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