Re: [Collectl-interest] Are measurements cumulative?

[Mark S. <mj...@gm...>]

On Fri, Feb 17, 2012 at 9:59 AM, Frank Heckes <f.h...@fz...>wrote:

> Hi all,
>
> sorry if this has been asked and answered before or if I was to blind to
> find it inside the documentation and the mail archive.
>

could be you're blind OR it could be it's not there.  ;)
lot's of documentation and when you're a one person team you miss things.


> How does the tool calculate
>
> -a- quantities (like number of slabs, memory allocated, queuelength,...)
>

these are simply instantaneous values as reported in /proc/xxx


> -b- utilisation (CPU used%,...)
>

all cpu times reported in /proc/stat are in jiffies, so I just look at the
change between to samples and add up the user, system and other times
ignoring iowait since that's not real cpu.  this then gives me the total
number of jiffies in the interval.  now it's simply a matter of something
like

user/total*100 to tell me the % time spend in user time

some tools actually report based on a single core and so a cpu bound 8 core
system would be reported as 800% but collectl reports 100%


> -c- rates (disks read/write, iops,...)
>

that's simply the different between start/finish divided by seconds.  if
you include -on, it divides by 1 to give absolute numbers


> when using different sampling intervals (-i ...)?
>

numbers should all be the same unless -on specified


> For rates it seems obvious(?) to divide the difference of counter values
> at the end and and the start by the sampling time.
>

correct


> Could someone explain what is the effect on choosing longer sampling
> intervals and how 'averaging' is done and which meas are considered in
> the calculation?
>

the only different in using longer sampling intervals is loss of accuracy.
 my favorite example is if you have a 30 second spike in the network and
are only sampling every couple of minutes, you'll see an elevation but
never know you were saturated for 1/2 minute.  even at 10 seconds you'll
miss shorter spiked, but 10 seconds has shown to be a good compromise,
though some user choose 5 or even 1 second.

hope this helps

-mark


> Many thanks in advance.
>
> Cheers
>
> -Frank
>
>
>
>
>
>
>
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