Re: power-efficient scheduling design

From: Arjan van de Ven
Date: Fri Jun 21 2013 - 17:34:07 EST

On 6/21/2013 2:23 PM, Catalin Marinas wrote:

oops sorry I misread your mail (lack of early coffee I suppose)

I can see your point of having a thing for "did we ask for all the performance
we could ask for" prior to doing a load balance (although, for power efficiency,
if you have two tasks that could run in parallel, it's usually better to
run them in parallel... so likely we should balance anyway)

Not necessarily, especially if parallel running implies powering up a
full cluster just for one CPU (it depends on the hardware but for
example a cluster may not be able to go in deeper sleep states unless
all the CPUs in that cluster are idle).

I guess it depends on the system

the very first cpu needs to power on
* the core itself
* the "cluster" that you mention
* the memory controller
* the memory (out of self refresh)

while the second cpu needs
* the core itself
* maybe a second cluster

normally on Intel systems, the memory power delta is quite significant
which then means the efficiency of the second core is huge compared to
running things in sequence.
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