Re: [PATCH v2 net-next] tcp: avoid expensive pskb_expand_head() calls

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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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