netdev - Re: [net-next PATCH 2/5] ixgbe: increase default TX ring buffer to 1024

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Message-Id: <20140514.155437.750583011696684472.davem@davemloft.net>
Date:	Wed, 14 May 2014 15:54:37 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	brouer@...hat.com
Cc:	alexander.h.duyck@...el.com, netdev@...r.kernel.org,
	jeffrey.t.kirsher@...el.com, dborkman@...hat.com, fw@...len.de,
	shemminger@...tta.com, paulmck@...ux.vnet.ibm.com,
	robert@...julf.se, greearb@...delatech.com,
	john.r.fastabend@...el.com, danieltt@....se, zhouzhouyi@...il.com
Subject: Re: [net-next PATCH 2/5] ixgbe: increase default TX ring buffer to
 1024

From: Jesper Dangaard Brouer <brouer@...hat.com>
Date: Wed, 14 May 2014 21:09:35 +0200

> On Wed, 14 May 2014 13:49:50 -0400 (EDT)
> David Miller <davem@...emloft.net> wrote:
> 
>> From: Alexander Duyck <alexander.h.duyck@...el.com>
>> Date: Wed, 14 May 2014 09:28:50 -0700
>> 
>> > I'd say that it might be better to just add a note to the documentation
>> > folder indicating what configuration is optimal for pktgen rather then
>> > changing everyone's defaults to support one specific test.
>> 
>> We could have drivers provide a pktgen config adjustment mechanism,
>> so if someone starts pktgen then the device auto-adjusts to a pktgen
>> optimal configuration (whatever that may entail).
> 
> That might be problematic because changing the TX queue size cause the
> ixgbe driver to reset the link.

A minor issue for someone firing up a specialized network test tool
like pktgen.

> Notice that pktgen is ignoring BQL.  I'm sort of hoping that BQL will
> push back for real use-cases, to avoid the bad effects of increasing
> the TX size.

I think we need to think carefully about drivers configuring such huge
per-queue TX queue sizes.

If anything, we should be decreasing their size in the default
configuration.
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