performance: opengl vs directx, why such a gap?

[scavenger]

hello there

I have a performance related stupid question, which may be irrelevant since everyone uses DirectX.

In the past, I used to play KSP 1.0.5 with opengl because it was so much faster and used less ram, when moded with EVE.

Indeed the rendering is slightly not as good as with directx but honestly, for a 20-30% FPS boost I wouldn't care less.

Now I switched to 1.2.1 and thanks to my G15 keyboard, I can monitor in real time the RAM and CPU while loading and playing.

Here is the point:

When launching KSP in opengl:

start KSP_x64.exe -force-opengl -popupwindow

the loading time is 2 times faster, and my i3770 CPU peaks at 80% whereas it is stuck at 34% with directX. The HDD is not the issue as I launch KSP from a RAM drive.

Why is there such a difference? Could that be because of DirectX 11? I am using SEVEN and I know most of you guys switched on Windows 10 + DX12 but, well, could that really be the problem?? Why would the CPU be caped at 34% btw? Has anyone experienced this?

[sal_vager]

This is actually a very difficult question @scavenger, and not one I can give a full answer to.

Something to consider is the very different render pipelines between DirectX 9 and OpenGL, even something as seemingly simple as loading the .DDS textures from the RAM to the graphics card can be very different depending on the drivers and the cards being used.

It is more likely that it just doesn't take a lot of CPU to load the textures when using DirectX, rather than your CPU being capped, there might be firmware routines to handle this on the card itself, where an OpenGL driver might not have these and relies on the CPU to to a lot of the work, a direct comparison is difficult as they would use very different processes.

Similarly, performance may vary depending on your CPU, GPU, drivers etc, it is well known that Nvidia adds a lot of optimizations for specific games, allowing them to run faster/better, but where these optimizations don't exist it may be that the OpenGL driver is "better".

To really answer this question would require examining every step of the DirextX 9 and OpenGL render pipelines used by Unity, and how these are affected by your hardware and drivers.