Why does Lightroom not use the performance cores (M4Max) during export??

[chfilm]

Hi,

I was wondering if anyone could shed some light on why Lightroom Classic (14.3.1) doesnt seem to use the performance cores of my m4max macbook pro AT ALL during export? it's slowly chugging along on the 4 efficiency cores, while the real workhorses sit idle. 

The Gpu is not maxed out either, nor is the Ram. 

I found many sources stating this behavior on the m4 macbook air, but couldn't really find sources about the macbook pro and find this really odd. Anybody else noticed this?

[johnrellis]

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By default on newer hardware, the GPU does all the heavy computation for Export, and the CPU cores do all the lighter-weight bookkeeping.  So in your instance, running the CPU threads on efficiency cores is sufficient to keep up with the GPU.

If the GPU isn't being maxed out, there could be other limiting factors, e.g. reading the source photos from a slow disk or network volume or writing the exported photos to slow destination disk.

1. Do the LR menu command Help > System Info and copy/paste the entire contents here so we can see exactly which versions of hardware and software LR thinks you're running and important LR options that are set.

2. Select several dozen raws and start an export.  After it's been running for 30 seconds or more, post a screenshot of Activity Monitor showing CPU History and GPU History, e.g. 

[chfilm]

Pretext: all photos are on a samsung 4TB SSD capable of 1000mb/s.

It's very interesting, I did some more testing..  and I found that it seems to be some sort of thermal throttling behavior that's going on.. The first 230 images or so get exported super fast, also using all cores pretty fully. But when I then went on to export another 600 images, I saw the same odd behavior again of only the efficiency cores being maxed out while the rest is at 30-40% utilization. Same for the GPU. I then manually ramped up the fans (which were almost unnoticable up to this point) and oh voila- suddenly, the cpus are working much harder again too! 

What kind of brought me on this track was that I watched a review of the m4 macbook air, which doesnt have active cooling, and shows the exact same behavior after a short time of LR exporting. But I find it really super odd that my macbook doesnt ramp up the fans by itself to give the CPU more headroom to use its full potential. Especially since the fans ARE spinning super loudly by themselves when I do something like AI retouching with Arams that uses the GPU to it's maximum. The fans are at absolute full blast right now while the gpu is crunching... 

Can that be something LR related or is that more like a problem with my specific machine or Mac OS even? 

Please have a loot at the attached screenshots, and find below the log:

Lightroom Classic version: 14.3.1 [ 202505061331-b063faef ]
License: Creative Cloud
Language setting: en
Operating system: Mac OS 15
Version: 15.5.0 [24F74]
Application architecture: arm64
Logical processor count: 14
Processor speed: NA
SqLite Version: 3.36.0
Power Source: Plugged In, 75%, Charging
Built-in memory: 36.864,0 MB
Dedicated GPU memory used by Lightroom: 395,5MB / 27.648,0MB (1%)
Real memory available to Lightroom: 36.864,0 MB
Real memory used by Lightroom: 2.446,1 MB (6,6%)
Virtual memory used by Lightroom: 414.904,6 MB
Memory cache size: 3.661,2MB
Internal Camera Raw version: 17.3.1 [ 2227 ]
Maximum thread count used by Camera Raw: 5
Camera Raw SIMD optimization: SSE2
Camera Raw virtual memory: 188MB / 18431MB (1%)
Camera Raw real memory: 190MB / 36864MB (0%)

Cache1:
NT- RAM:0,0MB, VRAM:0,0MB, Combined:0,0MB

Cache2:
m:3.661,2MB, n:0,0MB

U-main: 126,0MB

Standard Preview Size: 2880 pixels
Displays: 1) 5120x2880, 2) 3024x1964

Graphics Processor Info:
Metal: Apple M4 Max
Init State: GPU for Export supported by default
User Preference: Auto
Enable HDR in Library: OFF

Application folder: /Applications/Adobe Lightroom Classic
Library Path: /Volumes/4TB SSD/01_Lightroom/Lightroom Catalog-v13-3.lrcat
Settings Folder: /Users/Nastya/Library/Application Support/Adobe/Lightroom

Installed Plugins:
1) AdobeStock
2) Aperture/iPhoto Importer Plug-in
3) Flickr
4) Logi Plugin
5) Retouch4me Arams

Config.lua flags:

[johnrellis · Accepted solution]

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Very interesting. I observed the same behavior on my Macbook M2 Max exporting 1000 raws from and to the internal SSD:

(See the attached spreadsheet.)

After 364 files, the Mac throttled back on the GPU and performance cores. (Maybe the GPU was throttled and the performance cores were limited by that; or maybe the performance cores were throttled, and the GPU limited by that; or maybe both were throttled.)

Overall, keeping the fans on increased the rate of export by 23%.  But just comparing the time when the GPU and cores were throttled (after the first 364 files), the rate of export was 41% faster with the fans on.

* * *

I don't think this is under the direct control of LR (or any typical app). It's the operating system and firmware that decide when to throttle.

* * *

I didn't think there were thermal issues with the Apple Silicon Macbook Pros. The last years of the Intel Macbook Pros were notorious for their bad thermal engineering, with throttled CPUs due to overheating and fans that got easily  clogged.

It may be that the Apple designers have deliberately chosen to run the fans less aggressively in order to keep the Macbooks quieter. Many people really don't like to hear the fans running (like my wife).

[chfilm]

Wow, thanks for testing this on your machine! It seems that on the m4 since they get even hotter, the problem is emphasized even more. But alright, knowing this it's easy to solve with a manual fan override. 

[chfilm · Accepted solution]

Hey, I did another test while actually measuring the time difference and it's crazy! 
650 pics export with the standard mac os fan control: 13 minutes.

The same run but with manual fan control: 7:30 minutes! 

So almost twice as fast!! 

Something can't be right here? I think the fans used to spin up by themselves eventually but now, not at all! Only during GPU only tasks..

[chfilm]

aaaaand I have another update:

I ran the creation of 1:1 previews of the same 650 images- and interestingly, here all the cores were used to the max and the fans went up to almost 100% by themselves. So it must be that LRC decides during export to not force the performance cores into full use if that would create substantial heat, and instead decides to rely on the efficiency cores, therefore unneccesarily prolonging the export time, even though more performance would be available! 

Super weird. I'm not a developer, so I don't fully understand the logic behind it, but that's what's going on. Preview generation: LR uses all power, Export: LR decides to use only the E-cores if the P cores are not manually cooled. WTF adobe? 

[johnrellis]

LR doesn't use the GPU for generating previews, while for exports it tries to get the GPU do all the heavy computation. GPUs use a lot of of power and generate a lot of heat, so it may be that during export Mac OS is throttling the GPU rather than running the fan, which in turn lets it schedule the LR threads on the efficiency cores, which are fast enough to keep up with a throttled GPU.  

I honestly don't think LR is making policy decisions about when to use performance versus efficiency cores, much less about when to run the fans. These are very hardware-dependent functions, and the operating systems (Mac and Windows) and CPU firmware (at least with Intel CPUs) are responsible for scheduling threads on the cores and managing heat.