Re: [SAGE] 1000 Mbps networking question

[Guy Dawson <guy@crossflight.co.uk>]

Eric Sorenson wrote:

> Please, this is simply not true. The only advantage to jumbo frames
> is fewer driver interrupts on slow-clock-speed CPU, on a "backbone" link
> where there's a jumbo-mtu path all the way (otherwise you'll end up 
> fragmenting down to 1500-byte frames *anyway*). Here's why the "too
> much overhead" argument is fallacious: (ref. Siefert _The Switch Book_,
> p119 footnotes)
> 
> Maximum channel efficiency for 1500-byte MTU ethernet:
> 
>      MTU             1500
> --------------- = ----------  = 97.5%
> MTU+O_mac+O_phy   1500+26+12
> 
> Maximum channel efficiency for 9000-byte "jumbo frame" ethernet:
> 
>    9000
> ---------- = 99.6% 
> 9000+26+12
> 
> 6:1 increase in MTU size, 2.1% efficiency improvement. If it
> was the case that "to get full speed you need jumbo frames", 
> non-jumbo frames would give you 979 megabits out of 1000, no
> big deal.

The above tell us how efficiently the network bandwith is used.

However, when using 9000 byte packets we only send 1/6th the number
of packets and thus reduce the TCP overheads by 5/6ths. A big
saving in processing.

> Most off the shelf gige cards can and will use the full gigabit if
> they need to.  The card is almost never the bottleneck; getting
> 100 megabytes a second to or from your I/O subsystem is.

Agreed.

Guy
-- --------------------------------------------------------------------
Guy Dawson                    I.T. Manager              Crossflight Ltd
guy@crossflight.co.uk         07973  797819                01753 776104





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