netdev - Re: Network performance forwarding tests on RT

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Date:	Thu, 6 Nov 2008 18:50:29 -0600
From:	Clark Williams <williams@...hat.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	linux-rt-users@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: Network performance forwarding tests on RT

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On Wed, 5 Nov 2008 11:52:05 -0800
Stephen Hemminger <shemminger@...tta.com> wrote:

> As an experiment, I rebuilt a version of Vyatta using 2.6.26-rt11 kernel.
> This required some fixes to unionfs and aufs which I'll send to anyone who wants.

I'd like to see those fixes.

> 
> The performance of the RT PREEMPT kernel is worse than non-PREEMPT kernel.
> 
> Running RFC2544, frame loss test we the loss rate is worse on RT than non RT.
> Ideally, there would be no loss, but on this platform, the best we have
> seen is 70% loss at 64 bytes.
> 

We've seen that as you push the workloads up to max, the additional overhead of rt_mutexes starts to show and the performance of the RT kernel drops off.  So if you're trying to push the maximum amount of bits across a wire and you don't care about event latency, then I wouldn't recommend an RT kernel. 

> 
> Size	2.6.26		2.6.26-rt11
> 64	80.5%		99%
> 128	67		99
> 256	43		92
> 512	0		54
> 1024	0		3
> 1280	0		0
> 1518	0		0
> 
> More importantly, with RT PREEMPT, the driver gets stuck and times out
> under heavy load (see 99% loss above). It appears the change to network
> scheduling related to NAPI doesn't work well under load.
> 

Did you do anything with the priorities of interrupt threads? We generally boost hard IRQ threads (show up as [IRQ-xxx] in a ps) to SCHED_FIFO 80-85 and boost the softirq threads to between 70-75.

Since interrupt processing in RT takes place in SCHED_FIFO kernel threads, if you push the load up high enough, it's entirely possible to starve lower priority softirq/hardirq threads in the system.

Clark
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