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

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Date:	Thu, 19 Apr 2012 16:10:52 +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 15:52 +0200, Eric Dumazet wrote:

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

That insane. 

Performance drops so much because we _drop_ incoming ACKS :

<     TCPSackShifted: 39117
<     TCPSackMerged: 16500
<     TCPSackShiftFallback: 5092
<     TCPBacklogDrop: 27965
---
>     TCPSackShifted: 35122
>     TCPSackMerged: 16368
>     TCPSackShiftFallback: 4889
>     TCPBacklogDrop: 23247

Hmm, maybe we should reduce skb->truesize for small packets before
queueing them in socket backlog...



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