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I've got a Core i7 950.

Which eats a bit too much (20 Watt at idle, 133 Watt under load), and I would like to get the lowest energy consumption possible. If I turn off some of the cores. Will that save me some energy (in idle)?

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Probably not. Though it should be fairly simple to test. – ChrisF Mar 7 '12 at 23:19
I think you can't save electricity, only energy. – Oliver Salzburg Mar 7 '12 at 23:20
Don't have a watt-hour meter or anything like that. :-/ | Fixed Oliver, sorry. – Shiki Mar 7 '12 at 23:20
@Hennes - Yes, the first gen Core CPU. – Shiki Sep 11 '13 at 16:37

You may also want to look into your BIOS. What will save more energy than limiting Cores would be to down clock your CPU. For example, I have a dual core phone and it's battery life is significantly better than the single core predecessor. Not because it has a larger battery (same mAH) but because it is able to efficiently use two cores to get the same task done. The single core would need to run at a higher speed in order to do the same task in the same amount of time which means basically that it's using more energy and not to mention getting hotter which is a sign of inefficiency (wasted energy).

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True. Got a point there. I just need my CPU to run in the lowest possible speed (lowest energy consumption) possible for a given time. – Shiki Mar 8 '12 at 0:02

Yes. Cores consume a substantial amount of power, and that is also the reason why when you unplug from the mains and go to battery power, Windows will turn off as many cores as it can in order to save power.

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How much power cores use varies a lot. From 5 Watt Atom/E350's up to 150 Watt for top of the line server chips. Sometimes the actual CPU uses a lot less than even the chipset (e.g. in the first Asus E701 where the ULV CPU used 3-ish Watt, the chipset 5 Watt and the screen even more). Thus there is no one single answer. Also modern CPU's do stop the cores they do not use, but only modern CPU. THus your answer might benifit from adding some detail. – Hennes Sep 9 '13 at 14:33

The feature is called "Core Parking" More info here

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But they jump up all the time. I can't really disable and track all the things down which cause the interrupt (the rise in MHz). – Shiki Mar 8 '12 at 1:01
If you're running an OS that supports it AND your BIOS supports it (and the CPUs support it, which tends to but not always goes with the BIOS supporting it), then core parking is your answer. Microsoft implemented this to reduce the amount of electricity used by a system. It's POSSIBLE (even probably Linux and other operating systems will/already do support this, but it's fairly new technology so they might not. – Multiverse IT Mar 8 '12 at 2:35
Linux has multicore-aware scheduler for ages, once enabled it runs on as little cores on as little CPUs as possible for long. BIOS support determines whether core runs idle (best option) or at minimum frequency (not bad either) Windows core parking will do same thing - only that BIOS and motherboard should be able to not route interrupts to parked processor cores, otherwise it will keep unused cores at minimum frequency. – ZaB Mar 9 '12 at 9:48

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