netdev - Re: TX pre-headers...

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Message-ID: <m3zlgvaabf.fsf@intrepid.localdomain>
Date:	Mon, 09 Feb 2009 16:39:48 +0100
From:	Krzysztof Halasa <khc@...waw.pl>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: TX pre-headers...

Herbert Xu <herbert@...dor.apana.org.au> writes:

> Yes tunneling (especially when it's hidden by netfilter) is the
> hard part.
>
> Here's a thought, what if we had your global maximum, and every
> time we have to reallocate a packet because of a failed head
> room check, we increase the maximum by the needed amount (we should
> add a ceiling in case some buggy path ignores this maximum when
> allocating a new packet).
>
> This way even tunnels could benefit from not having to copy all
> the time.
>
> As a failed check should be rare (given that we're continuously
> increasing it) the overhead in updating the maximum should be
> reasonable.

I agree.

OTOH and FWIW my stuff (= WAN) alone without stacking fits in 32 bytes,
the typical max case is 10 bytes for Frame-Relay header + 14 bytes for
Ethernet header + 4 bytes for 802.1Q tag = 28 bytes.

OTOH I wonder if these changes make the dev->hard_header_len no longer
needed.
-- 
Krzysztof Halasa
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