Nelson's log

Ubuntu 12.04 power savings

This is kind of neat; Ubuntu 12.04 is saving me 10-15 watts or about 25% of my server’s power consumption. I upgraded from Ubuntu 11.10 and my UPS is telling me my server uses 45W now instead of 58W, or thereabouts. The system is a few degrees cooler, too.

I suspect the important change is from the 3.0 Linux kernel to the 3.2 kernel, and the subsequent Power Saving Tweak where the Intel chipset is told to shut down the built in GPU. That’s a win for a lot of folks; headless servers (without weird code using the GPU) and desktop computers with “real” graphics cards. I’m surprised it’s so much an improvement though.

A second possibility is CPU affinity; there’s been some work on multicore systems trying to keep some cores shut down more often if the workload is light. Details on Less Watts tips. I enabled sched_smt_power_savings a few hours after upgrading Ubuntu, but I didn’t see any difference, so maybe not. The CPU frequency scaling graph suggests that maybe something’s changed.

The one alarming thing is that the IO Service Time went way up, from .00001s to .05s. Huh? I’m not doing much IO, so maybe this is a bogus measurement, and I can’t really understand what Munin is measuring anyway. This may be an artifact of the disks being put to sleep more often, or maybe the measurement changed scale, or who knows what. Simple disk benchmarks suggest nothing’s really wrong.

Update: turns out this was a change in the Munin scripts, a fix for an eight year old bug. The Ubuntu 11.10 version was reporting bogus numbers that were backwards and too small. Supposedly the new numbers are correct, and also report no data correctly now. Just reading the numbers off the graph, my system now says writes take about 20ms when before it was saying 0.1ms. Disk seeks are roughly 10ms, so I’m inclined to believe the new numbers are more correct. The bug comments also note that the various disk stats graphs contain similar information but are better.