netdev - RE: [PATCH 2.6.25 1/1]S2io: Multiqueue network device support implementation

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Date:	Thu, 24 Jan 2008 22:19:50 -0500
From:	"Ramkrishna Vepa" <Ramkrishna.Vepa@...erion.com>
To:	"David Miller" <davem@...emloft.net>, <andi@...stfloor.org>
Cc:	"Sreenivasa Honnur" <Sreenivasa.Honnur@...erion.com>,
	<netdev@...r.kernel.org>, <jeff@...zik.org>,
	"support" <support@...erion.com>
Subject: RE: [PATCH 2.6.25 1/1]S2io: Multiqueue network device support implementation

> From: netdev-owner@...r.kernel.org
[mailto:netdev-owner@...r.kernel.org]
> On Behalf Of David Miller
> Sent: Wednesday, January 23, 2008 11:30 PM
> 
> From: Andi Kleen <andi@...stfloor.org>
> Date: Thu, 24 Jan 2008 08:25:49 +0100
> 
> > > have done away with handling tx completion in the interrupt
handler,
> and
> > > are instead handling them in the context of the transmit. The slow
> path,
> > > straggling transmit completions will be handled in the timer
context.
> >
> > Ok -- hopefully you don't have bad corner cases from this when the
pipe
> > is not fully filled and then causing longer latencies on completion.
> > Old NAPI sometimes suffered from such problems.
> 
> BTW, such a TX completion timer will stall TCP sockets when the TX
> queue of the device is partially filled and then sending stops.
> 
> TX ring SKB liberation really must be done in a very small finite
> amount of time in order to avoid this problem properly.
> 
> Deferring it to HZ granular timers doesn't give a quick enough
> response, especially at high packet rates.
[Ram] We could add a lower granular tx completion interrupt instead, to
handle all the transmit fifos. But this will be a fall back in the event
the completions do not happen in the context of the transmit.

Ram
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