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

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Date:	Tue, 3 Feb 2009 13:25:35 +0100
From:	Willy Tarreau <w@....eu>
To:	Evgeniy Polyakov <zbr@...emap.net>
Cc:	Herbert Xu <herbert@...dor.apana.org.au>,
	Jarek Poplawski <jarkao2@...il.com>,
	David Miller <davem@...emloft.net>, 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 Tue, Feb 03, 2009 at 03:18:36PM +0300, Evgeniy Polyakov wrote:
> On Tue, Feb 03, 2009 at 11:12:09PM +1100, Herbert Xu (herbert@...dor.apana.org.au) wrote:
> > The only change we need to make is at receive time.  Instead of
> > always pushing the received skb into the stack, we should try to
> > allocate a linear replacement skb, and if that fails, allocate
> > a fragmented skb and copy the data into it.  That way we can
> > always push a linear skb back into the ring buffer.
> 
> Yes, that's was the part about 'reserve' buffer for the sockets you cut
> :)
> 
> I agree that this will work and will be better than nothing, but copying
> 9kb into 3 pages is rather CPU hungry operation, and I think (but have
> no numbers though) that system will behave faster if MTU is reduced to
> the standard one.

Well, FWIW, I've always observed better performance with 4k MTU (4080 to
be precise) than with 9K, and I think that the overhead of allocating 3
contiguous pages is a major reason for this.

> Another solution is to have a proper allocator which will be able to
> defragment the data, if talking about the alternatives to the drop.
>
> So:
> 1. copy the whole jumbo skb into fragmented one
> 2. reduce the MTU

you'll not reduce MTU of established connections though. And trying to
advertise MSS changes in the middle of a TCP connection is an awful
hack which I think will not work everywhere.

> 3. rely on the allocator
> 
> For the 'good' hardware and drivers nothing from the above is really needed.
> 
> -- 
> 	Evgeniy Polyakov

Willy

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