netdev - Re: UDP regression with packets rates

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Date:	Tue, 15 Sep 2009 16:25:43 -0400 (EDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
cc:	netdev@...r.kernel.org
Subject: Re: UDP regression with packets rates < 10k per sec

On Tue, 15 Sep 2009, Eric Dumazet wrote:

> Once I understood my 2.6.31 kernel had much more features than 2.6.22 and that I tuned
> it to :
>
> - Let cpu run at full speed (3GHz instead of 2GHz) : before tuning, 2.6.31 was
> using "ondemand" governor and my cpus were running at 2GHz, while they where
> running at 3GHz on my 2.6.22 config

My kernel did not have support for any governors compiled in.

> - Dont let cpus enter C2/C3 wait states (idle=mwait)

Ok. Trying idle=mwait.

> - Correctly affine cpu to ethX irq (2.6.22 was running ethX irq on one cpu, while
>  on 2.6.31, irqs were distributed to all online cpus)

Interrupts of both 2.6.22 and 2.6.31 go to cpu 0. Does it matter for
loopback?

> Then, your mcast test gives same results, at 10pps, 100pps, 1000pps, 10000pps

loopback via mcast -Ln1 -r <rate>

		10pps	100pps	1000pps	10000pps
2.6.22(32bit)	7.36	7.28	7.15	7.16
2.6.31(64bit)	9.28	10.27	9.70	9.79

What a difference. Now the initial latency rampup for 2.6.31 is gone. So
even w/o governors the kernel does something to increase the latencies.

We sacrificed 2 - 3 microseconds per message to kernel features, bloat and
64 bitness?
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