netdev - Re: RFC: issues concerning the next NAPI interface

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Message-Id: <200708281319.03903.ossthema@de.ibm.com>
Date:	Tue, 28 Aug 2007 13:19:03 +0200
From:	Jan-Bernd Themann <ossthema@...ibm.com>
To:	David Miller <davem@...emloft.net>
Cc:	jchapman@...alix.com, 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 Monday 27 August 2007 22:37, David Miller wrote:
> From: Jan-Bernd Themann <ossthema@...ibm.com>
> Date: Mon, 27 Aug 2007 11:47:01 +0200
> 
> > So the question is simply: Do we want drivers that need (benefit
> > from) a timer based polling support to implement their own timers
> > each, or should there be a generic support?
> 
> I'm trying to figure out how an hrtimer implementation would
> even work.
> 
> Would you start the timer from the chip interrupt handler?  If so,
> that's taking two steps backwards as you've already taken all of the
> overhead of running the interrupt handler.

I'm also still trying to understand how hrtimer work exactly. 
The implementation of hrtimers for P6 has not been finished yet, so
I can't do experiments with hrtimers and eHEA now.

I will try the following scheme (once we get hrtimers):
Each device (queue) has a hrtimer.
Schedule the timer in the poll function instead of reactivating IRQs
when a high load situation has been detected and all packets have
been emptied from the receive queue.
The timer function could then just call netif_rx_schedule to register
the rx_queue for NAPI again. 

The advantages of this scheme (if it works as I understood it) would be:
- we don't have to modify NAPI
- benefit from fairness amoung rx_queues / network devices 
- The poll function can decide how long to stick to the timer based
  polling mode, and when to switch back to it's HW IRQs.
- driver can determine the time to wait based on the receive queue length and
  speed

Regards,
Jan-Bernd
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