netdev - Re: [PATCH 1/4] net: skb_orphan on dev_hard_start

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Message-Id: <200906012157.29465.rusty@rustcorp.com.au>
Date:	Mon, 1 Jun 2009 21:57:28 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	Divy Le Ray <divy@...lsio.com>,
	Roland Dreier <rolandd@...co.com>,
	Pavel Emelianov <xemul@...nvz.org>,
	Dan Williams <dcbw@...hat.com>,
	libertas-dev@...ts.infradead.org
Subject: Re: [PATCH 1/4] net: skb_orphan on dev_hard_start_xmit

On Sat, 30 May 2009 12:41:00 am Eric Dumazet wrote:
> Rusty Russell a écrit :
> > DaveM points out that there are advantages to doing it generally (it's
> > more likely to be on same CPU than after xmit), and I couldn't find
> > any new starvation issues in simple benchmarking here.
>
> If really no starvations are possible at all, I really wonder why some
> guys added memory accounting to UDP flows. Maybe they dont run "simple
> benchmarks" but real apps ? :)

Well, without any accounting at all you could use quite a lot of memory as 
there are many places packets can be queued.

> For TCP, I agree your patch is a huge benefit, since its paced by remote
> ACKS and window control

I doubt that.  There'll be some cache friendliness, but I'm not sure it'll be 
measurable, let alone "huge".  It's the win to drivers which don't have a 
timely and batching tx free mechanism which I aim for.

> , but an UDP sender will likely be able to saturate
> a link.

I couldn't see any difference in saturation here (with default scheduler and an 
100MBit e1000e).  Two reasons come to mind: firstly, only the hardware queue is 
unregulated: the tx queue is still accounted.  And when you add scheduling to 
the mix, I can't in practice cause starvation of other senders.

Hope that clarifies,
Rusty.
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