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I am looking at various laptops at the moment and it's appealing to get a quad core (2720QM) when I need the power (yes, I have an application that can utilize many cores in parallel just fine). But, most of the time those extra cores will just there eating battery. Is it possible to switch a few off?

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I have no idea if this will save power, but you could add the "maxcpus" kernel parameter when you boot. This will control the number of CPUs on which processes may be scheduled. But I have no idea what the power implications are or if it is possible to "switch off" (in the sense of removing their current draw from the circuit completely) a core.

http://www.cyberciti.biz/tips/linux-limiting-or-restricting-smp-cpu-activation-in-smp-mode.html

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It's possible to do this in the BIOS, if it supports it. Obviously YMMV depending on the exact machine, I would be surprised to find many that support it. I don't think there is any way to do it on the fly, though. (Also, if those cores aren't loaded, I doubt you'll see a significant difference in power consumption anyway.)

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