Re: [PATCH 3/4] libmultipath: path latency: simplify getprio()

[Martin Wilck <mwilck@suse.com>]

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@suse.com>, Tel. +49 (0)911 74053 2107
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton
HRB 21284 (AG Nürnberg)

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