In article ,
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So conceivably couldn't they just not crank the clock to the max and save power and generate less heat ? Or
do they already do that ? My laptops have a setting, performance, balanced and battery life. Is that possibly an indirect "underclocking"
control ? If not, what is it ?

Yes, almost certainly it is just that (and perhaps more as well).

Most modern CPUs/motherboards/chipsets support multiple CPU clock
rates - the CPU's own internal clock signal is created via a
PLL/multiplier system, based on a slower fixed-rate crystal clock.
The multiplier system is under processor control, so it's possible for
the CPU to switch speeds "on the fly" (with a momentary pause while
the PLL re-locks). I believe this is usually managed via the ACPI
layer in the BIOS/UEFI.

At the higher (user-visible) layer, this is usually set up via a
performance setting of the sort you mention. The setting then
controls a set of policies managed by the operating system, which
specify when to change CPU clock speeds (and sometimes voltages as
well) based on your usage patterns.

"Battery life" would probably lock the speed at the lowest supported
value, or at least to range of the slower values. "Performance" might
lock it to the highest speed at all times. "Balanced" would either be
a fixed speed in the middle of the range, or a dynamic system which
increases the CPU speed in increments when the CPU is mostly busy, and
reduces it when the CPU is idle more than a certain fraction of the
time.

On the Linux laptop I use, I have a choice of several such dynamic
policies... some are more aggressive about increasing CPU speed, some
are more conservative. The CPU speed can be varied over a range of
about 2:1, on a per-core basis (and a core which is currently sitting
idle isn't using much current at any clock speed, although consumption
is less at the lower clock speeds even when idle).

The current should not be that high, all that current is cumulative right ? Seems like the main issue is charging and discharging the input
capacitance. As such,lower clock speeds should be quite effective. If they could get it down to the point where no heatsink is needed at all,
wouldn't there be enough advantage in cost to justify a slightly lower clock speed ? A phone accessing the internet for example, how much does
that clock speed really mean then ? I am not being sarcastic here, that is a valid question, (I think) how processor intensive is all this ?

Depends what you're doing. Just downloading a file is probably not
CPU-expensive. Rendering a web page full of fancy animated graphics
and video, considerably more so. Doing full-screen video may require
little from the CPU, but may push the GPU quite hard (MPEG-4 or
similar video decoding).

Phones _tend_ to be designed to optimize battery life, as this is a
key selling point... and so they'll be somewhat more conservative
about speeding up their CPUs.

And, phones do have heatsinks. They're called "hands" :-)