Cycles renders SSS is noticeably slower in 3.4 than in 3.3 with multiple GPUs #103023
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Reference: blender/blender#103023
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System Information
Operating system: Windows-10-10.0.19044-SP0 64 Bits
Graphics card: 2x NVIDIA GeForce RTX 2080 Ti/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 526.98 (SLI)
Blender Version
Broken: version: 3.4.0, branch: blender-v3.4-release, commit date: 2022-12-06 18:46, hash:
a95bf1ac01Worked: 3.3
Short description of error
Cycles render images with SSS noticeably slower in 3.4 than in 3.3
Exact steps for others to reproduce the error
I noticed that some of my scenes render in 3.4 noticeably slower than in 3.3.
After some analysis, I figured out that only images with SSS shaders are affected.
I created a simplified test scene, this scene takes on my computer in Blender 3.3 4:58 min and in Blender 3.4 it's 6:10 min.
If SSS is disabled, the test file renders equal fast in 3.3 and 3.4.
Probably this change is unintended and might be a bug.
speed test.blend
Test render 3.4 (OptiX) 6:13 min:
Test render 3.3 (OptiX) 5:12min:
Added subscriber: @Rincewind3D-1
Cycles renders SSS is noticeably slower in 3.4 then in 3.3to Cycles renders SSS is noticeably slower in 3.4 than in 3.3Added subscriber: @OmarEmaraDev
Haven't tested yet, but do both version produce the exact same render, or are there differences?
Changed status from 'Needs Triage' to: 'Needs User Info'
I can't reproduce the slow down with CPU rendering. Does this happen with GPUs only in your testing?
I tested it only with GPUs. I have 2x 2080Ti.
Asked another artist I know with also 2x 2080Ti and he could reproduce the same slowndown.
I added test renders for 3.4 and 3.3. Both look the same.
But can you test if the same slow down occur with CPU only or using a single GPU? I am trying to narrow down the hardware configuration where the slowdown happens.
I just tested with CPU.
Therefore I reduced the Max samples to 512 and the resolution from 300% to 100%.
Results CPU:
3.4 5:16 min
3.3 5:00 min
Not sure what this means. This could be still in natural fluctuations.
Changed status from 'Needs User Info' to: 'Needs Triage'
I can detect a small difference on CPU, so I will leave this to someone with a GPU to triage.
I just did a 1x GPU test (2080Ti).
Results are
3.4 10:13 min
3.3 10:51 min
So, I guess it's a multi GPU issue with SSS.
Cycles renders SSS is noticeably slower in 3.4 than in 3.3to Cycles renders SSS is noticeably slower in 3.4 than in 3.3 with multiple GPUsAdded subscriber: @pmoursnv
Maybe @pmoursnv can take a look?
Added subscriber: @mano-wii
Changed status from 'Needs Triage' to: 'Confirmed'
I can notice performance loss with CPU+GPU:
It may take a while to bisect.
I'm confirming for now.
System Information
Operating system: Windows-10-10.0.22621-SP0 64 Bits
Graphics card: NVIDIA GeForce RTX 3060 Laptop GPU/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 516.94