Cycles - Fixed Threads Cap UX inconsistency #80080

New Issue

Andres Stephens · 2020-08-24T16:24:48+02:00

Andres Stephens commented

2020-08-24 16:24:48 +02:00

Issue

In Blender 2.83, we had fixed thread limits increase beyond the core count, and if your PC was multi-threaded you could take advantage of that.

But now in 2.91+ builds, this thread limit is now is limited to CPU cores, and not threads.

Example:

Cycles CPU mode: you have an i7 with 8 cores, but then if you increase the thread count to 12, 12 tiles would render in Blender 2.83. The thread limits are working as expected.
Cycles CPU mode: in Blender 2.9+now with the same hardware, the thread count is capped at 8 threads = 1 thread per core. Even if you add more threads, it will still cap to 8 threads.

The "fixed threads" naming is misleading, or now it is limited to cores and/or broken.
Image

Ultimately, this adds a performance hit or cap on multi-threaded CPU's. It is also a misleading UX as what you want to take advantage of more threads, you wouldn't be able to even if the number extends beyond your core count.

Task

Check and see why older versions of Blender works with better multithreading, but 2.9+ is now limited to CPU cores instead of threads.
Rename "Threads" and Tooltip information to be about "Cores" instead of "Threads" to reflect the hardware limits of 2.9+ versions of the source.
If there is a feature inconsistency with the UX and indeed it should be about threads that can instance more than physical cores per multi-threaded CPU architecture, then remove the core count cap to truly reflect multi-threading.

System Information
Operating system: WIndows 10, i7 CPU
Graphics card: Nvidia GTX 1060

Blender Version
Broken: (2.91 Alpha, 21fec95139)
Worked: (2.83.4)

Short description of error
Fixed Threads is limited to physical cores instead of multi-threaded virtual cores.

Exact steps for others to reproduce the error

Change render setting to Cycles
Go to Performance panel and choose "fixed" Threads
Set Threads to a higher number than physical CPU cores (eg, +4 or +8 from CPU cores).
Count tiles

5.a - in 2.83 builds, render thread tiles count will extend beyond physical CPU cores, reflecting the fixed threads number.
5.b - in 2.90+ build, render thread count will cap at physical CPU cores, even if the fixed thread number is more.

# Issue In Blender 2.83, we had fixed thread limits increase beyond the core count, and if your PC was multi-threaded you could take advantage of that. But now in 2.91+ builds, this thread limit is now is limited to CPU cores, and not threads. Example: 1. Cycles CPU mode: you have an i7 with 8 cores, but then if you increase the thread count to 12, 12 tiles would render in Blender 2.83. The thread limits are working as expected. 2. Cycles CPU mode: in Blender 2.9+now with the same hardware, the thread count is capped at 8 threads = 1 thread per core. Even if you add more threads, it will still cap to 8 threads. The "fixed threads" naming is misleading, or now it is limited to cores and/or broken. [Image](https://user-images.githubusercontent.com/35357409/90912219-d7ea8e80-e39f-11ea-81a6-fe355da6dbfc.png) Ultimately, this adds a performance hit or cap on multi-threaded CPU's. It is also a misleading UX as what you want to take advantage of more threads, you wouldn't be able to even if the number extends beyond your core count. ## Task 1. Check and see why older versions of Blender works with better multithreading, but 2.9+ is now limited to CPU cores instead of threads. 2. Rename "Threads" and Tooltip information to be about "Cores" instead of "Threads" to reflect the hardware limits of 2.9+ versions of the source. 3. If there is a feature inconsistency with the UX and indeed it should be about threads that can instance more than physical cores per multi-threaded CPU architecture, then remove the core count cap to truly reflect multi-threading. **System Information** Operating system: WIndows 10, i7 CPU Graphics card: Nvidia GTX 1060 **Blender Version** Broken: (2.91 Alpha, 21fec9513969) Worked: (2.83.4) **Short description of error** Fixed Threads is limited to physical cores instead of multi-threaded virtual cores. **Exact steps for others to reproduce the error** 1. Change render setting to Cycles 2. Go to Performance panel and choose "fixed" Threads 3. Set Threads to a higher number than physical CPU cores (eg, +4 or +8 from CPU cores). 4. Count tiles 5.a - in 2.83 builds, render thread tiles count will extend beyond physical CPU cores, reflecting the fixed threads number. 5.b - in 2.90+ build, render thread count will cap at physical CPU cores, even if the fixed thread number is more.

Andres Stephens commented

2020-08-24 16:24:48 +02:00

Added subscriber: @AndresStephens

Bent Hillerkus commented

2020-08-24 22:17:01 +02:00

Added subscriber: @bent

Richard Antalik commented

2020-08-26 07:20:13 +02:00

Added subscribers: @brecht, @iss

Richard Antalik commented

2020-08-26 07:20:13 +02:00

I can't confirm this but I would blame e50f1ddc65.

@AndresStephens can you provide exact CPU model?

@brecht any idea what can be an issue here?

I can't confirm this but I would blame e50f1ddc65. @AndresStephens can you provide exact CPU model? @brecht any idea what can be an issue here?

blender-admin commented

2020-08-26 19:00:02 +02:00

This issue was referenced by bd678918ae

This issue was referenced by bd678918aefc58f1a9c0c807755010b7282e1be9

Brecht Van Lommel commented

2020-08-26 19:28:35 +02:00

I'll improve the tooltip, but other than that I don't think there is anything to be fixed here. This feature does not exist to increase the number of threads beyond the number of cores (since that will generally not performance, but rather make it worse). It's there to use a lower the number of threads, to free up the CPU for other tasks.

Brecht Van Lommel commented

2020-08-26 19:29:36 +02:00

Changed status from 'Needs Triage' to: 'Resolved'

Brecht Van Lommel closed this issue

2020-08-26 19:29:36 +02:00

Brecht Van Lommel self-assigned this 2020-08-26 19:29:36 +02:00

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Cycles - Fixed Threads Cap UX inconsistency #80080

Issue

Task