netdev - RE: Performance regression on kernels 3.10 and newer

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Date:	Mon, 18 Aug 2014 09:03:21 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Alexander Duyck' <alexander.h.duyck@...el.com>,
	Tom Herbert <therbert@...gle.com>
CC:	David Miller <davem@...emloft.net>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Rick Jones <rick.jones2@...com>
Subject: RE: Performance regression on kernels 3.10 and newer

From: Alexander Duyck
> ...
> Another test I tried was to hack the nettest_bsd.c file in netperf to
> perform a poll() based receive.  That resolved the issue and had all the
> performance of the tcp_low_latency case.  I may see if I can work with
> Rick to push something like that into netperf as I really would prefer
> to avoid having to advise everyone on how to setup the sysctl for
> tcp_low_latency.

Doesn't that generate 2 system calls per receive?
Unless it now returns more data per receive I'm surprised that
it actually faster.

OTOH I've some code that runs a lot better when I run while :; do :; done
for all but one of the cpus.
I think that is because the processes spinning in userspace don't
get pre-empted.

	David