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Cycles preview with multiple GPUs is only as fast as the lowest GPU
Closed, ResolvedPublic

Description

System Information
Windows 7, 64 bit
NVIDIA GTX 780, 580, 560 Ti

Blender Version
Broken: 2.71, 5202fca (2014-09-10)

Short description of error
When setting the compute device to all GPUs, cycles preview is only as fast as the slowest GPU (560 Ti in my case).
This problem does not occur when rendering.
I understand that it might be hard to parallelize the preview across multiple GPUs, but it should be at least as fast as the fastest GPU.
An easy solution would be to create an extra setting "Preview Compute Device" which you can set independently of the normal compute device.

Event Timeline

Philipp Holl (bvh345) raised the priority of this task from to 90.
Philipp Holl (bvh345) updated the task description. (Show Details)
Philipp Holl (bvh345) edited a custom field.

Hi, cant verify on my system:

Opensuse 13.1/64
Intel i5 3770K
GTX 760 4 GB (Display)
GTX 560Ti 1.28 GB 448 Cores
Driver 340.32

Cycles use all GPU for preview render and final render.
Blender Hash: c9f0c20 from 16.09.2014

Cheers, mib

The problem also occurs with only the 780 and 560 Ti plugged in.
I should note, that only the 780 is connected via PCIe x16. The others are only PCIe x1.
My CPU is an AMD Phenom II X6 1090T.

Sergey Sharybin (sergey) changed the task status from Unknown Status to Resolved.Sep 17 2014, 10:03 AM
Sergey Sharybin (sergey) claimed this task.

This is a known behavior of Cycles scheduler, which requires preserving the compute device for the tile when doing viewport and progressive refine renderers. Currently it's done by simply slicing the image on the equally sized parts. Things here could be improved, but it's not considered a bug.

Thanks for the report, but closing as a TODO now.