[SirMaster]

What about 2 workloads that both register 100% CPU usage, but one workload draws significantly more power and heats the CPU up way more? Seems like that workload is utilizing more of the CPU, more of the transistors or something.

[inetknght]

Indeed, and there's a thing called "race to sleep". That is, you want to light up as much of the core as possible as fast as possible so you can get the CPU back to idle as soon as possible to save on battery power, because having the CPU active for more time (but not using as many circuits as it "could") draws a lot more power.

[MBCook]

At the same time, it takes a certain amount of time for a CPU to switch power levels, and I remember it being surprisingly slow on some (older?) processors.

So in Linux (and I assume elsewhere) there were attempts to figure out if the cost in time/power to move up to a higher power state would be worth the faster processing, or if staying lower power but slower would end up using less power because it was a short task.

I think the last chips I remember seeing numbers for were some of the older Apple M-series chips, and they were lightning fast to switch power levels. That would certainly make it easier to figure out if it was worth going up to a higher power state, if I’m remembering correctly.

[magicalhippo]

I deliberately put my govenor to the conservative one, as I hated fans spinning up for a second and then down again repeatedly. Much rather sacrifice a bit of battery and speed for quiet.

[SirMaster]

Can't you just cap the fan speed? Or does it actually get too hot at a lower fan speed to where it would throttle or crash?

[magicalhippo]

I wanted the full power when doing long compiles and such. Just not the fan jojo acion when neowsing the web or writing.

Also swapping the governor was trivial and reliable. Modifying fan profiles has always been a bit of a struggle for me, with huge differences in hardware support, persistence etc.

[porridgeraisin]

> jojo acion when neowsing

jumping into action when browsing

[0x457]

It's just bad fan curve for given workload. Sharp rises and drops are annoying to hear, curve that does that + race-to-sleep do not work together. Low speeds do cause throttling.

[MBCook]

Smart. That would drive me nuts too.

[saagarjha]

Yes, this is pretty normal; your processor will downclock to accommodate. For HPC where the workloads are pretty clearly defined it’s possible to even measure how close you’re coming to the thermal envelope and adjust the workload.

[throwaway31131]

Percent utilization for most operating systems is the amount of time the idle task is not scheduled. So for both workloads the idle task was never scheduled, hence 100% "utilization".

[BrendanLong]

Some esoteric methods of measuring CPU utilizations are to calculate either the current power usage over the max available power, or the current temperature over the max operating temperature. Unfortunately these are typically even more non-linear than the standard metrics (but they can be useful sometimes).

[gblargg]

Like measuring RMS of an AC voltage by running it through a heating element: https://wikipedia.org/wiki/True_RMS_converter#Thermal_conver...

[PunchyHamster]

except it doesn't really tell you much, because having some parts of CPUs underutilized doesn't mean adding load will utilize them. Like if load underutilizes floating point units and you have nothing else that uses them

[inetknght]

> Like if load underutilizes floating point units

This is why I sigh really hard when people talk about some measurement of FLOPS per second, as if it's the only thing that matters.

It matters. Perhaps it matters a lot for specific workloads. But most general workloads are integer-based.

[colejohnson66]

But *sparkle emoji* AI *sparkle emoji*

[kqr]

Also there's dark silicon to consider – the CPU simply cannot for thermal reasons run power to all parts of itself at the same time.