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

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Date:	Mon, 27 Aug 2007 18:02:47 +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 Monday 27 August 2007 17:51, James Chapman wrote:

> In the second half of my previous reply (which seems to have been 
> deleted), I suggest a way to avoid this problem without using hardware 
> interrupt mitigation / coalescing. Original text is quoted below.
> 
>  >> I've seen the same and I'm suggesting that the NAPI driver keeps
>  >> itself in polled mode for N polls or M jiffies after it sees
>  >> workdone=0. This has always worked for me in packet forwarding
>  >> scenarios to maximize packets/sec and minimize latency.
> 
> To implement this, there's no need for timers, hrtimers or generic NAPI 
> support that others have suggested. A driver's poll() would set an 
> internal flag and record the current jiffies value when finding 
> workdone=0 rather than doing an immediate napi_complete(). Early in 
> poll() it would test this flag and if set, do a low-cost test to see if 
> it had any work to do. If no work, it would check the saved jiffies 
> value and do the napi_complete() only if no work has been done for a 
> configurable number of jiffies. This keeps interrupts disabled longer at 
> the expense of many more calls to poll() where no work is done. So 
> critical to this scheme is modifying the driver's poll() to fastpath the 
> case of having no work to do while waiting for its local jiffy count to 
> expire.
> 

The problem I see with this approach is that the time that passes between
two jiffies might be too long for 10G ethernet adapters. (I tried to implement
a timer based approach with usual timers and the result was a disaster).
HW interrupts / or HP timer avoid the jiffy problem as they activate softIRQs
as soon as you call netif_rx_schedule. 
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