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

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Message-ID: <46E50F6E.5010503@katalix.com>
Date:	Mon, 10 Sep 2007 10:33:34 +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

Mandeep Singh Baines wrote:
>> Why would using a timer to hold off the napi_complete() rather than 
>> jiffy count limit the polls per packet to 2?
>>
> I was thinking a timer could be used in the way suggested in Jamal's
> paper. The driver would do nothing (park) until the timer expires. So
> there would be no calls to poll for the duration of the timer. Hence,
> this approach would add extra latency not present in a jiffy polling
> approach.

Ah, ok. I wasn't planning to test timer-driven polling. :)

>> Why wouldn't it be efficient? It would usually be done by reading an 
>> "interrupt pending" register.
>>
> Reading the "interrupt pending" register would require an MMIO read.
> MMIO reads are very expensive. In some systems the latency of an MMIO
> read can be 1000x that of an L1 cache access.

Agreed. Testing for any work being available should be as efficient as 
possible and would be driver specific.

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

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