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

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Date:	Tue, 3 Feb 2009 23:33:12 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Evgeniy Polyakov <zbr@...emap.net>, david@...g.hm,
	Jarek Poplawski <jarkao2@...il.com>,
	David Miller <davem@...emloft.net>, w@....eu,
	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 Tuesday 03 February 2009 23:18:08 Herbert Xu wrote:
> On Tue, Feb 03, 2009 at 03:12:19PM +0300, Evgeniy Polyakov wrote:
> > It is a solution, but I think it will behave noticebly worse than
> > with decresed MTU.
>
> Not necessarily.  Remember GSO/GRO in essence are just hacks to
> get around the fact that we can't increase the MTU to where we
> want it to be.  MTU reduces the cost over the entire path while
> GRO/GSO only do so for the sender and the receiver.
>
> In other words when given the choice between a larger MTU with
> copying or GRO, the larger MTU will probably win anyway as it's
> optimising the entire path rather than just the receiver.
>
> > That's the main point: how to deal with broken hardware? I think (but
> > have no strong numbers though) that having 6 packets with 1500 MTU
> > combined into GRO/LRO frame will be processed way faster than copying 9k
> > MTU into 3 pages and process single skb.
>
> Please note that with my scheme, you'd only start copying if you
> can't allocate a linear skb.  So if memory fragmentation doesn't
> happen then there is no copying at all.

This sounds like a really nice idea (to the layman)!

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