netdev - Re: RFC: possible NAPI improvements to reduce interrupt rates for low traffic rates

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Message-ID: <20070906163703.4fc12d32@oldman>
Date:	Thu, 6 Sep 2007 16:37:03 +0100
From:	Stephen Hemminger <shemminger@...ux-foundation.org>
To:	James Chapman <jchapman@...alix.com>
Cc:	netdev@...r.kernel.org, hadi@...erus.ca, davem@...emloft.net,
	jeff@...zik.org, mandeep.baines@...il.com, ossthema@...ibm.com
Subject: Re: RFC: possible NAPI improvements to reduce interrupt rates for
 low traffic rates

On Thu, 06 Sep 2007 16:30:30 +0100
James Chapman <jchapman@...alix.com> wrote:

> Stephen Hemminger wrote:
> 
> > What about the latency that NAPI imposes? Right now there are certain applications that
> > don't like NAPI because it add several more microseconds, and this may make it worse.
> 
> Latency is something that I think this approach will actually improve, 
> at the expense of additional polling. Or is it the ksoftirqd scheduling 
> latency that you are referring to?

The problem is that you leave interrupts disabled, right. Also you are busy during
idle which kills powersaving and no hz clock.

> > Maybe a per-device flag or tuning parameters (like weight sysfs value)? or some other
> > way to set low-latency values.
> 
> Yes. I'd like to think good defaults could be derived though, perhaps 
> based on settings like CONFIG_PREEMPT, CONFIG_HIGH_RES_TIMER, CONFIG_HZ 
> and maybe even bogomips / nr_cpus.
> 
> -- 
> James Chapman
> Katalix Systems Ltd
> http://www.katalix.com
> Catalysts for your Embedded Linux software development
> 
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