netdev - Re: UDP regression with packets rates

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Date:	Tue, 15 Sep 2009 10:07:01 -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:

> 2.6.31 is actually faster than 2.6.22 on the bench you provided.

Well at high packet rates which were not the topic.

> Must be specific to the hardware I guess ?

Huh? Even your loopback numbers did show the regression up to 10k.

> As text size presumably is bigger in 2.6.31, fetching code
> in cpu caches to handle 10 packets per second is what we call
> a cold path anyway.

Ok so its an accepted regression? This is a significant reason not to use
newer versions of kernels for latency critical applications that may have
to send a packet once in a while for notification. The latency is doubled
(1G) / tripled / quadrupled (IB) vs 2.6.22.

> If you want to make it a fast path, you want to make sure code its
> always hot in cpu caches, and find a way to inject packets into
> the kernel to make sure cpu keep the path hot.

Oh, gosh.

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