Date Fri, 12 Aug 2011 21:19:54 +0800 From Wu Fengguang <> Subject Re: [PATCH 2/5] writeback: dirty position control
`On Wed, Aug 10, 2011 at 01:20:27AM +0800, Peter Zijlstra wrote:> On Tue, 2011-08-09 at 12:32 +0200, Peter Zijlstra wrote:> > >                     origin - dirty> > >         pos_ratio = --------------> > >                     origin - goal > > > > > which comes from the below [*] control line, so that when (dirty == goal),> > > pos_ratio == 1.0:> > > > OK, so basically you want a linear function for which:> > > > f(goal) = 1 and has a root somewhere > goal.> > > > (that one line is much more informative than all your graphs put> > together, one can start from there and derive your function)> > > > That does indeed get you the above function, now what does it mean? > > So going by:> >                                          write_bw>   ref_bw = dirty_ratelimit * pos_ratio * -------->                                          dirty_bw> > pos_ratio seems to be the feedback on the deviation of the dirty pages> around its setpoint.Yes.> So we adjust the reference bw (or rather ratelimit)> to take account of the shift in output vs input capacity as well as the> shift in dirty pages around its setpoint.However the above function should better be interpreted as                                            write_bw    ref_bw = task_ratelimit_in_past_200ms * --------                                            dirty_bwwhere        task_ratelimit_in_past_200ms ~= dirty_ratelimit * pos_ratioIt would be highly confusing if trying to find the direct "logical"relationships between ref_bw and pos_ratio in the above equation.> From that we derive the condition that: > >   pos_ratio(setpoint) := 1Right.> Now in order to create a linear function we need one more condition. We> get one from the fact that once we hit the limit we should hard throttle> our writers. We get that by setting the ratelimit to 0, because, after> all, pause = nr_dirtied / ratelimit would yield inf. in that case. Thus:> >   pos_ratio(limit) := 0> > Using these two conditions we can solve the equations and get your:> >                         limit - dirty>   pos_ratio(dirty) =  ---------------->                       limit - setpoint> > Now, for some reason you chose not to use limit, but something like> min(limit, 4*thresh) something to do with the slope affecting the rate> of adjustment. This wants a comment someplace.Thanks to your reasoning that lead to the more elegant                             setpoint - dirty 3   pos_ratio(dirty) := 1 + (----------------)                            limit - setpointThanks,Fengguang`

Last update: 2011-08-12 15:23    [from the cache]