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Memory power management power savings testMemory power management power savings testPosted Jun 10, 2011 15:05 UTC (Fri) by ccurtis (guest, #49713)In reply to: Memory power management power savings test by mjg59 Parent article: Memory power management
Perhaps I was a bit too terse. The amount of power actually saved is a bit unclear; estimates seem to run in the range of 5-15% of the total power used by the memory subsystem. [...] A recent patch set from Ankita Garg does not attempt to solve the whole problem; instead, it creates an initial infrastructure which can be used for future power management decisions. Before creating this extensive infrastructure, perhaps it would be better to get an idea of what kind of power savings this actually provides. Code would still need to be written to power down the memory bank, and code would also likely need to be written to isolate the excluded RAM from the boot parameter, but this seems like a relatively easy way to answer the question before embarking on the endeavor. Of course, this may be a done deal and it's just a matter of time before the code gets written, but it would still be interesting to see how much power is actually going to be saved. A patch like this would allow individuals to measure the power savings of their own systems as well, in case they wanted to control the trade of any overhead the new memory management changes might impose.
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Memory power management power savings test Posted Jun 10, 2011 16:20 UTC (Fri) by etienne (subscriber, #25256) [Link]
Maybe just reboot your PC with different number of memory modules and measure the power difference?
Memory power management power savings test Posted Jun 11, 2011 3:19 UTC (Sat) by willy (subscriber, #9762) [Link]
The problem is that pages are interleaved across DIMMs. If you have 3 channels with a single DIMM each, cacheline 0 is on DIMM 0, cacheline 1 on DIMM 1, cacheline 2 on DIMM 2 and cacheline 3 on DIMM 0. Removing a DIMM causes the interleaving to change, which will also cause performance to change, and your measurements are now invalid.
As I understand PASR, one would not power down an entire DIMM, but rather sections of each DIMM, thus preserving the performance benefits of interleaving.
Memory power management power savings test Posted Jun 16, 2011 4:54 UTC (Thu) by Ankita (subscriber, #39147) [Link]
Yes, PASR support seems to be at bank level. From a few documents I have read, I find that interleaving can be configured to control the number of banks that have to be kept open for every memory access, typically the minimum being 2 banks. Thus, if only two banks are interleaved, the other banks can potentially be turned off or not refreshed. Power v performance benchmarking will be needed to decide on the best interleaving scheme though.
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