IPC and VR - How it works and can it be improved without an upgrade?

[lee1981]

Hi all.

In a nut shell I use my VR (Quest 2) in a flight sim (il2:Bos) and originally had a few issues with maps where FPS dropped considerably in certain areas. The confusing part I originally had was my graphics card utilisation was around 60%.. my CPU around 30% (The head scratching started).

After numerous hours looking at videos, settings, adjustments , forums etc a gentlemen mentioned to me about the IPC and that it can have an effect on certain VR pc titles.

I was wondering if someone could explain the correlation and if there was a way I could possibly improving my performance?

Just for the rundown I upgraded from my trusty 3700,X to 5600x. Less cores but slightly higher speed and higher IPC. I did actually see an improvement. G'card is a 3080.

Thanks in advance for any explanation and help in understanding how it all works.

Lee

 

[Zeeflyboy]

Some games, especially sim games, are very heavy on a single main thread… you can throw as many cores as you like at the problem but that one thread will always be the limiting factor for how fast your CPU can run that game.

VR makes this even worse, because you have two viewpoints and without doing anything special that means twice the number of objects to draw for the CPU… so if a game is already heavy on the CPU main thread it is likely to be even more so in VR.

The only way to improve the situation is to step up to a cpu that can “do more” operations per second in a single thread, aka single thread performance. You can either achieve this with a higher clock frequency (so you do the same amount of work each clock cycle, but there are more cycles per second) or you can use a CPU that has a higher IPC and in simple terms can process more things per cycle, even though the cycles per second remain the same. Or of course a combination of both.

So previously you were CPU limited in IL2, the fact that it was at 30% CPU is actually irrelevant as it isn’t like looking at GPU utilisation - if for example you had only one thread in the game being used and it was at 100%, then on your 16 thread 3700X would show a total of 6.25% utilisation (as the other 15 threads would be sitting idle) despite being the limiting factor. When you upgraded you changed to a CPU that can both do more per clock (higher IPC) and can do more of those clock clues per second (high frequency), resulting in that bottleneck being improved. Having less cores didn’t impact you because the game can’t utilise all those cores effectively anyway and it was the main thread that was likely holding back the entire cpu.

 

[BigBANGtheory]

yeah the issue is single threaded performance, so when you see a CPU utilisation of 30% that is across all cores where as you're more interested in how utilised the core(s) running the gamer renderer are.

Its because sims tend to use their own proprietary game engine and its tough to re-engineer them for multi-core multi-threaded work loads you almost have to rebuild the whole thing. Add to that DirectX 11 only accepts (I believe) api draw calls from one thread the upshot is you need a sim to have a modern game engine utilising Vulkan or DX12 to have CPU cores and GPU horse power scale well. You also get further complications with sims having to make more in depth calculations before making draw calls which can easily add latency and effect performance...

That's why Intel CPUs historically fair well in these situations cos of high frequency and raw single thread performance, but I'd say Zen2 closed that gap finally. Its also worth saying that low latency RAM can have a positive effect here as well and I think gamers getting themselves hyped up for DDR5 are going to be disappointed.

 

[forteh]

Following on from the RAM comment above, my old i5 2500k @4.6ghz saw a hefty increase in fps (around 25-30%) in Arma 3 simply by switching from 1600mhz to 2133mhz memory. Arma is a notorious single thread hog and whilst the ipc or frequency didn't increase there was obviously a memory bottleneck.

I upgraded to an R5 3600, overclocked it to 4.4ghz all cores and it got maybe 4-5% more fps in Arma but makes a much bigger difference with 6c12t over 4c in modern games that are properly multithreaded.

I'd look at overclocking the CPU and see if you can release more from the GPU. What speed is your RAM running at currently?

 

[lee1981]

Totally missed these comments and never got an update to reply.

For what it's worth just wanted to say thanks for the replies. Explains allot. Sadly overclocking my CPU is a big no no as AM4 doesn't appreciate it allot and single core hits around 4.55 which compared to Intel isn't great but about average for the CPU.

I've got 32gig of Corsair DDR4 3600mhz. Sadly its not expensive 18-22-22-42 timings but I don't feel this would have much effect with lower tighter timings.

Thanks again for the replies.