Date: Mon, 31 Jul 2006 12:09:22 -0700 From: John-Mark Gurney <gurney_j@resnet.uoregon.edu> To: Robert Watson <rwatson@FreeBSD.org> Cc: arch@FreeBSD.org, net@FreeBSD.org, John Polstra <jdp@polstra.com> Subject: Re: Changes in the network interface queueing handoff model Message-ID: <20060731190922.GJ96589@funkthat.com> In-Reply-To: <20060731180643.E71432@fledge.watson.org> References: <XFMail.20060731100533.jdp@polstra.com> <20060731180643.E71432@fledge.watson.org>
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Robert Watson wrote this message on Mon, Jul 31, 2006 at 18:08 +0100:
> >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.
This work tends to lead to a generic ethernet card framework that I've
been thinking about.. where instead of cards doing all the handling of
a ring buffer, the card registers a few functions to manipulate a ring
buffer (if it has one), and does the necessary work... Though encoding
all the different style of ring buffers may be interesting, per packet
instead of per segment (if_re)...
The other part is to digest the current monolithic lock structure that
the ethernet cards have, into three (or four) different locks, tx head,
tx tail, rx head & tail...
--
John-Mark Gurney Voice: +1 415 225 5579
"All that I will do, has been done, All that I have, has not."
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