linux-kernel - Re: RFC: issues concerning the next NAPI interface

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Message-Id: <200708291043.14380.ossthema@de.ibm.com>
Date:	Wed, 29 Aug 2007 10:43:13 +0200
From:	Jan-Bernd Themann <ossthema@...ibm.com>
To:	James Chapman <jchapman@...alix.com>
Cc:	David Miller <davem@...emloft.net>,
	shemminger@...ux-foundation.org, akepner@....com,
	netdev@...r.kernel.org, raisch@...ibm.com, themann@...ibm.com,
	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org,
	meder@...ibm.com, tklein@...ibm.com, stefan.roscher@...ibm.com
Subject: Re: RFC: issues concerning the next NAPI interface

On Wednesday 29 August 2007 10:15, James Chapman wrote:
> Jan-Bernd Themann wrote:
> > What I'm trying to improve with this approach is interrupt
> > mitigation for NICs where the hardware support for interrupt
> > mitigation is limited. I'm not trying to improve this for NICs
> > that work well with the means their HW provides. I'm aware of
> > the fact that this scheme has it's tradeoffs and certainly
> > can not be as good as a HW approach.
> > So I'm grateful for any ideas that do have less tradeoffs and
> > provide a mechanism to reduce interrupts without depending on
> > HW support of the NIC.
> > 
> > In the end I want to reduce the CPU utilization. And one way
> > to do that is LRO which also works only well if there are more
> > then just a very few packets to aggregate. So at least our
> > driver (eHEA) would benefit from a mix of timer based polling
> > and plain NAPI (depending on load situations).
> 
> Wouldn't you achieve the same result by enabling hardware interrupt 
> mitigation in eHEA in combination with NAPI? Presumably a 10G interface 
> has hardware mitigation features?

Quote from above: "What I'm trying to improve with this approach 
is interrupt mitigation for NICs where the hardware support for
interrupt mitigation is limited"

So guess why I'm doing that ;-)

> 
> > If there is no need for a generic mechanism for this kind of
> > network adapters, then we can just leave this to each device
> > driver.
> 
> I've been looking at this from a different angle. My goal is to optimize 
> NAPI packet forwarding rates while minimizing packet latency. Using 
> hardware interrupt mitigation hurts latency so I'm investigating ways to 
> turn it off without risking NAPI poll on/off thrashing at certain packet 
> rates.
> 
> Jan-Bernd, I think I've found a solution to the issue that you 
> highlighted with my scheme yesterday and it doesn't involve generating 
> other interrupts using hrtimers etc. :) Initial results are very 
> encouraging in my setups. Would you be willing to test it with eHEA? I 
> don't have a 10G setup. If results are encouraging, I'll post an RFC to 
> ask for review / feedback from the NAPI experts here. What do you think?
> 

I'm not sure which solution you mean. If you post your RFC, please create
a new thread (other title)

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