netdev - Re: [PATCH v2] tcp: splice as many packets as possible at once

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Date:	Wed, 4 Feb 2009 20:28:20 +0100
From:	Willy Tarreau <w@....eu>
To:	Roland Dreier <rdreier@...co.com>
Cc:	David Miller <davem@...emloft.net>, herbert@...dor.apana.org.au,
	zbr@...emap.net, jarkao2@...il.com, dada1@...mosbay.com,
	ben@...s.com, mingo@...e.hu, linux-kernel@...r.kernel.org,
	netdev@...r.kernel.org, jens.axboe@...cle.com
Subject: Re: [PATCH v2] tcp: splice as many packets as possible at once

On Wed, Feb 04, 2009 at 11:19:06AM -0800, Roland Dreier wrote:
>  > And the arbitrary choice of 9k for jumbo frames was total crap too.
>  > It's clear that no hardware designer was involved in the process.
>  > They have to stuff 16kB of RAM on a NIC to use only 9. And we need
>  > to allocate 3 pages for slightly more than 2. 7.5 kB would have been
>  > better in this regard.
> 
> 9K was not totally arbitrary.  The CRC used for checksumming ethernet
> packets has a probability of undetected errors that goes up about
> 11-thousand something bytes.  So the real limit is ~11000 bytes, and I
> believe ~9000 was chosen to be able to carry 8K NFS payloads + all XDR
> and transport headers without fragmentation.

Yes I know that initial motivation. But IMHO it was a purely functional
motivation without real considerations of the implications. When you
read Alteon's initial proposal, there is even a biased analysis (they
compare the fill ratio obtained with one 8k frame with that of 6 1.5k
frames). Their own argument does not stand with their final proposal!
I think that there might already have been people pushing for 8k and
not 9 by then, but in order to get wide acceptance in datacenters,
they had to please the NFS admins.

Now it's useless to speculate on history and I agree with Davem that
we're wasting our time with this discussion, let's go back to the
keyboard ;-)

Willy

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