LinuxSA Mailing list archives

  From: Daryl Tester <Daryl.Tester@iocane.com.au>
  To  : Michael <michael@home.lyppard.com.au>
  Date: Tue, 01 Apr 2003 21:28:51 +0930

Re:

Michael wrote:

> On Tue, 01 Apr 2003 20:37:28 +0930
> steve@centraldocuments.com wrote:
> 
> > Well guys, I hate to be a party pooper but I only user realtek (no
> > real reason they just work and are cheap)
> > and use them 1, 2's and 3's eg broadband, private network and DMZ I
> > have had zero problems with them in any configuration!
> >
> > Steve D
> 
> I can't say I've had any problems either. I think the problem is that
> anything this cheap can't be any good can it?

The problem is the 8129s (and to some extent, the 8139s, but I can't
remember the differences).  From driver/net/rtl8139.c (2.2 kernel):

<quote>

The receive unit uses a single linear ring buffer rather than the more
common (and more efficient) descriptor-based architecture.  Incoming frames
are sequentially stored into the Rx region, and the host copies them into
skbuffs.

Comment: While it is theoretically possible to process many frames in place,
any delay in Rx processing would cause us to drop frames.  More importantly,
the Linux protocol stack is not designed to operate in this manner.

IIIb. Tx operation

The RTL8129 uses a fixed set of four Tx descriptors in register space.
In a stunningly bad design choice, Tx frames must be 32 bit aligned.  Linux
aligns the IP header on word boundaries, and 14 byte ethernet header means
that almost all frames will need to be copied to an alignment buffer.

</quote>

The alignment copy issue means that the kernel must do an additional
copy of all/most outbound data, which means more work on the CPUs part
to get around a brain-dead design defect.

> I've got one on my backup-cache machine that processes a 33Gb backup
> file every night. Never misses a beat.

More power to you.  It's still a cheap card, in every sense of the word.


-- 
Regards,
  Daryl Tester,  Software Wrangler and Bit Herder, IOCANE Pty. Ltd.

"Um ... those are IP addresses the quarterback guy yells out, right?"
"No, A.J. ... they're wild guesses at your I.Q." --  User Friendly

-- 
LinuxSA WWW: http://www.linuxsa.org.au/ IRC: #linuxsa on irc.freenode.net
To unsubscribe from the LinuxSA list:
  mail linuxsa-request@linuxsa.org.au with "unsubscribe" as the subject