On Mon, 21 Mar 2016, Oliver Neukum wrote: > On Mon, 2016-03-21 at 14:24 -0400, Alan Stern wrote: > > On Mon, 21 Mar 2016, Oliver Neukum wrote: > > > > > On Mon, 2016-03-21 at 10:57 -0400, Alan Stern wrote: > > > > > > > One possible solution is to export a sysfs parameter to prevent > > > > statistics collection (or more generally, to change the interval at > > > > which it occurs). > > > > > > Surely, not performing a task can hardly be beaten in terms of power > > > consumption. That is not meant to be flippant, but I think these > > > issues are orthogonal. The question of how much to do doesn't > > > solve the question of doing efficiently what we do. > > > > In other words, what's the best way to collect the statistics without > > interfering with runtime PM, right? > > Yes. > > > If the device is suspended, presumably we know there's nothing to > > collect -- especially if we already collected the statistics at the > > time the device got suspended. Hence my suggestion to avoid querying > > the device while it is suspended. > > That is perfectly alright if we just collect statistics. > As a generic mechanism it is bad. Think about the polling > for media detection.
True. Here I'm talking specifically about collecting statistics. Media detection has its own requirements. > > But this leaves open the issue that querying the device too often will > > prevent it from going into autosuspend. It seems to me that the best > > way to deal with this is to make sure that the autosuspend timeout is > > shorter than the interal between queries, not to make the querying > > conditional on !runtime_auto. > [..] > > > If we know when the next activity will come, why not pass this > > > information down? > > We have an autosuspend timeout because we think that IO, if it will > come at all, is likeliest to come soon. If, however, the IO is > periodic that heuristics is false. > To save most power the driver must either decide that the interval > is too short or suspend immediately. So if we are lucky enough > to have the frequency in the kernel, we should use that information. The autosuspend timeout is set by userspace. The kernel may assign a default value, but the user can always override it. Given this, I don't see how the kernel can use frequency information (and I'm not sure where that information would come from in the first place). Alan Stern
