[dm-devel] [PATCH 3/4] libmultipath: path latency: simplify getprio()

[Guan Junxiong]

On 2017/11/18 8:11, Martin Wilck wrote:
> The log standard deviation can be calculated much more simply
> by realizing
> 
>    sum_n (x_i - avg(x))^2 == sum_n x_i^2 - n * avg(x)^2
> 

I derive the equation:
 sum_n {(x_i - avg(x))^2} = sum_n{x_i^2 -2*x_i*avg(x) + avg(x)^2}
                          = sum_n{x_i^2} - 2*avg(x)*sum_n{x_i} + sum_n{avg(x)^2}
                          = sum_n{x_i^2} - 2*avg(x)*avg(x) + n*avg(x)^2
                          =  sum_n{x_i^2} + (n-2)*avg(x)^2

> Also, use timespecsub rather than the custom timeval_to_usec,
> and avoid taking log(0).
> 

Great.


> +	pp_pl_log(3, "%s: latency avg=%.2e uncertainty=%.1f prio=%d\n",

latency avg -> latency geometric avg ? Because in most cases,
avg means arithmetic avg , but in this case, it means geometric avg.


> +		  pp->dev, exp(lg_avglatency * lg_base),
> +		  exp(standard_deviation * lg_base), rc);

How can you get the uncertainty of Log-normal distribution
is the exp(standard_deviation * lg_base) ?

Thanks.
Regards
Guan

.

> +
>  	return rc;
>  }
>