Memory power management power savings test [LWN.net]

Memory power management power savings test

Posted Jun 13, 2011 14:11 UTC (Mon) by nye (guest, #51576)
In reply to: Memory power management power savings test by ccurtis
Parent article: Memory power management

So, back in 2003 I built my first system with 1GB of RAM. Initially I thought I would save money by re-using a number of existing components, including the power supply.

Sadly, the existing power supply proved inadequate and the system would crash shortly after booting, if it even booted at all. The memory was a single DIMM so there wasn't anything I could remove to reduce power consumption, however it turned out that telling the kernel to use only about 800M allowed the system to remain completely stable for the few days it took to get a new power supply.

Thus I conclude that the most likely explanation is that RAM which the OS doesn't believe even exists actually does use less power than RAM which is simply not in use at the time.

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Memory power management power savings test

Posted Jun 16, 2011 18:26 UTC (Thu) by Pc5Y9sbv (guest, #41328) [Link]

You may have just been lucky that the most voltage-sensitive circuits were active in the "upper" region you excluded. It's not likely that the memory was statically powered down because it wasn't in use, but there are many dynamic power loads in digital circuits as they change states.

It could even have to do with certain combinations of address and data bits that required more power to configure the addressing logic and route the data signals, and it didn't stabilize within the configured access timings.

The end result is that certain memory addresses in a given module will tend to show corruption before others as the power supply sags or the timings get too tight. This is why people advocate long runs with a dedicated memory test program to try to validate parts in situ. Just running an OS may not exercise combinations of address and data bits with sufficient testing coverage, at least not for many hours (or weeks!) of operation.