I'm considering options for a new portable PC. I'd like to keep power utilization at a minimum, while being able to run batch jobs as quickly as possible. Batch jobs I can generally parallelize using xargs in cases where the processing program itself does not know how to take advantage of multi-core.

So a low-voltage, multi-core chip seems pretty appealing. However, if a dual-core chip is going to have twice the (minimum) power consumption of a single-core chip, I might instead opt for a single core with a higher maximum clock speed. Are multi-core CPUs capable of temporarily shutting down one or more cores in order to lower power consumption?

What's the typical power consumption penalty for the addition of a core (or 3 cores for quad-core chips)? And by contrast, what is the typical minimum power consumption penalty for increased per-core processing speed? By "minumum" I mean to indicate that I'd like the machine to use as little power as possible when the cpu(s) is/are scaled down to the minimum frequency.

  • portable pc == labtop? or small/lightweight pc? – madmaze Dec 18 '10 at 0:29

Nowadays processors technology are really advanced. When you need one core, you can just run it on one core and park the others (on intel cpus, at least).

The reason behind the multicore computing is that for a given architecture (e.g. core i7), given process (e.g. 45nm), given yield, if you scale up the frequency, on the high end of the spectrum the power will scale up much more than linear.

e.g. i3 processor: (**system** peak power)
i3-530 @ 2.93 GHz = 127 Watt
i3-530 @ 3.3 GHz = 133 Watt
i3-530 @ 4.4 GHz = 171 Watt

Note that it is system peak power, not CPU peak power that is measured. Taking the idle power of the system (80 minuses the ~5-7 watt idle power that the CPU draws) you get around 54-100 Watt of peak CPU draw. going from 2.9 GHz to 4.4 GHz is a ~50% overclock but power almost double.

For most consumer level CPUs sold today the frequency is at a fairly low level (e.g. 2.93GHz above) so that a modest overclock generally would not cause much increase in peak power, but at the higher end of the spectrum (enthusiest CPU market) you are going into the steeper part of the curve.

Multicore CPUs deliver relatively linear performance gain (usually, 0.6-0.7x per additional core) at a usually lower power budget.

All in all, scaling less frequency and more core will generally give you better performance in a well-parallelized task.

p.s. <rant>if you had a problem you need to think of parallelizing it to make it work reasonably, perhaps portable PC would not be the answer. how about connecting to a power hungry computer at home to do some remote number crunching?</rant>

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  • Thanks, quite informative. In my case the performance gain would tend to be just about 1x, since in many cases each core will have a separate process to run. – intuited Dec 20 '10 at 6:28
  • I don't really have a need for a desktop box; I'm figuring out what to get to upgrade from my current pentium M machine so I can relegate it to being a file server. I find that running it with the frequency locked at 600mHz is more or less adequate for most tasks, and that the exceptions tend to be things that are readily parallizable; for example, converting a directory full of .mp3 files to .ogg format. – intuited Dec 20 '10 at 6:37

There are hardly any single core processors out there these days anyhow. If you look at Intel's specfinder, you'll see that a Core 2 Solo uses significantly less power (look at max TDP) than a quad core i7, but a low-voltage dual-core Core i7 UM uses only a little bit more than the core 2 solo, and will be significantly faster. Single-core processors are not worthwhile these days.

All modern processors can shut down cores and go into lower power states. Intel processors call this SpeedStep, and basically what it does is runs the processor at a much lower clock speed, and only runs at a higher clock speed when that processor power is actually needed.

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