netdev - Re: Drops in qdisc on ifb interface

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Date:	Mon, 25 May 2015 20:17:26 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"John A. Sullivan III" <jsullivan@...nsourcedevel.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Drops in qdisc on ifb interface

On Mon, 2015-05-25 at 22:52 -0400, John A. Sullivan III wrote:

> Hmm . . . I've been thinking about that SFQ leaf qdisc.  I see that
> newer kernels allow a much higher "limit" than 127 but it still seems
> that the queue depth limit for any one flow is still 127.  When we do
> something like GRE/IPSec, I think the decrypted GRE traffic will
> distribute across the queues but the IPSec traffic will collapse all the
> packets initially into one queue.  At 80ms RTT a 1 Gbps wire speed, I
> would need a queue of around 7500.  Thus, can one say that SFQ is almost
> useless for high BDP connections?

I am a bit surprised, as your 'nstat' output showed no packet
retransmits. So no packets were lost in your sfq.

> 
> Is there a similar round-robin type qdisc that does not have this
> limitation?

fq_codel limit 10000 

> 
> If I recall correctly, if one does not attach a qdisc explicitly to a
> class, it defaults to pfifo_fast.  Is that correct? Thanks - John
> 

That would be pfifo.

pfifo_fast is the default root qdisc ( /proc/sys/net/core/default_qdisc
)


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