Re: [Qemu-devel] [PATCH v2 3/3] util: Add an utility infrastructure used to compute an average on a time slice

["Benoît Canet" <benoit.canet@nodalink.com>]

On Mon, Sep 15, 2014 at 01:13:08PM +0200, Markus Armbruster wrote:
> Benoît Canet <benoit.canet@nodalink.com> writes:
> 
> > On Mon, Sep 08, 2014 at 05:09:38PM +0200, Paolo Bonzini wrote:
> >> Il 08/09/2014 16:49, Benoît Canet ha scritto:
> >> >> > - create two windows, with twice the suggested expiration period, and
> >> >> > return min/avg/max from the oldest window.  Example
> >> >> > 
> >> >> >        t=0          |t=1          |t=2          |t=3          |t=4
> >> >> >        wnd0: [0,1)  |wnd0: [1,3)  |             |wnd0: [3,5)  |
> >> >> >        wnd1: [0,2)  |             |wnd1: [2,4)  |             |
> >> >> > 
> >> >> > Values are returned from:
> >> >> > 
> >> >> >        wnd0---------|wnd1---------|wnd0---------|wnd1---------|
> >> > 
> >> > This is neat.
> >> 
> >> Alternatively, you can make it probabilistically correct:
> >> 
> >>     t=0            |t=0.66           |t=1.33             |t=2              |t=2.66
> >>                    |wnd0: [0.66,2)   |                   |wnd0: [2,3.33)   |
> >>     wnd1: [0,0.66) |                 |wnd1: [1.33,2.66)  |                 |
> >> 
> >> Return from:
> >> 
> >>     wnd1-----------|wnd1-------------|wnd0---------------|wnd1-------------|wnd0
> >> 
> >> So you always have 2/3 seconds worth of data, and on average exactly 1 second
> >> worth of data.
> >> 
> >> The problem is the delay in getting data, which can be big for the minute-
> >> and hour-based statistics.  Suppose you have a spike that lasts 10 seconds,
> >> it might not show in the minute-based statistics for as much as 30 seconds
> >> after it ends (the window switches every 40 seconds).
> >> 
> >> For min/max you could return min(min0, min1) and max(max0, max1).  Only the
> >> average has this problem.
> >> 
> >> Exponential smoothing doesn't have this problem.  IIRC uptime uses that.
> >
> > I am writing this so cloud end users can programatically get informations about
> > their vms disk statistics.
> >
> > Cloud end users are known to use their cloud API to script the
> > elasticity of their
> > architecture.
> > Some code will poll system statistics to decide if new instances must
> > be launched
> > or existing instances must be pruned.
> > This means introducing a delay in the accounting code would slow down their
> > decisions.
> >
> > min and max is also useful to know since it gives an idea of the deviation.
> 
> For what it's worth, the algorithm in the Dr. Dobb's Paolo referenced
> can compute a standard deviation.  Can we figure out what users really
> want, standard deviation, min/max, or both?

I'll ask to my test subject.

> 
> [...]