netdev - Re: Bogus frames transmitted with r8169 & fragmentation & large mtu

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Message-ID: <20120216161442.GA10857@elliptictech.com>
Date:	Thu, 16 Feb 2012 11:14:42 -0500
From:	Nick Bowler <nbowler@...iptictech.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Francois Romieu <romieu@...zoreil.com>, netdev@...r.kernel.org,
	Realtek linux nic maintainers <nic_swsd@...ltek.com>
Subject: Re: Bogus frames transmitted with r8169 & fragmentation & large mtu

On 2012-02-16 05:19 +0100, Eric Dumazet wrote:
> 1) Are you using SLUB, SLAB, or SLOB ?

SLUB.

> 2) Problem with MTU=9000 is that frames span several 4K pages.
> 
> Maybe some versions of r8169 hardware have problems with that ...

Could be, but probably not the whole story.  I tested some >4k MTU values
(all with 60K packets here because it failed more reliably than 30K).
6500 byte MTU appears to work OK (<1% loss).  7500 fails (>99% loss).
A largish range of MTU settings inbetween achieve a roughly 50% loss
rate.

The corruption is not always as significant as the original trace: some
of my tests had all the fragments correct but a mere two bytes of payload
were zero (resulting in the reassembled datagram being dropped due to a
checksum failure).

Recall that the original trace also had this "two bytes zeroed" problem
in the second fragment, in addition to all its other problems.

Cheers,
-- 
Nick Bowler, Elliptic Technologies (http://www.elliptictech.com/)

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