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

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Message-ID: <535ddc6b0709120947x3458c87dk7dac9a2b8edf26f6@mail.gmail.com>
Date:	Wed, 12 Sep 2007 09:47:46 -0700
From:	"Mandeep Baines" <mandeep.baines@...il.com>
To:	"Stephen Hemminger" <shemminger@...ux-foundation.org>
Cc:	"James Chapman" <jchapman@...alix.com>, hadi@...erus.ca,
	"Bill Fink" <billfink@...dspring.com>, netdev@...r.kernel.org,
	davem@...emloft.net, jeff@...zik.org, ossthema@...ibm.com
Subject: Re: RFC: possible NAPI improvements to reduce interrupt rates for low traffic rates

On 9/12/07, Stephen Hemminger <shemminger@...ux-foundation.org> wrote:
> But if you compare this to non-NAPI driver the same softirq
> overhead happens. The problem is that for many older devices disabling IRQ's
> require an expensive non-cached PCI access. Smarter, newer devices
> all use MSI which is pure edge triggered and with proper register
> usage, NAPI should be no worse than non-NAPI.

Why would disabling IRQ's be expensive on non-MSI PCI devices?
Wouldn't it just require a single MMIO write to clear the interrupt
mask of the device. These are write-buffered so the latency should be
minimal. As mentioned in Jamal's UKUUG paper, any MMIO reading could
be avoided by caching the interrupt mask.
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