netdev - Re: qdisc spin lock

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Date:	Wed, 20 Apr 2016 15:34:44 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Michael Ma <make0818@...il.com>
Cc:	Cong Wang <xiyou.wangcong@...il.com>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: qdisc spin lock

On Wed, 2016-04-20 at 14:24 -0700, Michael Ma wrote:
> 2016-04-08 7:19 GMT-07:00 Eric Dumazet <eric.dumazet@...il.com>:
> > On Thu, 2016-03-31 at 16:48 -0700, Michael Ma wrote:
> >> I didn't really know that multiple qdiscs can be isolated using MQ so
> >> that each txq can be associated with a particular qdisc. Also we don't
> >> really have multiple interfaces...
> >>
> >> With this MQ solution we'll still need to assign transmit queues to
> >> different classes by doing some math on the bandwidth limit if I
> >> understand correctly, which seems to be less convenient compared with
> >> a solution purely within HTB.
> >>
> >> I assume that with this solution I can still share qdisc among
> >> multiple transmit queues - please let me know if this is not the case.
> >
> > Note that this MQ + HTB thing works well, unless you use a bonding
> > device. (Or you need the MQ+HTB on the slaves, with no way of sharing
> > tokens between the slaves)
> 
> Actually MQ+HTB works well for small packets - like flow of 512 byte
> packets can be throttled by HTB using one txq without being affected
> by other flows with small packets. However I found using this solution
> large packets (10k for example) will only achieve very limited
> bandwidth. In my test I used MQ to assign one txq to a HTB which sets
> rate at 1Gbit/s, 512 byte packets can achieve the ceiling rate by
> using 30 threads. But sending 10k packets using 10 threads has only 10
> Mbit/s with the same TC configuration. If I increase burst and cburst
> of HTB to some extreme large value (like 50MB) the ceiling rate can be
> hit.
> 
> The strange thing is that I don't see this problem when using HTB as
> the root. So txq number seems to be a factor here - however it's
> really hard to understand why would it only affect larger packets. Is
> this a known issue? Any suggestion on how to investigate the issue
> further? Profiling shows that the cpu utilization is pretty low.

You could try 

perf record -a -g -e skb:kfree_skb sleep 5
perf report

So that you see where the packets are dropped.

Chances are that your UDP sockets SO_SNDBUF is too big, and packets are
dropped at qdisc enqueue time, instead of having backpressure.