Change Details

**System Information** Operating system: Linux-5.10.0-8-amd64-x86_64-with-glibc2.31 64 Bits Graphics card: NVIDIA GeForce RTX 3090/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 470.63.01 **Blender Version** Broken: version: 3.0.0 Alpha, branch: cycles-x, commit date: 2021-09-14 15:01, hash: `rB58ab616d4c6e` Worked: Prior to Cycles-X **Short description of error** The Spring demo scene shows large performance regressions on GPUs compared to normal Cycles. For example, normal Cycles may only take about a minute on my CPU, but Cycles-X takes 30 minutes. This was reported by Raimund Klink in blender chat: |{F10418747}|{F10418750}| |Master with OptiX|Cycles-X with OptiX| **Exact steps for others to reproduce the error** 1. Open the Spring demo file and render it on a GPU, comparing performance to normal Cycles. https://www.blender.org/download/demo-files/ Upon investigation I have found what seems to be the cause. The hair on Spring seems to be the issue. It is made with a shader like this: {F10418757} And there are a lot of intersecting hairs and a lot of "transparent bounces". Looking at just this culprit object, you can simplify the scene with something like this: {F10418762} Here are some steps to recreate the file above: 1. Create an object with a lot of hair (a cube with 400,000 strands of hair works) 2. Get the hairs to overlap with each other (Using the "Brownian" value in the "Physics -> Forces" tab of the particle settings (with "Advanced" enabled at the top of the particle setting section) worked for me). 3. Give the cube a material with the culprit material (mix shader with a "Transparent BSDF") 4. Change the "Light Paths" count for "Transparent" to something high (64) for the biggest effect 5. Position the camera and objects in such a way that the object with the hair only takes up a small portion of the frame 6. Render

**System Information** Operating system: Linux-5.10.0-8-amd64-x86_64-with-glibc2.31 64 Bits Graphics card: NVIDIA GeForce RTX 3090/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 470.63.01 **Blender Version** Broken: version: 3.0.0 Alpha, branch: cycles-x, commit date: 2021-09-14 15:01, hash: `rB58ab616d4c6e` Worked: Prior to Cycles-X **Short description of error** The Spring demo scene shows large performance regressions on GPUs compared to normal Cycles. For example, normal Cycles may only take about a minute on my GPU, but Cycles-X takes 30 minutes. This was reported by Raimund Klink in blender chat: |{F10418747}|{F10418750}| |Master with OptiX|Cycles-X with OptiX| **Exact steps for others to reproduce the error** 1. Open the Spring demo file and render it on a GPU, comparing performance to normal Cycles. https://www.blender.org/download/demo-files/ Upon investigation I have found what seems to be the cause. The hair on Spring seems to be the issue. It is made with a shader like this: {F10418757} And there are a lot of intersecting hairs and a lot of "transparent bounces". Looking at just this culprit object, you can simplify the scene with something like this: {F10418762} Here are some steps to recreate the file above: 1. Create an object with a lot of hair (a cube with 400,000 strands of hair works) 2. Get the hairs to overlap with each other (Using the "Brownian" value in the "Physics -> Forces" tab of the particle settings (with "Advanced" enabled at the top of the particle setting section) worked for me). 3. Give the cube a material with the culprit material (mix shader with a "Transparent BSDF") 4. Change the "Light Paths" count for "Transparent" to something high (64) for the biggest effect 5. Position the camera and objects in such a way that the object with the hair only takes up a small portion of the frame 6. Render

**System Information** Operating system: Linux-5.10.0-8-amd64-x86_64-with-glibc2.31 64 Bits Graphics card: NVIDIA GeForce RTX 3090/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 470.63.01 **Blender Version** Broken: version: 3.0.0 Alpha, branch: cycles-x, commit date: 2021-09-14 15:01, hash: `rB58ab616d4c6e` Worked: Prior to Cycles-X **Short description of error** The Spring demo scene shows large performance regressions on GPUs compared to normal Cycles. For example, normal Cycles may only take about a minute on my CPUGPU, but Cycles-X takes 30 minutes. This was reported by Raimund Klink in blender chat: |{F10418747}|{F10418750}| |Master with OptiX|Cycles-X with OptiX| **Exact steps for others to reproduce the error** 1. Open the Spring demo file and render it on a GPU, comparing performance to normal Cycles. https://www.blender.org/download/demo-files/ Upon investigation I have found what seems to be the cause. The hair on Spring seems to be the issue. It is made with a shader like this: {F10418757} And there are a lot of intersecting hairs and a lot of "transparent bounces". Looking at just this culprit object, you can simplify the scene with something like this: {F10418762} Here are some steps to recreate the file above: 1. Create an object with a lot of hair (a cube with 400,000 strands of hair works) 2. Get the hairs to overlap with each other (Using the "Brownian" value in the "Physics -> Forces" tab of the particle settings (with "Advanced" enabled at the top of the particle setting section) worked for me). 3. Give the cube a material with the culprit material (mix shader with a "Transparent BSDF") 4. Change the "Light Paths" count for "Transparent" to something high (64) for the biggest effect 5. Position the camera and objects in such a way that the object with the hair only takes up a small portion of the frame 6. Render