netdev - performance regression with skb_add_data

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Date:	Fri, 12 Oct 2012 16:29:17 +0200
From:	Animesh K Trivedi1 <ZRLATR@...ibm.com>
To:	netdev@...r.kernel.org
Cc:	Bernard Metzler <BMT@...ich.ibm.com>,
	Animesh K Trivedi1 <ZRLATR@...ibm.com>
Subject: performance regression with skb_add_data_nocache

Hi all,

I recently upgraded from 2.6.36 to 3.2.28 and saw performance regression
for
TCP performance. Upon further investigation it looked that
skb_add_data_nocache()
was the culprit.

I am getting following performance numbers on my Nehalem (Xeon E7520) box
connected using 10GbE cards (transmit side, and netperf client). Server is
a another
box with E5540 CPU (receiver of the request) . For my netperf TCP_RR tests:

-  1,400 bytes request, 1 byte response:
No cache copy (enabled)  : 26,623 tps, 22.72% utilization
No cache copy (disabled) : 26,710 tps, 21.76% utilization

- 14,000 bytes request, 1 byte response:
No cache copy (enabled)  : 14,245 tps, 23.04% utilization
No cache copy (disabled) : 14,850 tps, 21.6% utilization

and for even larger buffer the performance lag is increases with
significant CPU load

- 1 MBytes request, 1 byte response:
No cache copy (enabled)  : 1,032 tps, 98.96% utilization
No cache copy (disabled) : 1,081 tps, 74.86% utilization

Though there isn't a lot performance difference, but notice the significant
CPU
utilization in case of nocache copy for 1MB buffer size. Thoughts?

Thanks,
--
Animesh

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