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

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Date:	Thu, 19 Apr 2012 10:20:21 -0700
From:	Rick Jones <rick.jones2@...com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	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 04/19/2012 07:10 AM, Eric Dumazet wrote:
> 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...

By copying them to smaller buffers? Or just by altering truesize? 
Wasn't the whole point of fixing all the broken truesize settings to 
accurately account for memory consumed?

rick
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