On Thu, 2017-12-07 at 12:26 +0800, Guan Junxiong wrote: > Hi Martin, > > I have tested this patch and found something needed to be correct. My > comments inline. > > > > @@ -340,8 +323,14 @@ int getprio(struct path *pp, char *args, > > unsigned int timeout) > > ".It is recommend to be set larger", > > pp->dev, base_num); > > > > + standard_deviation = sqrt((sum_squares - lg_toldelay * > > lg_toldelay) > > + / (io_num -1)); > > > > This assignment is wrong. It gets a "NAN" for standard_deviation. > It should be the following equation according to sum_n (x_i - > avg(x))^2 == sum_n x_i^2 - n * avg(x)^2 > standard_deviation = sqrt((sum_squares - io_num* lg_avglatency* > lg_avglatency)/ (io_num -1)); > = sqrt((sum_squares - lg_toldelay* > lg_avglatency)/ (io_num -1)); That's of course correct. I'll submit an update asap. > I still have the doubt about the computation of the "uncertainty" > item. > Because I have observed that the uncertainty is in the range of 1.3 ~ > 1.6 whenever > the base_num varies from 1.1 to 10. > Do you mean the uncertainty as the "Error function" of (log) normal > distribution? > Here is the definition of > https://en.wikipedia.org/wiki/Error_function > > I prefer to using "Error function" that describes accumulated > probability of how prio > locates in the (-inf, prio-1) and (prio+1, +inf). There's a misunderstanding here. My "uncertainty factor" describes the width of the distribution of the *measured* latencies. It roughly represents the accuracy of the measurement (indicating that 68%, or error_function (sqrt(1/2)), of the measured values lie in the interval [avg/F, avg*F]). IOW, it tells you how stable your latencies for a certain path are. It has nothing to do with the artificial prio "bins" that the latency prioritizer sets up. Therefore it *has to be independent* of base_num. It just gives an indication how large base_num should be. I hope this makes it clear. Martin -- Dr. Martin Wilck <mwilck@xxxxxxxx>, Tel. +49 (0)911 74053 2107 SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton HRB 21284 (AG Nürnberg) -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
