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

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Message-ID: <ada63jqm2mt.fsf@cisco.com>
Date:	Wed, 04 Feb 2009 11:19:06 -0800
From:	Roland Dreier <rdreier@...co.com>
To:	Willy Tarreau <w@....eu>
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

 > 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.

 - R.
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