netdev - Re: [net-next PATCH 1/3] net: make default tx_queue

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Message-Id: <20150729.143731.834219016906219771.davem@davemloft.net>
Date:	Wed, 29 Jul 2015 14:37:31 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	phil@....cc
Cc:	fw@...len.de, netdev@...r.kernel.org, brouer@...hat.com,
	cwang@...pensource.com, eric.dumazet@...il.com
Subject: Re: [net-next PATCH 1/3] net: make default tx_queue_len
 configurable

From: Phil Sutter <phil@....cc>
Date: Wed, 29 Jul 2015 23:34:28 +0200

> On Wed, Jul 29, 2015 at 11:06:18PM +0200, Florian Westphal wrote:
>> Phil Sutter <phil@....cc> wrote:
> [...]
>> > +config DEFAULT_TX_QUEUE_LEN
>> > +	prompt "Default TX queue length (in packets)" if EXPERT
>> > +	int
>> > +	default 1000	# Ethernet wants good queues
>> > +	help
>> > +	  Set the default value of tx_queue_len for newly created network
>> > +	  interfaces. It is used by queueing disciplines to determine how many
>> > +	  packets to keep in backlog before starting to drop new ones.
>> > +
>> > +	  The default value of 1000 packets is there for a very long time and
>> > +	  in combination with GSO way too big.
>> > +
>> 
>> I can't see how this could be used in a meaningful way.
>> 
>> No distro is going to touch this.
>> 
>> I don't think sysctl value would help either.
> 
> I just didn't want to introduce yet another magic value assignment. It's
> merely a #define with a little flexibility and a subtle note that the
> default should be changed attached.

Like others have mentioned, fix the _REAL_ issue.

Which is that there are devices (virtual or whatever) which don't want
a qdisc attached no matter what.  Flag those devices as such and
adjust the qdisc attachment logic to check that new flag.

Anything is better than hacking the queue len.
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