What does 95th %tile mean?

Fri Apr 20 00:34:53 UTC 2001

> In the extreme I cited, (full rate for 5 minutes, idle for five minutes, 
> repeated),  the five minute average rate oscillates between zero and full 
> line rate. The period of oscillation is 10 minutes (i.e. five minutes for 
> the five minute rate to decay from line rate to zero and fine minutes to 
> build back to line rate).

I do think you are confused, Geoff. 

The 5-minute average is not being sampled every five minutes.

The raw number of octets is being sampled every five minutes, and
divided by the time since the previous sample (5 minutes). Then,
95th percentile is taken of that.

So, if you have 5-min FULL, 5-min IDLE, continuously all day, then a
given 5 minute burst either falls entirely within one period, in which
case that period has maximum utilization, or it falls half in one and
half in another, in which case you have half utilization. Or somewhere
inbetween. But in no case would the measured utilization for the
greater of the two periods be less than half.

So your 95th percentile will fall somewhere between 50% and 100%,
not beteen 0% and 100%.

I think that real world traffic is extremely unlikely to meet this
precise synchronization case, and that it is not terribly worth
worrying about. If it were to become a source of billing disputes
between providers and customers, then presumably we would work out a
better system.

--jhawk