VR zoom makes CPU frame time (bad performance) better, why?

[darkman222]

I have a small replay from a server I frequently play on, where the CPU frame time was really bad, right when I joined the server.

Around 16 - 17 ms CPU frame time. When I replay it in VR and press the VR spyglass Zoom , for that time being zoomed the CPU frame time improves significantly. See the video with fpsVR on the left. Same behavior in singlethreading too. But with much worse frame times of course.

Is that behavior worth noticing? Or is it just what will be expected using VR zoom?

Can that be used to optimized something in DCS that helps me get better CPU frame times?

Video and track file attached.

System Specs below, using VR with steamVR and Varjo Aero.

dsa

mobettametas_Dogfight_Arena_v1.73.1-20230528-223449 16 ms permanent.trk

Edited May 31, 2023 by darkman222

[speed-of-heat]

you are rending less things by looking closely ... so i think it is  expected behaviour

[darkman222]

Sure. But its the CPU frame time that improves drastically, which is the bottleneck on my system.

[speed-of-heat]

less things for the CPU to do... 

[WipeUout]

2 hours ago, darkman222 said:

Sure. But its the CPU frame time that improves drastically, which is the bottleneck on my system.

Your CPU should not be the botleneck, an i9-12900kf is pretty good.  Unless you are not using DCS-MT? 

[darkman222]

I am using DCS-MT. The ST test was just for comparison, and performance is much worse. Its the second part of the video.

CPU frame times explode when using VR. Even with a high end PC.

I hope for more MT optimisation from ED. Also interesting is that in MT if you watch fpsVR closely there is one Core that jumps up to full load. Whereas in ST there is no single core in full load.

Edited May 31, 2023 by darkman222

[WipeUout]

3 hours ago, darkman222 said:

Also interesting is that in MT if you watch fpsVR closely there is one Core that jumps up to full load. Whereas in ST there is no single core in full load.

Are you sure about that?  It should be the other way around.  Try to monitor your core load with something else like the performance tab of task manager.

[darkman222]

Looks like this is the one maxed out core
 

Edited May 31, 2023 by darkman222

[zildac]

Your CPU should not be the botleneck, an i9-12900kf is pretty good.  Unless you are not using DCS-MT? 

With a 4090 any 12th or 13th gen Intel CPU will absolutely be the bottleneck in VR with a modern HMD, based on the current MT implementation. And the existing DCS gfx engine. This fact is well understood and accepted, albeit with some disdane. The same applies to AMD CPUs with high end GFX cards.

[WipeUout]

6 hours ago, zildac said:

With a 4090 any 12th or 13th gen Intel CPU will absolutely be the bottleneck in VR with a modern HMD, based on the current MT implementation. And the existing DCS gfx engine. This fact is well understood and accepted, albeit with some disdane. The same applies to AMD CPUs with high end GFX cards.

Since MT was introduced, in all my testing, my CPU FT never exceeded 4.5ms in DCS.  This was the case also with my i7-12700KF.

If your CPU is bottlenecked in DCS, then you have something eating CPU cycles in the background. Maybe you should consider a fresh windows install.

Edited June 1, 2023 by WipeUout

[zildac]

Since MT was introduced, in all my testing, my CPU FT never exceeded 4.5ms in DCS.  This was the case also with my i7-12700KF.
If your CPU is bottlenecked in DCS, then you have something eating CPU cycles in the background. Maybe you should consider a fresh windows install.

Main thread limited in the DCS FPS counter is pretty self explanatory I think. No extraneous background processes and the sim runs fine mostly.

[mbucchia]

19 hours ago, darkman222 said:

Sure. But its the CPU frame time that improves drastically, which is the bottleneck on my system.

It varies from engine to engine but the most likely explanation is that there is more geometry being culled.

Zooming reduces the field of view as mentioned in the post above yours, and before attempting to render anything, the engine will perform what is called a "culling pass", and it will completely remove from its list of things to draw anything that is not at all visible due to that lower field of view. Less geometry to draw will translate into less work to do on the CPU (because the CPU is responsible for enqueuing commands to the GPU for all the triangle lists to draw).

It won't affect as much your GPU because in the end the same number of pixels end up being filled (can still vary due to poorly optimized z-ordering, but let's ignore that). It's all about how complex it was to assemble the geometry that produces these pixels.

You can't really leverage that property to optimize things no, unless you are willing to significantly reduce your field of view (there are options to do that in SteamVR/OpenXR, though they probably produce much less gain since they won't reduce FOV as much as zooming does).

[darkman222]

Well, that was a detailled explanation from a professional Thanks @mbucchia