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

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Date:	Tue, 1 Mar 2011 14:37:25 -0500
From:	Albert Cahalan <acahalan@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	David Miller <davem@...emloft.net>, johnwheffner@...il.com,
	linville@...driver.com, jussi.kivilinna@...et.fi, swmike@....pp.se,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: txqueuelen has wrong units; should be time

On Tue, Mar 1, 2011 at 2:26 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> Le mardi 01 mars 2011 à 01:54 -0500, Albert Cahalan a écrit :
>> On Mon, Feb 28, 2011 at 11:18 PM, David Miller <davem@...emloft.net> wrote:
>> > From: Albert Cahalan <acahalan@...il.com>

>> >> It sounds like you need a callback or similar, so that TCP can be
>> >> informed later that the drop has occurred.
>> >
>> > By that point we could have already sent an entire RTT's worth
>> > of data, or more.
>> >
>> > It needs to be synchronous, otherwise performance suffers.
>>
>> Ouch. OTOH, the current situation: performance suffers.
>>
>> In case it makes you feel any better, consider two cases
>> where synchronous feedback is already impossible.
>> One is when you're routing packets that merely pass through.
>> The other is when some other box is doing that to you.
>> Either way, packets go bye-bye and nobody tells TCP.
>
> So in a hurry we decide to drop packets blindly because kernel took the
> cpu to perform an urgent task ?

Yes. If the system can't handle the load, it needs to fess up.

> Bufferbloat is a configuration/tuning problem, not a "everything must be
> redone" problem. We add new qdiscs (CHOKe, SFB, QFQ, ...) and let admins
> do their job. Problem is most admins are unaware of the problems, and
> only buy more bandwidth.

We could at least do as well as Windows. >:-)

You can not expect some random Linux user to tune things
every time the link changes speed or the app mix changes.
What person NOT ON THIS MAILING LIST is going to mess
with their qdisc when they connect to a new access point
or switch from running Skype to running Netflix? Heck, how
many have any awareness of what a qdisk even is? Linux
networking needs to be excellent for people with no clue.

> We might need some changes (including new APIs).

If an app can't specify latency, adding the ability could
be nice. Still, stuff needs to JUST WORK more of the time.

> ECN is a forward step. Blindly dropping packets before ever sending them
> is a step backward.

Last I knew, ECN defaulted to a setting of "2" which means
it is only used in response. Perhaps it's time to change that.
It's been a while, with defective firewalls being replaced
by faster hardware.

> We should allow some trafic spikes, or many applications will stop
> working. Unless all applications are fixed, we are stuck.

Such applications would stop working...

1. across a switch
2. across an older router

We certainly should allow some traffic spikes. 1 to 10 ms of
traffic ought to do nicely. Hundreds or thousands of ms is
getting way beyond "spike".
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