Why gpu slower than cpu

[Jin.7787]

The GPU rendering should have been fast but here it is happening the opposite. Am I doing something wrong?

[vladimir_krastev]

Hi, thanks for posting. Could you share a bit more details? What are the render times, which render is rendered with V-Ray CPU? Are you using RTX or CUDA? What is you hardware?
Rendering speed depends on the performance of your hardware. If you have a very powerful CPU that you compare to a old model weak GPU then V-Ray CPU might render faster.

[Jin.7787]

Rendertime

[Jin.7787]

My pc specification

[Jin.7787]

I tried the same setup on redshift also. Its render time was very fast. And I don't want to shift to any other render engine. But now because of Unreal, clients are asking for faster render. Now everyone wants real time.

[Muhammed_Hamed]

Hi

For performance testing I recommend using longer tests, +5 minutes to get a meaningful result. It is also better to use production scenes for such comparisons, the scene you are using now has mostly empty frame, other than the statue. Here is a better scene to test with,

Could you please send me the V-Ray scene to my Email, amgquads@gmail.com
I have the same hardware setup as you, I will test on my side

Best,
Muhammed

[Jin.7787]

I rendered your scene. There is very little difference in CPU and GPU render time. There should be at least 5 to10 minute difference in GPU, RTX is not able to render.

[vladimir_krastev]

Hi, make sure your GPU setup works up to spec. You could easily do that with the V-Ray Benchmark Tool. Please run it on your setup and compare with the statistics for a single RTX3090. If your result differs a lot from the average there might be something wrong with your video card. Could be the driver or anything else. Of course the render settings are crucial as well. Please for testing use the default settings for V-Ray CPU and V-Ray GPU.

[Jin.7787]

My hardware is working fine..there are no problems, and the drivers are also updated And even before this I had done GPU render in Vray 5 and faster than this

[Muhammed_Hamed]

I'm investigating this now, it seems that the new drivers have 556.xx has performance issues
I will update the thread soon with my conclusion

Best,
Muhammed

[Jin.7787]

Thank you, I like V-Ray very much, we have worked on it since the beginning, but now it is a bit difficult to beat the time line of the client, please make VRay GPU at least 30-40% faster than CPU.

[Muhammed_Hamed]

Please downgrade to the recommended studio driver 551.86 for now, it will have around 30% better performance than what you have now
We will communicate this driver issue with Nvidia ASAP
I will test the Max scene with a very similar setup to yours today, to see how the performance compares between V-Ray and V-Ray GPU. I think your GPU should be at least 2x faster than your CPU
I will post my testing here shortly

Best,
Muhammed

[Jin.7787]

Today was the last day of my redshift software trial and tried to match your setup and I think it matches well, and the render time of redshift is 1:18 min.

[Jin.7787]

Even in your Vantage it is rendered in 1.2 min

[Muhammed_Hamed]

Here are my results, with the recommended driver
RTX 3090: 10m 9s
Ryzen 5950x 16-Cores: 17m 13.1 s

The 5950X is faster than your 3950X by around 20%, so the difference between CPU and GPU in your case should be bigger(close to 2x)
Another note is that you need to adjust the bump values for the standard V-Ray CPU to match the GPU result, which will increase render times

I don't understand how your 3090 renders my scene in 27 minutes, something is not right! Do you have Low priority turned on for your GPU?
I'm using Progressive mode for GPU and Buckets for CPU, please check this article and this article about
You could also try to monitor your GPU power draw like this article, it should be above 300 Watts during rendering
If you are still getting this decreased performance, I suggest a remote session. I will take a look at your setup to see why it renders slowly

About Redshift, it doesn't seem to use Random Walk SSS in your setup. There is also the bowl itself, which uses Chaos Scans which adds detail and variety to the wood shader. With this being added, RS render times will be longer for sure

Same about Chaos Vantage, it doesn't use SSS yet

all the best,
Muhammed