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

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Date:	Mon, 10 Sep 2007 08:12:52 -0400
From:	jamal <hadi@...erus.ca>
To:	Mandeep Singh Baines <mandeep.baines@...il.com>
Cc:	James Chapman <jchapman@...alix.com>, netdev@...r.kernel.org,
	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

On Sat, 2007-08-09 at 09:42 -0700, Mandeep Singh Baines wrote:

> 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.

Indeed.

> However, work_done() doesn't have to be inefficient. For newer
> devices you can implement work_done() without an MMIO read by polling
> the next ring entry status in memory or some other mechanism. Since
> PCI is coherent, acceses to this memory location could be cached
> after the first miss. For architectures where PCI is not coherent you'd 
> have to go to memory for every poll. So for these architectures has_work()
> will be moderately expensive (memory access) even when has_work() does
> not require an MMIO read. This might affect home routers: not sure if MIPS or
> ARM have coherent PCI.

I think the effect would be clearly experimentally observable in smaller
devices e.g the Geode you seem to be experimenting on.
One other suggestion i made in the paper is to something along the lines
of cached_irq_mask for the i8259

cheers,
jamal

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