Super User is a question and answer site for computer enthusiasts and power users. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

When purchasing a new MBP I want to optimize it for mobile usage.

There are different quad-core i7 processor available for the 15'' MBP. See the list here. While they have different features, all of them have the same maximum thermal design power:

enter image description here

  • How does their power drain compare at their base frequencies?
  • Will a processor of the same generation, but lower base frequency, draw less power?
share|improve this question
Modern processors spend most of their time not working. E.g. when you're using a word processor, up to 90% of the time between keypresses your CPU will be asleep. Faster processors will spend more of their time asleep. Spending 90% of the time at 0 Hz makes the notion of a "base frequency" quite meaningless. – MSalters Feb 23 '12 at 12:26
I don't understand. I've never noticed my CPU not working. I have a CPU monitor currently running and it's never 100% idle. – user111964 Feb 23 '12 at 12:39
The feature is intentionally not noticeable. The CPU is not in a "deep" sleep, so it can respond immediately. As you note, your CPU is often 90% idle - that's the average over 1000 milliseconds. In practice, this means it's 100% idle during 900 milliseconds, and 100% busy during the other 100 ms. – MSalters Feb 23 '12 at 12:44
Ah ok. So would you assume, that the high-frequncy processors would consumer less power overall because they sleep more? – user111964 Feb 23 '12 at 12:47
No, because this is precisely offset by their higher power draw when they're not asleep. See my answer. – MSalters Feb 23 '12 at 12:53
up vote 0 down vote accepted

In general, within the same family, CPUs draw almost the same amount of energy per instruction. Faster CPU's can execute more instructions per second, so they can draw more power if they're fully loaded.

share|improve this answer
This makes sense, because the architecture of the CPU determines the power consumption of an instruction, right? I'm close to accepting this. I would just like some literature for confirming your first sentence. – user111964 Feb 23 '12 at 13:06
For a given architecture, the transistors used by a ssingle instruction are fixed. This paper shows that the energy used per transistor per operation is pretty much constant, therefore the total energy use per instruction is fixed too. (BTW PC CPU's are classified as high-performace in that paper) – MSalters Feb 23 '12 at 13:19
Great @MSalters! Thank you very much for your help :) – user111964 Feb 23 '12 at 14:07

You must log in to answer this question.