Don’t Trust Activity Monitor on M1 Macs

https://eclecticlight.co/2022/05/02/dont-trust-activity-monitor-on-m1-macs/

by KubikPixel • 4 years ago

112 28 4 years ago

jeroenhd• 4 years ago

I don't get it. CPU graphs don't show power usage and they never have; they show how much time, percentually, a CPU was busy executing a certain type of task. You can never get a power estimate from a CPU usage graph at all. Even during normal operation purely on performance cores the clock speed will go up and down as the OS idles or new tasks get spawned. Whatever 10% is in your graph changes depending on the state of the system. This means the energy value in monitoring tools like these are bogus and should never have been trusted in the first place!

It's like trying to read the amount of gas left in the tank by looking at the odometer. Distance and fuel remaining are related, but there's no 1-to-1 connection going on there.

Different cores having different speeds is a difficult problem to solve for reporting CPU usage. The author seems to have missed the latest Intel CPUs which also have a performance/effeciency core setup, but I bet those suffer from the same "problem" as well.

tmikaeld• 4 years ago

Since Apple have full control over each Mac's hardware, couldn't they within a quite narrow average know how much power is consumed?

You also have hardware control chips like the T1/T2 that could help narrow this down even further.

I would get it for PC's, but here, it's quite different.

lkxijlewlf2• 4 years ago

Yes.

But why would they? There's no benefit for them.

jeffbee• 4 years ago

They wouldn’t need to guess. CPUs measure their own current consumption and demand specific voltage levels from their regulators. They know the instantaneous power consumption to at least three significant digits.

wongarsu2• 4 years ago

On Windows (or Linux) you can get power consumption numbers down to the core using tools like HWMonitor. My CPU is currently using 95W, and my GPU uses 15W. Of course then you have fans, motherboard etc. On server hardware (where software-queriable power meters in the power supply are commonplace) having as much power draw from the fans as from the CPU isn't uncommon.

avianlyric2• 4 years ago

They do, and their software knows as well. It’s just Activity Monitor that isn’t currently using the data for some reason.

matheusmoreira• 4 years ago

Can't the CPU just measure how much power it's drawing?

Doctor_Fegg• 4 years ago

CPU graphs don't show power usage and they never have

Sure, but Activity Monitor has separate CPU and Energy graphs, and the author here appears to be talking specifically about the Energy graph ("According to Activity Monitor’s Energy pane") rather than the CPU one.

Someone• 4 years ago

I don’t think the article only looked at CPU graphs (FTA: “According to Activity Monitor’s Energy pane”)

https://support.apple.com/en-gb/guide/activity-monitor/actmn...:

- In the Activity Monitor app on your Mac, click Energy (or use the Touch Bar).

To display more columns, choose View > Columns, then choose the columns you want to show.

The energy use of individual apps and their processes is displayed in the upper part of the Activity Monitor window.

- Energy Impact: A relative measure of the current energy consumption of the app (lower is better).

coldtea• 4 years ago

You can never get a power estimate from a CPU usage graph at all.

Yes, but you could (or should) get one from Activity Monitor's energy usage graph (under the "Energy" tab).

zinekeller• 4 years ago

If it's already messy in macOS, it's even messier in Windows:

https://devblogs.microsoft.com/oldnewthing/20210629-00/?p=10...

https://aaron-margosis.medium.com/task-managers-cpu-numbers-...

https://illuminati.services/2021/03/17/windows-10-task-manag...

dijit• 4 years ago

I'm torn, or maybe I misunderstood, from what I know about energy efficiency of CPU's, it has always been: Spend less time on the CPU.

If your task is short lived, or constrained by I/O, affining it to E cores is going to be better so that you don't have to "race to a stop-light".

But, to take an extreme example: having a 10w CPU awake for 24hrs vs a 30w one for 7 hours is going to be less energy efficient.

avianlyric2• 4 years ago

Race-to-idle is a good strategy in homogenous CPU setups.

But chips like the M1 with its E and P cores (or ARMs BIG.little designs) significantly change the playing field, and race-to-idle makes less sense.

Two of the places you can look to understand why running on an E core for longer is better than running on a P core for shorter, is power consumption as frequency increases, and number of computational units in each core.

Power consumption in CPU generally increases exponentially with clock frequency. So double the frequency, if 4x the power (as a very rough guiding principle), so going slow and steady can be more power efficient.

Additionally E cores will have fewer computational units (ALUs and other basic computing blocks) compared to a P core. Running a core generally means you’re powering all of it computational units, regardless of if they’re actually doing any work (modern CPU have very good power gating, but there’s a limit). So running a core with less silicon is more efficient, because portion of powered silicon doing useful work is larger.

Put all of the above together, throw in sophisticated power gating, CPU clock scaling, various on die peripherals (like media engines, crypto engines etc) and all of a sudden, simple strategies like race-to-idle stop being a good approach.

nicoburns• 4 years ago

100% active residency of an E core running at maximum frequency uses around 100 mW, and the same active residency of a P core running at its higher maximum frequency uses ten times that amount, 1000 mW.

Damn, that's a lot more efficient!

rocqua• 4 years ago

just as they would on an Intel processor with its identical cores.

Whilst its true that all intel macs have identical cores, the latest intel architecture also has Efficiency cores that are different from the performance cores.

bloopernova• 4 years ago

I was curious about this, and I found this explanation of the Intel Alder Lake series:

https://superuser.com/questions/1677692/what-are-performance...

jmull• 4 years ago

This is good to know.

I wonder if Xcode Instruments could be used to profile energy usage instead?

(Well, actually, for the stuff I do I almost certainly want to leave it to the OS to decide when to use which core.)

mc4ndr3• 4 years ago

There's an argument that says to prefer performance cores, because once a non-looping task completes, the cores can idle again.

This allows the system to more quickly reach a very power efficient state, compared to leaving a weak stove on to cook a large pot of chili.

nfriedly• 4 years ago

I always thought that part of the reason that Apple measured energy impact in arbitrary units was because they're trying to take into account the energy used by the rest of the system. So, for example, if a task takes half the energy when running on E cores, but causes the screen, wifi, disk, etc. to stay on longer, then perhaps it really is causing more total power to be used.

But, OTOH, if it's happening in the background while you're already using the computer, then those parts were probably going to be on anyways, so I suppose it's fairly context-dependent.

Don’t Trust Activity Monitor on M1 Macs

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