For osx it’s very important to tweak opencl preferences in 
.config/darktable/darktablerc.
I found my macbookpro 16 gives best performance if used in opencl mode 
‚default‘ with forcing full pixelpipe being processed by amd gpu (1) and 
preview pixel pipe with cpu (-1) 
(opencl_device_priority=1,0,*/!1,!0,*/1,0,*/0,*/*) in combination with 
performance mode (new with upcoming 3.4) and reduced resolution of preview 
image. The other opencl modes prioritized the on chip gpu (0) at cost of speed 
...

You can evaluate the processing time and used gpu device runnig darktable from 
terminal:
darktable -d perf -d opencl | grep -e'processing took' -e'using device'
Happy tweaking ...

To enable opencl on osx 11 use the workaround described at 
https://github.com/darktable-org/darktable/issues/6918#issuecomment-727595566

Am 22.11.2020 um 19:05 schrieb Volker Lenhardt <[email protected]>:

Hi Marc,

when I started dt two years ago for the first time on my new iMac-5k-screen, in 
full screen as I had been used to on my former HD screen, I was shocked by the 
slow performance. My first idea was to blame Apple. But when I did realize that 
full screen even extended the pixel size of my pictures I changed the mode of 
my operation. I reduced both the dt window and my distance from the screen, put 
my reading glasses on and eventually got a decent dt performance.

The point is that you shouldn't so much pay your attention to higher resolution 
but to more free screen space. What's the use of a 5k screen when you remove 
yourself from it so that it looks the same as a HD screen? We don't talk of UHD 
movies, do we.

All the best
Volker

> Am 22.11.20 um 16:04 schrieb Marc Cabuy:
> Hi Volker,
> Further to that opencl issue, that slow response time you had/have may be 
> related to the high resolution display.  That's my experience with dt 3.2.1 
> on Windows.  I now reduce my display resolution prior to editing (feels 
> stupid, I know). See also here: 
> https://www.mail-archive.com/[email protected]/msg10305.html.
> I would be interested to hear whether that is an issue too on MAC.
> Marc.
> -----Oorspronkelijk bericht-----
> Van: Volker Lenhardt [mailto:[email protected]]
> Verzonden: zondag 22 november 2020 12:26
> Aan: [email protected]
> Onderwerp: [darktable-user] No Opencl on macOS 11 (Big Sur)
> After upgrading to macOS 11 (Big Sur) I experience very long response 
> behavior of DT on my 5K screen (iMac 2017), about at least double compared to 
> the already slow performance under macOS Catalina.
> Especially denoising is nerve-racking.
> I see that OpenCL isn't supported any more. There is Metal instead. Is there 
> a way out to sufferable working conditions?
> Stay healthy
> Volker
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