Re: Calculation of stdev

On Thu, Jul 19 2007, Erwan Velu wrote:
> Hey, I'm back with another note ;)
>
> I've been running the following job :
>
> job1: (groupid=0, jobs=1): err= 0: pid=9099
> Description : ["Read Seq 16k O/Direct"]
> read : io=16384KiB, bw=13315KiB/s, iops=812, runt= 1260msec
> clat (usec): min= 278, max=317965, avg=1225.76, stdev=13734.77
> bw (KiB/s) : min= 3996, max= 3996, per=30.01%, avg=3996.00, stdev=-1.00
> cpu : usr=0.00%, sys=5.40%, ctx=1029
> IO depths : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%,
> >=64=0.0%
> issued r/w: total=1024/0, short=0/0
> lat (usec): 500=91.31%, 750=2.54%, 1000=0.10% lat (msec): 2=2.44%,
> 4=3.03%, 10=0.20%, 20=0.10%, 100=0.10% lat (msec): 500=0.20%
>
> I'm really surprised that stdev for bw is set to -1 whereas the min, max &
> average are equal.
> Stdev in such situation should be 0 isn't it ?

Yep it should, that looks very odd. -1 is the start value, I'll see if I
can reproduce!

-- 
Jens Axboe