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Date:      Wed, 26 Apr 2000 10:46:00 +0200 (CEST)
From:      Luigi Rizzo <luigi@info.iet.unipi.it>
To:        "C. Stephen Gunn" <csg@waterspout.com>
Cc:        Archie Cobbs <archie@whistle.com>, freebsd-net@FreeBSD.ORG, luigi@FreeBSD.ORG, remy@boostworks.com, pavel@alum.mit.edu, nsayer@sftw.com, juliam@elischer.org
Subject:   Re: Proposal for ethernet, bridging, netgraph
Message-ID:  <200004260846.KAA23270@info.iet.unipi.it>
In-Reply-To: <20000426011134.A768@waterspout.com> from "C. Stephen Gunn" at "Apr 26, 2000 01:11:34 am"

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> Luigi -- Could you please remark about the optimizations in
> if_ed.c?  I want to fully udnerstand your motivations.
> 
> Is it simply due to having the ed device in promisc mode for
> bridging, and trying to optimize the "we don't need to forward
> this packet" case?

the problem is, "ed" transfers the packet using programmed I/O,
which is terribly slow on the ISA bus (twice wire speed i think)
and painfully slow on the PCI bus as well (i dont remember the
figures but i don't think we reach 10MB/s which means that you
monopolise the PCI bus and the CPU for 10-20% of the time (irrespective
of CPU speed, mind you!)  _just_ to transfer the packets from one
interface. Because there is some likelyhood that most packets need
not to be bridged, the optimization first transfers the 14-byte
header, then calls bridge_in(), and then transfers the rest of the
packet only if really necessary. I think you can have a 10x improvement
by doing this.

One could say "who cares about the 'ed' cards" but i think
this is wrong attitude being these cards so cheap, widespread, and
often (e.g. here) the only cards left on the market with a BNC
connector.

Anyways, my point for retaining the old semantics for ether_input
is so that one does need to fix all drivers at once (and some
are problematic, e.g. i am not really sure how to make "de" work.)

	cheers
	luigi
-----------------------------------+-------------------------------------
  Luigi RIZZO, luigi@iet.unipi.it  . Dip. di Ing. dell'Informazione
  http://www.iet.unipi.it/~luigi/  . Universita` di Pisa
  TEL/FAX: +39-050-568.533/522     . via Diotisalvi 2, 56126 PISA (Italy)
  Mobile   +39-347-0373137
-----------------------------------+-------------------------------------


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