netdev - Re: [PATCH v2 net-next] tcp: avoid expensive pskb_expand

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Message-ID: <1334843527.2395.182.camel@edumazet-glaptop>
Date:	Thu, 19 Apr 2012 15:52:07 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc:	Neal Cardwell <ncardwell@...gle.com>,
	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Tom Herbert <therbert@...gle.com>,
	Maciej Żenczykowski <maze@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH v2 net-next] tcp: avoid expensive pskb_expand_head()
 calls

On Thu, 2012-04-19 at 15:18 +0200, Eric Dumazet wrote:
> On Thu, 2012-04-19 at 13:30 +0200, Eric Dumazet wrote:
> 
> > I'll provide a v3 anyway with more performance data, I setup two cards
> > in PCI x8 slots to get full bandwidth.
> 
> Incidentally, using PCI x8 slots dont anymore trigger the slow path on
> unpatched kernel and a single flow (~9410 Mbits)
> 
> It seems we are lucky enough to TX complete sent clones before trying to
> tcp_trim_head() when processing ACK
> 
> Sounds like a timing issue, and fact that drivers batches TX completions
> and RX completions.
> 
> Also BQL might have changed things a bit here (ixgbe is BQL enabled)
> 
> Only if I start several concurrent flows I see the pskb_expand_head()
> overhead.
> 
> 

And disabling GRO on receiver definitely demonstrates the problem, even
with a single flow. (and performance drops from 9410 Mbit to 6050 Mbit)





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