linux-kernel - Re: txqueuelen has wrong units; should be time

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Message-ID: <AANLkTinFZu5AgfhO6PoA7C1NTGSfD8HH0KXq_o-mHEJF@mail.gmail.com>
Date:	Mon, 28 Feb 2011 23:11:13 -0500
From:	Albert Cahalan <acahalan@...il.com>
To:	John Heffner <johnwheffner@...il.com>
Cc:	"John W. Linville" <linville@...driver.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Jussi Kivilinna <jussi.kivilinna@...et.fi>,
	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 4:45 PM, John Heffner <johnwheffner@...il.com> wrote:
> On Mon, Feb 28, 2011 at 11:55 AM, John W. Linville
> <linville@...driver.com> wrote:
>> 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...?
>
> Notably, TCP is one caller that uses the error code.  The error code
> is functionally equivalent to ECN, one of whose great advantages is
> reducing delay jitter.  If TCP didn't get the error, that would
> effectively double the latency for a full window of data, since the
> dropped segment would not be retransmitted for an RTT.

It sounds like you need a callback or similar, so that TCP can be
informed later that the drop has occurred.
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