pontification bloat (was 10GE TOR port buffers (was Re: 10G switch recommendaton))

[bicknell at ufp.org (Leo Bicknell)]

In a message written on Sat, Jan 28, 2012 at 11:02:14AM +0900, Randy Bush wrote:
> one problem is that we do not have good tools to look at a link and
> suggest parms.  how did you derive those?

It's actually simple math, it just can get moderate complex.

Let's say you have a 10Mbps ethernet interface, and you want to set
the queue size (in packets).

10Mbps is ~1250000 bytes/sec.

Now, I pick an arbitrary value, this is where experience comes in.
For this example I'm going to say I want no more than 5ms queuing
latency.  5ms/1000sec/ms * 1250000 = 6250 bytes.

I then look at my MTU, we'll go with 1500 here.  6250 / 1500 4.16
packets.

So queueing around 4 full sized packets generates 0-5ms of jitter
on a 10Mbps ethernet, worst case.

How many ms is good?  Well, that depends, a lot.  However I suspect most
people here have seen enough pings they have some idea what is good and
what isn't.