netdev - RE: [PATCH net-next 1/3] mlx4_en: TX ring size default to 1024

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Date:	Fri, 24 Feb 2012 21:17:34 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Yevgeny Petrilin <yevgenyp@...lanox.com>
Cc:	David Miller <davem@...emloft.net>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: RE: [PATCH net-next 1/3] mlx4_en: TX ring size default to 1024

Le vendredi 24 février 2012 à 19:35 +0000, Yevgeny Petrilin a écrit :
> > > Signed-off-by: Yevgeny Petrilin <yevgenyp@...lanox.co.il>
> > 
> > This is rediculious as a default, yes even for 10Gb.
> > 
> > Do you have any idea how high latency is going to be for packets
> > trying to get into the transmit queue if there are already a
> > thousand other frames in there?
> 
> On the other hand, when having smaller queue with 1000 in-flight packets would mean queue would be stopped,
> how is it better?

Its better because you can have any kind of Qdisc setup to properly
classify packets, with 100.000 total packets in queues if you wish.

TX ring is a single FIFO, and that is just horrible, especially with big packets...

> Having bigger TX ring helps dealing better with bursts of TX packets, without the overhead of stopping and starting the queue,
> It also makes sense to have same size TX and RX queues, for example in case of traffic being forwarded from TX to RX.
> 

Really I doubt people using forwarding setups use default qdiscs.

Instead of bigger TX rings, they need appropriate Qdiscs.

> I did find number of 10Gb vendors that have 1024 or more as the default size for TX queue.

Thats a shame.



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