Hi all, sampling_down_factor is a tunable for the ondemand frequency governor. Intuitively, higher values of sampling_down_factor make ondemand act more like performance. For those computers where ondemand has a performance penalty compared to performance, this is good, with the obvious downside of higher power consumption.
Where does this performance penalty come from? Some of it is from the basic operation of ondemand, the checks that it does, and the lag between when a heavy cpu load hits and when ondemand actually turns up the clock frequency. Another, larger, performance penalty can come when (under load) ondemand seems to resonate the cpu frequency instead of keeping it high. My theory is that there is some insidious issue where the logic of ondemand sets up resonance with certain hardware specific setups instead of properly detecting the system as being under load. Question of this email: If the best value for sampling_down_factor is hardware specific, how would it be included into Ubuntu? Is it a kernel issue? distro issue? upstart issue? or something else? A few years ago, when I first noticed this issue, I tested a performance drop of about %30. Today it seems to be around %10. I found a value of about 400 for sampling_down_factor wiped out the performance penalty of ondemand for me. Here is an old bug report ( https://bugs.launchpad.net/ubuntu/+source/linux/+bug/862785) from someone else who had significant performance issues. Here is a relatively recent analysis that takes into account power usage ( http://phoronix.com/forums/showthread.php?86322-Tuning-pre-3-12-ondemand-governor-version-2-now-with-energy-efficiency) There, a value of 7 was determined to be the sweet spot. On a different webpage that I don't have in front of me, I saw a comment saying that a value of 200 caused a performance issue, which leads me to believe that intermediate values could cause other unforeseen resonance issues on certain hardware setups. So again, how could this be "fixed" in Ubuntu? If there are hardware specific optimal values for sampling_down_factor, how would that get included into Ubuntu? One thought I had would be to just have some conservative values tied to the powersaving: a relatively low one for anything on battery mode, and a relatively high one for everything else, where power usage is not a concern.
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