[Beowulf] extreme dynamic underclocking and undervolting

Chris Samuel csamuel at vpac.org
Mon Mar 5 02:55:05 EST 2007


On Sat, 3 Mar 2007, David Mathog wrote:

> So it would be nice if the range of underclocking / undervolting
> adjustments  provided on compute nodes extended quite a bit further
> towards the lower end than it currently does.

FWIW 2.6.21 looks like it will include i386 support for the clockevents and 
dyntick patches that have been developed out in the real time Linux world. 
Apparently they have AMD64 and ARM patches too, but these haven't been merged 
as of yet.

There's a nice LWN article that describes this work:

http://lwn.net/Articles/223185/

All of this is an improvement, but there is still one thing which could be 
better: there is no real need for a periodic tick in the system. That is 
especially true when the processor is idle. An idle CPU can save quite a bit 
of power, but waking that CPU up 100 times (or more) per second will hurt 
those power savings considerably. With a flexible timer infrastructure, there 
is no point in turning the CPU back on until it has something to do. So, when 
the (i386) kernel goes into its idle loop, it checks the next pending timer 
event. If that event is further away than the next tick, the periodic tick is 
turned off altogether; instead, the timer is is programmed to fire when the 
next event comes due. The CPU can then rest unharrassed until that time - 
unless an interrupt comes in first. Once the processor goes out of the idle 
state, the periodic tick is restored.
[...]

It quotes the developers saying:

The implementation leaves room for further development like full tickless 
systems, where the time slice is controlled by the scheduler, variable 
frequency profiling, and a complete removal of jiffies in the future.

-- 
 Christopher Samuel - (03)9925 4751 - VPAC Deputy Systems Manager
 Victorian Partnership for Advanced Computing http://www.vpac.org/
 Bldg 91, 110 Victoria Street, Carlton South, VIC 3053, Australia

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