Ethernet Flowcontrol...

Donald Becker becker at scyld.com
Thu Jan 17 18:49:33 EST 2002


On Sun, 17 Feb 2002, Bill Northrup wrote:

> Hello everyone. I was just about to begin some network tuning and wanted
> to get some input from the list. We have a few gig devices that talk to
> other gig devices as well as the lesser Fast Ethernet nodes. I am well
> aware of the gig frame padding, latency and such with gig e.

Frame padding shouldn't concern most people -- no one is running Gb
Ethernet with repeaters.  (IMHO, the MAC changes for Gb half duplex were
a waste of time.)

> However I
> was wondering if anyone is using flow control both rx and tx or asym to
> help with packet flow?

There is little down-side to flow control, and it's usually enabled by
default.  Link flow control is transparent, and can minimize overruns
and dropped packets.  For the 100Mb->1Gb direction, where presumably
flow control would never be triggered, it has no performance impact.

> For instance I have a gig master that is trying
> to shove everything down a fast e pipe at the switch. Should one just
> enable TX flow control on the gig segment? The other way from fast e to
> gig e wouldn't be an issue, right? Does relying on the network for flow
> control reduce the overhead encountered on any of the machines or
> possibly off load it to the network devices that may do it better? I'll
> report back to the list what I find, but everyone's mileage is
> different.

Donald Becker				becker at scyld.com
Scyld Computing Corporation		http://www.scyld.com
410 Severn Ave. Suite 210		Second Generation Beowulf Clusters
Annapolis MD 21403			410-990-9993

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