Hi,

On Wed, Jun 29, 2011 at 10:06:24AM -0700, Dave Hansen wrote:

I was kinda hoping for something a bit simpler than that.  I'd boil down
what you were saying to this:

     1. The kernel must be aware of how the pieces of hardware are
        mapped in to the system's physical address space
     2. The kernel must have a mechanism in place to minimize access to
        specific pieces of hardware 
     3. For destructive power-down operations, the kernel should have a
        mechanism in place to ensure that no valuable data is contained
        in the memory to be powered down.

	4. The kernel must have a mechanism to maintain utilization
	   statistics pertaining to a piece of hardware, so that it can
	   trigger the hardware to power it off
	5. Being able to group these pieces of hardware for purpose of
	   higher savings. 

Is that complete?

On Wed, 2011-06-29 at 18:30 +0530, Ankita Garg wrote:

quoted

1) Dynamic Power Transition: The memory controller can have the ability
to automatically transition regions of memory into lower power states
when they are devoid of references for a pre-defined threshold amount of
time. Memory contents are preserved in the low power states and accessing
memory that is at a low power state takes a latency hit.

2) Dynamic Power Off: If a region is free/unallocated, the software can
indicate to the controller to completely turn off power to a certain
region. Memory contents are lost and hence the software has to be
absolutely sure about the usage statistics of the particular region. This
is a runtime capability, where the required amount of memory can be
powered 'ON' to match the workload demands.

3) Partial Array Self-Refresh (PASR): If a certain regions of memory is
free/unallocated, the software can indicate to the controller to not
refresh that region when the system goes to suspend-to-ram state and
thereby save standby power consumption.

(3) is simply a subset of (2), but with the additional restriction that
the power off can only occur during a suspend operation.  

Let's say we fully implemented support for (2).  What would be missing
to support PASR?

Yes, PASR is a subset of (2) from implementation perspective.

-- 
Regards,
Ankita Garg (ankita at in.ibm.com)
Linux Technology Center
IBM India Systems & Technology Labs,
Bangalore, India