netdev - Re: Drops in qdisc on ifb interface

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Date:	Mon, 25 May 2015 22:52:50 -0400
From:	"John A. Sullivan III" <jsullivan@...nsourcedevel.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: Drops in qdisc on ifb interface

On Mon, 2015-05-25 at 15:31 -0700, Eric Dumazet wrote:
> On Mon, 2015-05-25 at 16:05 -0400, John A. Sullivan III wrote:
> > Hello, all.  One one of our connections we are doing intensive traffic
> > shaping with tc.  We are using ifb interfaces for shaping ingress
> > traffic and we also use ifb interfaces for egress so that we can apply
> > the same set of rules to multiple interfaces (e.g., tun and eth
> > interfaces operating on the same physical interface).
> > 
> > These are running on very powerful gateways; I have watched them
> > handling 16 Gbps with CPU utilization at a handful of percent.  Yet, I
> > am seeing drops on the ifb interfaces when I do a tc -s qdisc show.
> > 
> > Why would this be? I would expect if there was some kind of problem that
> > it would manifest as drops on the physical interfaces and not the IFB
> > interface.  We have played with queue lengths in both directions.  We
> > are using HFSC with SFQ leaves so I would imagine this overrides the
> > very short qlen on the IFB interfaces (32).  These are drops and not
> > overlimits.
> 
> IFB is single threaded and a serious bottleneck.
> 
> Don't use this on egress, this destroys multiqueue capaility.
> 
> And SFQ is pretty limited (127 packets)
> 
> You might try to change your NIC to have a single queue for RX,
> so that you have a single cpu feeding your IFB queue.
> 
> (ethtool -L eth0 rx 1)
> 
> 
> 
> 
> 
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?

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

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

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