Date: Mon, 31 Jul 2006 18:08:24 +0100 (BST) From: Robert Watson <rwatson@FreeBSD.org> To: John Polstra <jdp@polstra.com> Cc: arch@freebsd.org, net@freebsd.org Subject: RE: Changes in the network interface queueing handoff model Message-ID: <20060731180643.E71432@fledge.watson.org> In-Reply-To: <XFMail.20060731100533.jdp@polstra.com> References: <XFMail.20060731100533.jdp@polstra.com>
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On Mon, 31 Jul 2006, John Polstra wrote: >> Attached is a patch that maintains the current if_start, but adds >> if_startmbuf. If a device driver implements if_startmbuf and the global >> sysctl net.startmbuf_enabled is set to 1, then the if_startmbuf path in the >> driver will be used. Otherwise, if_start is used. I have modified the >> if_em driver to implement if_startmbuf also. If there is no packet backlog >> in the if_snd queue, it directly places the packet in the transmit >> descriptor ring. If there is a backlog, it uses the if_snd queue protected >> by driver mutex, rather than a separate ifq mutex. > > I question whether you need a fallback software if_snd queue at all for > modern devices such as the Intel and Broadcom gigabit chips. The hardware > transmit descriptor rings typically have sizes of the order of 256 > descriptors. I think if the ring fills up, you could simply drop the packet > with ENOBUFS. That's what happens if the if_snd queue fills up, and its > maximum size is comparable to the sizes of modern descriptor rings. It > would simplify things quite a bit to eliminate the if_snd queue entirely for > such devices. I tend to agree, but implemented full queueing support for if_em to make sure I understood to complexity implications of completely removing queueing from the ifnet side dispatch. I guess an interesting question for us is how we decide what the right threshold is to implement software queuing. Do any if_em cards need software queueing, or do they all have adequate in-hardware queues as is? Entirely cutting the queue code would significantly simplify em_startmbuf. Robert N M Watson Computer Laboratory University of Cambridge
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