netdev - Re: gso: Attempt to handle mega-GRO packets

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Message-ID: <20131107003658.GA27976@gondor.apana.org.au>
Date:	Thu, 7 Nov 2013 08:36:58 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Ben Hutchings <bhutchings@...arflare.com>,
	David Miller <davem@...emloft.net>,
	christoph.paasch@...ouvain.be, netdev@...r.kernel.org,
	hkchu@...gle.com, mwdalton@...gle.com, mst@...hat.com,
	Jason Wang <jasowang@...hat.com>
Subject: Re: gso: Attempt to handle mega-GRO packets

On Wed, Nov 06, 2013 at 07:01:10AM -0800, Eric Dumazet wrote:
> Have you thought about arches having PAGE_SIZE=65536, and how bad it is
> to use a full page per network frame ? It is lazy and x86 centered.

So instead if we were sending a full 64K packet on such an arch to
another guest, we'd now chop it up into 1.5K chunks and reassemble them.

> So after our patches, we now have an optimal situation, even on these
> arches.

Optimal only for physical incoming packets with no jumbo frames.

What's worse, I now realise that the coalesce thing isn't even
guaranteed to work.  It probably works in your benchmarks because
you're working with freshly allocated pages.

But once the system has been running for a while, I see nothing
in the virtio_net code that tries to prevent fragmentation.  Once
fragmentation sets in, you'll be back in the terrible situation
that we were in prior to the coalesce patch.

Jason/Michael (Tsirkin), am I missing something that would prevent
fragmentation of these buffers?

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt
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