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

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Date:	Mon, 15 Sep 2014 21:55:50 +0300
From:	Dave Taht <dave.taht@...il.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Tom Herbert <therbert@...gle.com>,
	Jesper Dangaard Brouer <brouer@...hat.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>,
	Tim Shepard <shep@...m.mit.edu>,
	Avery Pennarun <apenwarr@...gle.com>
Subject: Re: Qdisc: Measuring Head-of-Line blocking with netperf-wrapper

On Mon, Sep 15, 2014 at 8:24 PM, 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). Also, you probably want to make
>> sure to have enough antagonist flows to saturate all links when using
>> MQ.


> Right.
>
> Jesper, relevant netperf option is :
>
>     -y local,remote   Set the socket priority
>
>

Problems with TCP_RR and UDP_RR are that they are subject to a RTO on
packet loss.
The time around a packet loss is "interesting". Secondly, several of
my measurements show that icmp is actually quicker than other code
paths, I figure because a switch to userspace is not needed to handle
the packet.

Outside of ping, the only isochronous packet generation code out there
worth dinking with is owamp,

http://software.internet2.edu/owamp/

And d-itg, at the moment.

There are some neato tools under development for looking at bursty
loss and mixed classifications (see very buggy code in my isochronous
dir on github - I put it down to finish stablizing cerowrt, less buggy
versions are in gfiber's repo which I don't have a link to handy at
the moment)

It would be nice if netperf could sprout a "send a user specified
packet size on this isochronous interval (with a resolution below a
ms) over X transport" test. Or there was something simpler than owamp
but as secure.

-- 
Dave Täht

https://www.bufferbloat.net/projects/make-wifi-fast
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