netdev - Re: Qdisc: Measuring Head-of-Line blocking with netperf-wrapper

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Date:	Tue, 16 Sep 2014 08:52:00 -0700
From:	Tom Herbert <therbert@...gle.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Stephen Hemminger <stephen@...workplumber.org>,
	David Miller <davem@...emloft.net>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>,
	Daniel Borkmann <dborkman@...hat.com>,
	Florian Westphal <fw@...len.de>,
	Toke Høiland-Jørgensen <toke@...e.dk>,
	Dave Taht <dave.taht@...il.com>
Subject: Re: Qdisc: Measuring Head-of-Line blocking with netperf-wrapper

On Mon, Sep 15, 2014 at 11:30 PM, Jesper Dangaard Brouer
<brouer@...hat.com> wrote:
> On Mon, 15 Sep 2014 10:24:23 -0700
> Eric Dumazet <eric.dumazet@...il.com> wrote:
>
>> On Mon, 2014-09-15 at 10:10 -0700, Tom Herbert wrote:
>> > On Mon, Sep 15, 2014 at 9:45 AM, Jesper Dangaard Brouer
>> > <brouer@...hat.com> wrote:
>> > >
>> > > Hi Eric,
>> > >
>> > > I've constructed a "netperf-wrapper" test for measuring Head-of-Line
>> > > blocking, called "tcp_upload_prio", that I hope you will approve of?
>> > >
>> > >  https://github.com/tohojo/netperf-wrapper/commit/1e6b755e8051b6
>> > >
>> > > The basic idea is to have ping packets with TOS bit 0x10, which end-up
>> > > in the high-prio band of pfifo_fast.  While two TCP uploads utilize
>> > > all the bandwidth.
>> > >
>> > > These high-prio ping packet should then demonstrate the Head-of-Line
>> > > blocking occurring due to 1) packets in the HW TX ring buffer, or
>> > > 2) in the qdisc layers requeue mechanism.  Disgusting these two case
>> > > might be a little difficult.
>> > >
>> > >
>> > >
>> > > Special care need to be take for using this on the default
>> > > qdisc MQ which have pfifo_fast assigned for every HW queue.
>> > >
>> > > Setup requirements:
>> > >  1. IRQ align CPUs to NIC HW queues
>> > >  2. Force netperf-wrapper subcommands to run the same CPU
>> > >   E.g: taskset -c 2 ./netperf-wrapper -H IP tcp_upload_prio
>> > >
>> > > This will force all measurements to go through the same qdisc.  This
>> > > is needed so the ping/latency tests measures the real property of
>> > > the qdisc and Head-of-Line blocking effect.
>> > >
>> > >
>> > > Basically the same as:
>> > >  sudo taskset -c 2 ping -Q 0x10 192.168.8.2
>> > >  sudo taskset -c 2 ping         192.168.8.2
>> > >  sudo taskset -c 2 netperf   -H 192.168.8.2 -t TCP_STREAM -l 120
>> > >  sudo taskset -c 2 netperf   -H 192.168.8.2 -t TCP_STREAM -l 120
>> > > --
>> > ping is a very coarse way to measure latency and in network devices it
>> > doesn't follow same path as TCP/UDP (no 4-tuple for RSS, ECMP) so it's
>> > biased and not a very realistic workload. You might want to try using
>> > netperf TCP_RR at higher priority for a fairer comparison (this is
>> > what I used to verify BQL benefits).
>
> I worry about starvation, when putting too much/heavy traffic in the
> high prio queue.
>
Each TCP_RR flow only has one packet outstanding by default.

> I've played with UDP_RR (in high prio queue) to measure the latency, it
> worked well (much less fluctuations than ping) for GSO and TSO , but
> for the none-GSO case it disturbed the two TCP uploads so much, that
> they could not utilize the link.
>
> For TCP_RR I worry what happens if a packet loss and RTO happens, but I
> guess putting this in the high prio queue should make drops (a lot)
> less likely.
>
Are you actually seeing packet loss when you run tour test? If you're
testing between two hosts in a controlled environment, you should be
able to configure test parameters for no loss.

>> > Also, you probably want to make
>> > sure to have enough antagonist flows to saturate all links when using
>> > MQ.
>
> For the none-GSO case, I guess adding more TCP uploads might help, but
> they might just get starvated.  I'll give it a try.
>
>
>> Jesper, relevant netperf option is :
>>
>>     -y local,remote   Set the socket priority
>
> Check, netperf-wrapper already supports setting these.
>
>
> --
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Sr. Network Kernel Developer at Red Hat
>   Author of http://www.iptv-analyzer.org
>   LinkedIn: http://www.linkedin.com/in/brouer
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