netdev - Re: [PATCH v3 net-next] net: introduce dev_set

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Date:	Sun, 03 Nov 2013 22:05:55 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
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
Subject: Re: [PATCH v3 net-next] net: introduce dev_set_forwarding()

On Mon, 2013-11-04 at 13:23 +0800, Herbert Xu wrote:
> On Sun, Nov 03, 2013 at 09:00:54PM -0800, Eric Dumazet wrote:
> > On Mon, 2013-11-04 at 12:29 +0800, Herbert Xu wrote:
> >
> > > Have you actually measured this? The latency added by GRO is pure
> > > processing overhead.  This is tiny when compared to the time NAPI takes
> > > to wait.
> > 
> > Please take a look at 
> > 
> > 2e71a6f8084e net: gro: selective flush of packets
> 
> This is a different problem altogether.  I was worried about the
> latency in cases where we're idle and waiting for new data, while
> you're worried about the latency in the CPU-bound case.

Idle case, you very rarely cant keep up building skbs with 16 MSS.

> 
> I think we can definitely improve our behaviour the CPU-bound case.
> Right now if we encounter something we can't hold for GRO we
> start processing it right away.  Instead we can place it in a
> list for later processing together with the GRO packets.
> 
> This way GRO packets are not penalised by non-GRO packets.
> 
> You can then use the usual NAPI budget to minimise latency and
> ensure scheduling fairness.

We had these latencies only dealing with TCP packets, all GRO
candidates.

Really, I think we have used GRO at large scale here at Google ;)



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