI: 6.69GB/s.

 

[Feyd]

Sorry I have 3DMark demo only so I can' t run that test. I did Timespy and Firestrike both in MSI mode and INT mode. But it is as I though no difference (at least not noticeable) in FPS. Driver used is 441.87 DCH.

 

[Gedza91]

Guys i got huge problems,and its occur randomly. Its about audio cracking sound,and high latency,high cpu usage on youtube videos when watching in full hd,high latency (checked with latency mon),high gpu usage (much more then ussual) in gaming. So im now in "ALL IN" mode,live or die. xD Im prepairing to use MSI mode on my hardware,but i was reading on main forum guru 3d,it can cause big problems,even hardware to die (hope i understand well how it can be) on hardware whos not supp MSI mode. So lunching MSI utility mode,and i got this (see picture below). Im wondering on which device i can take advantage of MSI Mode.

 

[Feyd]

It can maybe solve your problems with audio (for some it did) and system latency but i don't think it is cause of your high cpu usage. It's not default option option on some device drivers (never on NVIDIA cards from what i have seen on other forums) so yes it can cause unexpected behavior and system instability. It shouldn't cause unrecoverable hardware failure in any circumstances but since i don't make any hardware i can't guarantee that so i am not liable for any damage it does. Every system is unique so results can be quite different but from i gathered on other forums for most users it solved audio issues and lowered system latency (in my case a lot) and you can easily revert the change back if you want to. Before you try anything make system restore point so you can revert in case of system failure. In the utility only change devices which utility reports that supports MSI mode so in your case only both HD Audio controllers and NVIDIA GeForce GTX 750 Ti. For those rows just tick MSI column and after ticking all you want click apply and restart your PC. This change is persistent until you change driver for those devices or you upgrade you Windows (only a possibility but it may stay same even after that) so you may need to do that change again in future.

 

[Gedza91]

It can maybe solve your problems with audio (for some it did) and system latency but i don't think it is cause of your high cpu usage. It's not default option option on some device drivers (never on NVIDIA cards from what i have seen on other forums) so yes it can cause unexpected behavior and system instability. It shouldn't cause unrecoverable hardware failure in any circumstances but since i don't make any hardware i can't guarantee that so i am not liable for any damage it does. Every system is unique so results can be quite different but from i gathered on other forums for most users it solved audio issues and lowered system latency (in my case a lot) and you can easily revert the change back if you want to. Before you try anything make system restore point so you can revert in case of system failure. In the utility only change devices which utility reports that supports MSI mode so in your case only both HD Audio controllers and NVIDIA GeForce GTX 750 Ti. For those rows just tick MSI column and after ticking all you want click apply and restart your PC. This change is persistent until you change driver for those devices or you upgrade you Windows (only a possibility but it may stay same even after that) so you may need to do that change again in future.

So,MSI mode is on on those three devices,audio stuttering is still here,while playing youtube at 1080p,also for gaming while cpu goes 50-60%. So i tried unistaling audio driver for my motherboard (Via HD drivers from msi site). Problem still here. So is there any suggestion guys?

 

[Feyd]

I don't know anything about what HW or OS you have so i can only guess which can be a problem. For audio problems i would first try to update your audio drivers (from maker of your audio chip or your motherboard maker) and check if your OS has latest updates installed. Next i would check if you have forced HPET in your OS and if so revert that to original state (run comand line as admin and copy in without commas of course "bcdedit /deletevalue useplatformclock" for Windows 7, 8, 8.1, 10 and reboot your PC after that) since that usually generates a lot of stutter especially on Intel CPUs. If you have Windows 10 you can also try this. CPU usage on YT videos in FHD resolution depends on your HW. So if you have slow older CPU and video uses AV1 or VP9 codec it's normal since your VGA doesn't support HW accelerated decoding of that two codecs so CPU has to do all the work hence high CPU usage.