Freebsd IP Forwarding performance (question, and some info) [7-stable, current, em, smp]

[Erik Trulsson]

On Mon, Jul 07, 2008 at 10:30:53PM +1000, Bruce Evans wrote:
> On Mon, 7 Jul 2008, Andre Oppermann wrote:
> 
> > Bruce Evans wrote:
> >> What are the other overheads?  I calculate 1.644Mpps counting the 
> >> inter-frame
> >> gap, with 64-byte packets and 64-header_size payloads.  If the 64 bytes
> >> is for the payload, then the max is much lower.
> >
> > The theoretical maximum at 64byte frames is 1,488,100.  I've looked
> > up my notes the 1.244Mpps number can be ajusted to 1.488Mpps.
> 
> Where is the extra?  I still get 1.644736 Mpps (10^9/(8*64+96)).
> 1.488095 is for 64 bits extra (10^9/(8*64+96+64)).

A standard ethernet frame (on the wire) consists of:
7 octets preamble
1 octet  Start Frame Delimiter
6 octets destination address
6 octets source address
2 octets length/type
46-1500 octets  data (+padding if needed)
4 octets Frame Check Sequence

Followed by (at least) 96 bits interFrameGap, before the next frame starts.

For minimal packet size this gives a maximum packet rate at 1Gbit/s of
1e9/((7+1+6+6+2+46+4)*8+96)/ = 1488095 packets/second

You probably missed the preamble and start frame delimiter in your
calculation.




-- 
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Erik Trulsson
ertr1013 at student.uu.se