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Comment by yjftsjthsd-h | Hacker News Reader

I don't think that's true; isn't the relationship between clock speed and power non-linear?

The real killer with energy usage in browsers is idle wake ups per second. If you've got a lot of tabs open and they're all running timers, waking up, hitting the network, etc. then they're keeping the CPU from going into low power state and thus wasting a lot of energy.

Even though I'd prefer to use Firefox I tend to stick with Safari due to the battery life advantage which really shows when you open a lot of tabs.

Indeed the most dramatic improvement I saw in battery life was when macOS and Safari started cooperating around timer coalescing using App Nap.

That's for max frequency on a given process node. Power scales with voltage squared. But that doesn't say anything about wasted power. And dynamic scaling screws that up in modern chips.

I believe I could summarize things by saying the only way you can really save energy* doing the same work+ is by using a different semiconductor process (either power/leakage-reduction-focused or smaller).

* For serious values of "energy"

+ Where the same work is not always true for a given task, if one optimizes an algorithm

Can you explain the voltage squared thing? To me, power = voltage * current.

For chips, power scales with voltage squared. Is also true that P=IV (since both are true, these observations cannot be in contradiction). Apparently, for chips, the current must be proportional to voltage also. Glossing over some details, turning on (off) a transistor is the same as charging (discharging) a capacitor. The energy stored on a capacitor is 1/2 C V^2. If you turn on and off the transistor periodically (say with frequency f) you use 1/2 C V^2 energy f times per second (energy per unit time is power). Normally the capacitance is ignored when discussing how power changes because for a given design the capacitance is a fixed quantity.

Running a processor at higher frequency also requires increasing voltage, which increases effective capacitance by that same formula.

That's not the primary cause of the power = frequency^2 rule, but actually adds a factor on top of that.

Even for purely resistive loads, power is proportional to the square of the voltage:

  P = V * I

But,

  I = V / R

So,

  P = V * (V / R)
    = V^2 / R

I think it is linear for frequency and non-linear for voltage, i.e. P~fCV^2. But in many current CPUS, the feature that adjusts frequency also adjusts voltage. That's why I stipulated that, for my comments to be true, such shenanigans as dynamic frequency (and voltage) scaling must be "turned off." I think the OP was asking, what happens to CPU energy if you load the web page with and without the optimized compilation. The OP was interested in core sleep states, but I think that dynamic frequency scaling is a confounding factor. It would be interesting to see the measurements w/ and w/out that feature perhaps.

To increase the frequency, you also have to increase the voltage so that the transistors charge faster, otherwise they won't be able to switch in the shorter time.

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