netdev - Re: Drops in qdisc on ifb interface

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Date:	Mon, 25 May 2015 15:31:04 -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 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)





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