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I have a Core 2 Duo laptop, and I want to extend my battery life as long as possible.

If one was to use an uniprocessor kernel, the second core should not be 'booted' for use by the OS (for lack of a better word, I know some of the low level technicals about multiprocessor systems and how they work but not everything).

If the second core is not started by the kernel for multiprocessor support, would it be shut down and further reduce power consumption? Or, would it be stuck waiting for a start sequence and thus be using more power (because it will be in a pre-boot wait loop before any power management is enabled)?

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Doesn't the BIOS have an option to disable the second core? This would solve the software problem since it would only see one processor. –  mprill Apr 24 '13 at 14:58
    
I don't recall seeing that option but I'll take another look. –  Burton Samograd Apr 24 '13 at 15:09

2 Answers 2

While you can have the OS only use one processor I am not sure that it will result in a huge power savings. You don't mention which specific Core 2 you have but most modern multi-core CPUs are able to throttle down cores that are not use for heat and power management.

Additionally depending on what you are using the laptop for you may consume more power. This is because (depending on the workload) one core may be running full-out for longer than two cores would be. See this tech-report review for an example of how this can play out.

power

My recommendation would be to use both cores and let the CPU manage the power.

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The CPU can throttle down to 800 MHz using an ondemand govenor, but I use a very low tech system (stumpwm and consoles) and I thought that under compile/run/debug programming I'm doing a single core would be acceptable. There won't be any big processing on this system so I think the would be a power reduction overall, but I won't know until I test and measure. I just wanted to make sure I wasn't completely out to lunch on the idea. –  Burton Samograd Apr 24 '13 at 15:08
    
Yes it will depend on workload and testing it will be your best bet. I know the recent CPUs will completely shutdown cores not in use. I think the Core 2 just throttles the cores. –  Brad Patton Apr 24 '13 at 15:10

You can just disable the second core at runtime by running

echo 0 > /sys/devices/system/cpu/cpu1/online

as root user and check if there is actually a reduction in power usage (e.g. using powertop).

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Thanks, I didn't realize I could do that at runtime. I'll give it a try and measure any difference. –  Burton Samograd Apr 24 '13 at 15:08

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