Cycles seems to not use more then 32 threads #84267

New Issue

shaun · 2020-12-30T11:49:00+01:00

shaun commented

2020-12-30 11:49:00 +01:00

System Information
Ryzen Threadripper 2970WX 24c 48t
Operating system:
Windows 10 PRO
Graphics card:
4X GTX 1070 8GB

Blender Version
2.83.3 = 100% CPU Usage, I can see 44 Rendering Threads on the screen, (Orange Boxes, 44 of them) another 4 Orange Boxes are the 4x GPUs rendering very quickly
2.91.0 = 75% CPU Usage, I can see 32 Threads, blender seems to be stuck using only 32 threads maximum, including 4 threads for GPU
2.92.0 ALPHA = 75% CPU Usage, I can see 32 Threads, blender seems to be stuck using only 32 threads maximum, including 4 threads for GPU

Short description of error
The latest blender versions are not using as many cpu core threads.

Exact steps for others to reproduce the error
create any blender scene anything., cycles render.
on a system with the threadripper setting the threads use to auto or 48 does not make any difference.

on a system with less then 48 threads force 48 threads..

**System Information** Ryzen Threadripper 2970WX 24c 48t Operating system: Windows 10 PRO Graphics card: 4X GTX 1070 8GB **Blender Version** 2.83.3 = 100% CPU Usage, I can see 44 Rendering Threads on the screen, (Orange Boxes, 44 of them) another 4 Orange Boxes are the 4x GPUs rendering very quickly 2.91.0 = 75% CPU Usage, I can see 32 Threads, blender seems to be stuck using only 32 threads maximum, including 4 threads for GPU 2.92.0 ALPHA = 75% CPU Usage, I can see 32 Threads, blender seems to be stuck using only 32 threads maximum, including 4 threads for GPU **Short description of error** The latest blender versions are not using as many cpu core threads. **Exact steps for others to reproduce the error** create any blender scene anything., cycles render. on a system with the threadripper setting the threads use to auto or 48 does not make any difference. on a system with less then 48 threads force 48 threads..

shaun commented

2020-12-30 11:49:00 +01:00

Added subscriber: @toxsickcity

Robert Guetzkow commented

2020-12-31 13:53:20 +01:00

Added subscribers: @brecht, @rjg

Robert Guetzkow commented

2020-12-31 13:53:20 +01:00

If I'm not mistaken you are intentionally limited to the number of physical cores since 2.91 according to #80080. However, the Ryzen Threadripper 2970WX has 24 cores, which doesn't match the reported 32 threads either. If I'm reading the specification of the CPU correctly it should be 24 + GPU threads.

@brecht I assume the lower utilization is expected due to the aforementioned changes, is that correct? I couldn't find any task that explains why this would now be bound to cores.

If I'm not mistaken you are intentionally limited to the number of physical cores since 2.91 according to #80080. However, the Ryzen Threadripper 2970WX has 24 cores, which doesn't match the reported 32 threads either. If I'm reading the specification of the CPU correctly it should be 24 + GPU threads. @brecht I assume the lower utilization is expected due to the aforementioned changes, is that correct? I couldn't find any task that explains *why* this would now be bound to cores.

shaun commented

2020-12-31 13:59:58 +01:00

Hi
Thanks for reply.

So my threadripper has 24cores. But 48 Threads.

So with blender 2.9x I can only see 32 threads active.

So what usually happens is that we full cores will be used then the rest of the 4 SMT threads to reach the 32 threads you speak of. That's why I have the 75% usage. Because having the 24 full cores is the performance part!

Any reason why they limit it?
Seems really really odd as so many people in production would be using threadrippers..

Hi Thanks for reply. So my threadripper has 24cores. But 48 Threads. So with blender 2.9x I can only see 32 threads active. So what usually happens is that we full cores will be used then the rest of the 4 SMT threads to reach the 32 threads you speak of. That's why I have the 75% usage. Because having the 24 full cores is the performance part! Any reason why they limit it? Seems really really odd as so many people in production would be using threadrippers..

Campbell Barton changed title from ~~Blender seems to not use more then 32 threads~~ to Cycles seems to not use more then 32 threads

2021-01-08 02:33:24 +01:00

Brecht Van Lommel commented

2021-01-11 16:24:05 +01:00

Changed status from 'Needs Triage' to: 'Needs User Info'

Brecht Van Lommel commented

2021-01-11 16:24:05 +01:00

This is not expected, it's supposed to detect all virtual/logical cores. This is likely related to the switch to Intel TBB. We use that for both Cycles and multithreading in Blender in general now.

With my Ryzen 2990WX it seems to be correctly detecting all 64 virtual cores. I guess it's somehow detecting the wrong number for this particular CPU model.

To help narrow down the problem, can you try running blender --threads 48 from the command line and check if that makes it use all virtual cores?
https://docs.blender.org/manual/en/latest/advanced/command_line/launch/windows.html

This is not expected, it's supposed to detect all virtual/logical cores. This is likely related to the switch to Intel TBB. We use that for both Cycles and multithreading in Blender in general now. With my Ryzen 2990WX it seems to be correctly detecting all 64 virtual cores. I guess it's somehow detecting the wrong number for this particular CPU model. To help narrow down the problem, can you try running `blender --threads 48` from the command line and check if that makes it use all virtual cores? https://docs.blender.org/manual/en/latest/advanced/command_line/launch/windows.html

shaun commented

2021-01-11 23:13:38 +01:00

Hi Brecht

Running blender via that command line does use 48 threads

The CPU does become fully utilised.

This is a good trick. Thankyou for letting me know about this one.
For some unknown reason running blender without the special command line is now using 48 threads normally.

I am suspecting a weird windows bug here.

I think maybe a windows update, or a power profile on the windows was limiting it.
All is good with the blender. Please close the case.

Hi Brecht Running blender via that command line does use 48 threads The CPU does become fully utilised. This is a good trick. Thankyou for letting me know about this one. For some unknown reason running blender without the special command line is now using 48 threads normally. I am suspecting a weird windows bug here. I think maybe a windows update, or a power profile on the windows was limiting it. All is good with the blender. Please close the case.

Brecht Van Lommel commented

2021-01-12 13:48:07 +01:00

Changed status from 'Needs User Info' to: 'Resolved'

Brecht Van Lommel closed this issue

2021-01-12 13:48:07 +01:00

Brecht Van Lommel self-assigned this 2021-01-12 13:48:07 +01:00

Brecht Van Lommel commented

2021-01-12 13:48:07 +01:00

Ok, thanks for testing.

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Cycles seems to not use more then 32 threads #84267