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Re: DP performance

From: Danial Thom <danial_thom@xxxxxxxxx>
Date: Sat, 10 Dec 2005 13:57:38 -0800 (PST)

--- "Martin P. Hellwig" <mhellwig@xxxxxxxxx>

> <cut>
> > What do you think the
> > switch is going to do with the traffic? Its
> going
> > to dump it. 
> The only argument you gave is false, read the
> full specs of any modern 
> switch (ie all 1Gb switches)
> -- 
> mph
If I relied on "specs" for my info I'd be in the
same boat you guys are in, so I don't. Specs are
a nice "upper limit" but you can hardly come to
any conclusions based on specs alone.

Why don't you post a snippet from one of your
"specs" to illustrate how long you can flow
control a switch before it starts dumping
packets. And remember that a spec is the max a
box can do, so thats with only 2 ports and large
packets. So then you can interpolate out
(assuming you want to use math to solve your
problems) to much smaller packets and many more
ports contending for bus bandwidth. 

Like I said, its time to get out of college-mode
and get yourself a test bed. Its the only way to
learn how things really work, rather than how
they're supposed to work. The truth is that
switches under load don't like to be flow
controlled, and they drop packets when their
queues are at relatively low watermarks. Christ,
some switches drop packets at 300K pps when
they're not flow controlled.

Besides, flow control isn't part of the argument.
"Performance" isn't about how gracefully you can
fail to perform a task; its about being able to
perform the task without having to resort to
using flow control. To me, a box that is issuing
flow control is no better than one that drops
packets. Both have failed to do the job required.

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