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why idle at 700mhz ???

Posted: Thu Nov 15, 2012 7:11 pm
by munklefish
Hi,

It recently occurred to me that excluding turbo mode the pi runs at a constant 700mhz even when idle. This got me wondering why the idle speed isnt lower, especially considering how the pis low power consumption seems to be one of its main selling points.

Now i can see the '[email protected] the polar icecaps' brigade screaming that its already lower power so whats the point in using less power.........

But seriously, if idle speeds can be lowered and less electricity used, then why not??????

This would be a great boost for the many solar pi'ers on the forum, plus it just seems to make sense.


Now i know the idle speed can be lowered via nano /boot/config.txt



So i guess my question is.... Why not???????

Re: why idle at 700mhz ???

Posted: Thu Nov 15, 2012 7:28 pm
by Frodo1B
See the comments by Flojer0 in this thread:

http://www.raspberrypi.org/phpBB3/viewt ... ng#p193341

Seems it might be possible. Now the can you test the wattage and report what it saves?

Re: why idle at 700mhz ???

Posted: Thu Nov 15, 2012 7:59 pm
by munklefish
Thanks for the response but that thread appears to be about underclocking and under volting, rather than reducing the idling speed.

Re: why idle at 700mhz ???

Posted: Thu Nov 15, 2012 8:23 pm
by jamesh
Underclocking is in effect reducing the idling speed, so that would give you some idea of the power saving to be gained. I don't think it's that much tbh.

The GPU already does lots of power saving by reducing clock frequency and turning off unused HW blocks.

Re: why idle at 700mhz ???

Posted: Thu Nov 15, 2012 9:07 pm
by munklefish
jamesh wrote:Underclocking is in effect reducing the idling speed, so that would give you some idea of the power saving to be gained. I don't think it's that much tbh.

The GPU already does lots of power saving by reducing clock frequency and turning off unused HW blocks.

Ah ok, i get the relevance now, sorry.

Tbh, i was thinking more along the lines of is there any linux or mechanical reasons not to reduce idle speeds. Not being a linux guy..... im doing my best to try and change my ways.

Re: why idle at 700mhz ???

Posted: Thu Nov 15, 2012 11:16 pm
by jojopi
When no other processes are runnable, Linux schedules the idle task. On most platforms this at least halts the pipeline, executing no further instructions until the next timer or hardware interrupt. It may also stop propagating the clock to parts of the core, or even power them down. So if a system is truly idle it probably does not matter too much what clock rate it was running at.

Having said that, if a machine is only 50% idle then it may indeed be more efficient to run it at half speed and undervolted, instead of at full speed half the time and half the time halted.

I have not seen or taken any detailed measurements, but I think the ARM core is not a big consumer of power in the Pi anyway, especially compared with the LAN chip, and certainly when idle. If you really wanted to hug the polar ice caps, you would probably start by replacing the linear voltage regulators.

Re: why idle at 700mhz ???

Posted: Sat Nov 17, 2012 6:23 pm
by Grumpy Mike
especially considering how the pis low power consumption seems to be one of its main selling points.
What! The Pi is an absolute power hog compared to virtually every other embedded controller.

Re: why idle at 700mhz ???

Posted: Sat Nov 17, 2012 9:01 pm
by munklefish
Grumpy Mike wrote:
especially considering how the pis low power consumption seems to be one of its main selling points.
What! The Pi is an absolute power hog compared to virtually every other embedded controller.
Oh sorry, i thought a few watts was somewhat of an improvement over a desktop pc. I guess some people just have to take things out of context inorder that they can find fault.

Re: why idle at 700mhz ???

Posted: Sat Nov 17, 2012 9:04 pm
by munklefish
jojopi wrote:If you really wanted to hug the polar ice caps, you would probably start by replacing the linear voltage regulators.
Surely that would cause them to melt anyway. :D

Re: why idle at 700mhz ???

Posted: Sat Nov 17, 2012 10:09 pm
by jamesh
Grumpy Mike wrote:
especially considering how the pis low power consumption seems to be one of its main selling points.
What! The Pi is an absolute power hog compared to virtually every other embedded controller.
It's not an embedded controller. :shock:

Re: why idle at 700mhz ???

Posted: Sat Dec 29, 2012 9:17 pm
by owenl
In config.txt set arm_freq_min=XXX, where XXX is the minimum frequence you want, I am running at 100Mhz. You also need to set the CPU governor to ondemand or conservative. The PI will then switch between the minimum and core frequeceny as required.

Re: why idle at 700mhz ???

Posted: Sat Dec 29, 2012 9:36 pm
by munklefish
owenl wrote:In config.txt set arm_freq_min=XXX, where XXX is the minimum frequence you want, I am running at 100Mhz. You also need to set the CPU governor to ondemand or conservative. The PI will then switch between the minimum and core frequeceny as required.
No offence but did you actually read the full thread before posting?

Re: why idle at 700mhz ???

Posted: Sat Dec 29, 2012 9:44 pm
by ghans
Perhaps he suggests playing with the governors ?
What are they anyway ? A feature of the kernel ?


ghans

Re: why idle at 700mhz ???

Posted: Sun Dec 30, 2012 12:14 am
by munklefish
ghans wrote:Perhaps he suggests playing with the governors ?
What are they anyway ? A feature of the kernel ?


ghans
The govenor is already set to ondemand isnt it, otherwise turbo mode would never kick in???

Re: why idle at 700mhz ???

Posted: Sun Dec 30, 2012 8:28 am
by owenl
You can check your current governor with this

Code: Select all

cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
You may need to add sudo to run this as root, I was logged in as root at the time of testing.

For more details take a look at

http://wiki.stocksy.co.uk/wiki/Raspberr ... t_in_a_box

scroll down to the frequency scaling section, I did this and it works.

Re: why idle at 700mhz ???

Posted: Sun Dec 30, 2012 12:11 pm
by munklefish
ghans wrote:Perhaps he suggests playing with the governors ?
What are they anyway ? A feature of the kernel ?


ghans
Yes. The governor is responsible for determining when and how the CPU should respond to system demands.