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

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Date:	Fri, 07 Sep 2007 10:38:15 +0100
From:	James Chapman <jchapman@...alix.com>
To:	Mandeep Singh Baines <mandeep.baines@...il.com>
CC:	netdev@...r.kernel.org, hadi@...erus.ca, davem@...emloft.net,
	jeff@...zik.org, ossthema@...ibm.com,
	Stephen Hemminger <shemminger@...l.org>
Subject: Re: RFC: possible NAPI improvements to reduce interrupt rates for
 low traffic rates

Hi Mandeep,

Mandeep Singh Baines wrote:
> Hi James,
> 
> I like the idea of staying in poll longer.
> 
> My comments are similar to what Jamal and Stephen have already said.
> 
> A tunable (via sysfs) would be nice.
> 
> A timer might be preferred to jiffy polling. Jiffy polling will not increase 
> latency the way a timer would. However, jiffy polling will consume a lot more
> CPU than a timer would. Hence more power. For jiffy polling, you could have 
> thousands of calls to poll for a single packet received. While in a timer 
> approach the numbers of polls per packet is upper bound to 2. 

Why would using a timer to hold off the napi_complete() rather than 
jiffy count limit the polls per packet to 2?

> I think it may difficult to make poll efficient for the no packet case because,
> at a minimum, you have to poll the device state via the has_work method.

Why wouldn't it be efficient? It would usually be done by reading an 
"interrupt pending" register.

> If you go to a timer implementation then having a tunable will be important.
> Different appications will have different requirements on delay and jitter.
> Some applications may want to trade delay/jitter for less CPU/power 
> consumption and some may not.

I agree. I'm leaning towards a new ethtool parameter to control this to 
be consistent with other per-device tunables.

> imho, the work should definately be pursued further:)

Thanks Mandeep. I'll try. :)

-- 
James Chapman
Katalix Systems Ltd
http://www.katalix.com
Catalysts for your Embedded Linux software development

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