netdev - Re: txqueuelen has wrong units; should be time

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Message-ID: <20110228165501.GC2515@tuxdriver.com>
Date:	Mon, 28 Feb 2011 11:55:01 -0500
From:	"John W. Linville" <linville@...driver.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Jussi Kivilinna <jussi.kivilinna@...et.fi>,
	Albert Cahalan <acahalan@...il.com>,
	Mikael Abrahamsson <swmike@....pp.se>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	netdev@...r.kernel.org
Subject: Re: txqueuelen has wrong units; should be time

On Mon, Feb 28, 2011 at 05:48:14PM +0100, Eric Dumazet wrote:
> Le lundi 28 février 2011 à 11:11 -0500, John W. Linville a écrit :
> > On Sun, Feb 27, 2011 at 09:07:53PM +0100, Eric Dumazet wrote:
> > 
> > > Qdisc should return to caller a good indication packet is queued or
> > > dropped at enqueue() time... not later (aka : never)
> > > 
> > > Accepting a packet at t0, and dropping it later at t0+limit without
> > > giving any indication to caller is a problem.
> > 
> > Can you elaborate on what problem this causes?  Is it any worse than
> > if the packet is dropped at some later hop?
> > 
> > Is there any API that could report the drop to the sender (at
> > least a local one) without having to wait for the ack timeout?
> > Should there be?
> > 
> 
> Not all protocols have ACKS ;)
> 
> dev_queue_xmit() returns an error code, some callers use it.

Well, OK -- I agree it is best if you can return the status at
enqueue time.  The question becomes whether or not a dropped frame
is worse than living with high latency.  The answer, of course, still
seems to be a bit subjective.  But, if the admin has determined that
a link should be low latency...?

John
-- 
John W. Linville		Someday the world will need a hero, and you
linville@...driver.com			might be all we have.  Be ready.
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