[c-nsp] Flow Control and 10GE interfaces

[Ross Vandegrift]

On Tue, Nov 24, 2009 at 09:00:51AM +0100, Marian ??urkovi?? wrote:
> On Mon, 23 Nov 2009 11:40:17 -0800 (PST), Kevin Graham wrote
> > My understanding of this must be broken... If the pause frame is sent
> > only sent when or immediately before RX buffers are exhausted, then
> > TX queuing is triggered (hopefully only briefly before those buffers
> > are exhausted). This would seem to trigger behavior consistent w/ a
> > congested interface (which in fact it is, just prior to reaching line
> > rate, as the receiver can't take it off interface buffers fast enough).
> 
> Yes, what you described is basically a case where the interface runs at faster
> speed than the data path behind it.
> 
> Some examples: oversubcribed 10GE card with only 8 Gbps bandwidth to the switch
> fabric or system bus, 100 Mpbs ethernet interface in front of 34 Mbps microware
> link. 
> 
> This is exactly the *only* situation, where classic flow control makes sense and
> does really help, since it properly triggers output queueing at the sending side
> when the real data-path speed is reached. Any other usage is likely to cause
> more problems than benefits.
 
But in these cases you're saturated!  So why not just drop the frame and
let the upper-layer figure out that it needs to back off?  You're just
delaying the inevitable by invoking flow control and hiding the
information from the upper layer.

-- 
Ross Vandegrift
ross at kallisti.us

"If the fight gets hot, the songs get hotter.  If the going gets tough,
the songs get tougher."
	--Woody Guthrie
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