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

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Date:	Thu, 07 Nov 2013 10:52:57 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	Herbert Xu <herbert@...dor.hengli.com.au>,
	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
Subject: Re: gso: Attempt to handle mega-GRO packets

On 11/07/2013 08:36 AM, Herbert Xu wrote:
> 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,

No. Maybe we can use per-queue buffers instead.
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