Energy-efficient cpu usage in multi-core systems

Question

I've a rather theoretical question: Is there a relation between cpu/core usage and the energy consumption. I try to explain it in an example: Lets say we have a set of tasks T and one core C1. C1 would complete the set T within 10 seconds while having a usage of 100%. Can it be more energy efficient to use two cores C1 and C2 and divide the tasks from T among them so it is still finished in 10 seconds but the usage of C1 and C2 during this time is 50%? (<-- just an example, the numbers are not real) Like a car has a certain speed where it does not waste to much gas (even though i am aware that a vehicles speed is more equivalet to a cores frequency). I although thought that when a cpu/core runs on 100% it gets warm than (lets say) 50%, which means it needs more energy for cooling.

I would be thankful for any comments on this topic

cheers

Answer 1

For CMOS circuits energy depends on number of switches. At the most basic level energy depends on total amount of work but not on CPU load percentage.

But there are some aspects:

1) energy is proportional to square of CPU voltage. OS could lower voltage using technologies like Intel EIST, making 2 x 50% better

2) clock net consumes high amounts of energy regardless of load. OS could lower frequency making 2 x 50% better

3) OS could disable inactive core, making 1 x 100% better. But I don't know such technologies. Probably it is possible for ARMs. I didn't find anything in Google.

2 x 50% looks better.