How do you (not how do I) calculate 95th percentile?

Tom Sands tsands at
Wed Feb 22 18:50:34 UTC 2006

Jo Rhett wrote:

> I am wondering what other people are doing for 95th percentile calculations
> these days.  Not how you gather the data, but how often you check the
> counter? Do you use averages or maximums over time periods to create the 
> buckets used for the 95th percentile calculation?

We use maximums, every 5 minutes.

> A lot of smaller folks check the counter every 5 min and use that same
> value for the 95th percentile.  Most of us larger folks need to check more 
> often to prevent 32bit counters from rolling over too often. 

Actually, a lot of people do 5 minutes... and I would say that larger 
companies don't check them more often because they are using 64 bit 
counters, as should anyone with over about 100Mbps of traffic.

  Are you larger
> folks averaging the retrieved values over a larger period?  Using the
> maximum within a larger period?  Or just using your saved values?

In our setup, as with a lot of people likely, any data that is older 
than 30 days is averaged.  However, we store the exact maximums for the 
most current 30 days.

> This is curiosity only.  A few years ago we compared the same data and the
> answers varied wildly.  It would appear from my latest check that it is
> becoming more standardized on 5-minute averages, so I'm asking here on Nanog 
> as a reality check.

> Note: I have AboveNet, Savvis, Verio, etc calculations.  I'm wondering
> if there are any other odd combinations out there.
> Reply to me offlist.  If there is interest I'll summarize the results
> without identifying the source.

Tom Sands			  				
Chief Network Engineer				
Rackspace Managed Hosting	    	

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