[Cerowrt-devel] Fwd: Throughput regression with `tcp: refine TSO autosizing`

Sat Jan 31 16:05:50 EST 2015

I would like to have somehow assembled all the focused resources to make a
go at fixing wifi, or at least having a f2f with a bunch of people in the
late march timeframe. This message of mine to linux-wireless bounced for
some reason and I am off to log out for 10 days, so...

see relevant netdev thread also for ore details.

---------- Forwarded message ----------
From: Dave Taht <dave.taht at gmail.com>
Date: Sat, Jan 31, 2015 at 12:29 PM
Subject: Re: Throughput regression with `tcp: refine TSO autosizing`
To: Arend van Spriel <arend at broadcom.com>
Cc: linux-wireless <linux-wireless at vger.kernel.org>, Michal Kazior <
michal.kazior at tieto.com>, Eyal Perry <eyalpe at dev.mellanox.co.il>, Network
Development <netdev at vger.kernel.org>, Eric Dumazet <eric.dumazet at gmail.com>

The wifi industry as a whole has vastly bigger problems than achieving
1500Mbits in a faraday cage on a single flow.

I encourage you to try tests in netperf-wrapper that explicitly test for
latency under load, and in particular, the RTT_FAIR tests against 4 or more
stations on a single wifi AP. You will find the results very depressing.
Similarly, on your previous test series, a latency figure would have been
nice to have. I just did a talk at nznog, where I tested the local wifi
with less than ambits of throughput, and 3 seconds of latency, filmed here:

https://plus.google.com/u/0/107942175615993706558/posts/CY8ew8MPnMt

Do wish more folk were testing in the busy real world environments, like
coffee shops, cities... really, anywhere outside a faraday cage!

I am not attending netconf - I was unable to raise funds to go, and the
program committee wanted something "new",

instead of the preso I gave the IEEE 802.11 working group back in
september. (
http://snapon.lab.bufferbloat.net/~d/ieee802.11-sept-17-2014/11-14-1265-00-0wng-More-on-Bufferbloat.pdf
)

I was very pleased with the results of that talk - the day after I gave it,
the phrase "test for latency" showed up in a bunch of 802.11ax (the next
generation after ac) documents. :) Still, we are stuck with the train wreck
that is 802.11ac glommed on top of 802.11n, glommed on top of 802.11g, in
terms of queue management, terrible uses of airtime, rate control and other
stuff. Aruba and Meraki, in particular took a big interest in what I'd
outlined in the preso above (we have a half dozen less well baked ideas -
that's just the easy stuff that can be done to improve wifi).  I gave a
followup at meraki but I don't think that's online.

Felix (nbd) is on vacation right now, as I am I. In fact I am going
somewhere for a week totally lacking internet access.

Presently the plan, with what budget (none) we have and time (very little)
we have is to produce a pair of proof of concept implementations for per
tid queuing (see relevant commit by nbd),  leveraging the new minstrel
stats, the new minstrel-blues stuff, and an aggregation aware codel with a
calculated target based on the most recently active stations, and a bunch
of the other stuff outlined above at IEEE.

It is my hope that this will start to provide accurate back pressure (or
sufficient lack thereof for TSQ), to also improve throughput while still
retaining low latency. But it is a certainty that we will run into more
cross layer issues that will be a pita to resolve.

If we can put together a meet up around or during ELC in california in
march?

I am really not terribly optimistic on anything other than the 2 chipsets
we can hack on (ath9k, mt76). Negotiations to get qualcomm to open up their
ath10k firmware have thus far failed, nor has a ath10k-lite got anywhere.
Perhaps broadcom would be willing to open up their firmware sufficiently to
build in a better API?

A bit more below.

On Jan 30, 2015 5:59 AM, "Arend van Spriel" <arend at broadcom.com> wrote:
>
> On 01/30/15 14:19, Eric Dumazet wrote:
>>
>> On Fri, 2015-01-30 at 11:29 +0100, Arend van Spriel wrote:
>>
>>> Hi Eric,
>>>
>>> Your suggestions are still based on the fact that you consider wireless
>>> networking to be similar to ethernet, but as Michal indicated there are
>>> some fundamental differences starting with CSMA/CD versus CSMA/CA. Also
>>> the medium conditions are far from comparable.

The analogy i now use for it is that switched ethernet is generally your
classic "dumbbell"

topology. Wifi is more like a "taxi-stand" topology. If you think about how
people

queue up at a taxi stand (and sometimes agree to share a ride), the inter
arrival

and departure times of a taxi stand make for a better mental model.

Admittedly, I seem to spend a lot of time, waiting for taxies, thinking
about

wifi.

>> There is no shielding so
>>> it needs to deal with interference and dynamically drops the link rate
>>> so transmission of packets can take several milliseconds. Then with 11n
>>> they came up with aggregation with sends up to 64 packets in a single
>>> transmit over the air at worst case 6.5 Mbps (if I am not mistaken). The
>>> parameter value for tcp_limit_output_bytes of 131072 means that it
>>> allows queuing for about 1ms on a 1Gbps link, but I hope you can see
>>> this is not realistic for dealing with all variances of the wireless
>>> medium/standard. I suggested this as topic for the wireless workshop in
>>> Otawa [1], but I can not attend there. Still hope that there will be
>>> some discussions to get more awareness.

I have sometimes hoped that TSQ could be made more a function of the

number of active flows exiting an interface, but eric tells me that's
impossible.

This is possibly another case where TSQ could use to be a callback
function...

but frankly I care not a whit about maximizing single flow tcp throughput
on wifi

in a faraday cage.

>>
>> Ever heard about bufferbloat ?
>
>
> Sure. I am trying to get awareness about that in our wireless
driver/firmware development teams. So bear with me.
>
>
>> Have you read my suggestions and tried them ?
>>
>> You can adjust the limit per flow to pretty much you want. If you need
>> 64 packets, just do the math. If in 2018 you need 128 packets, do the
>> math again.
>>
>> I am very well aware that wireless wants aggregation, thank you.

I note that a lot of people testing this are getting it backwards. Usually
it is the AP that is sending lots and lots of big packets, where the return
path is predominately acks from the station.

I am not a huge fan of stretch acks, but certainly a little bit of thinning
doesn't bother me on the return path there.

Going the other way, particularly in a wifi world that insists on treating
every packet as sacred (which I don't agree with at all), thinning acks can
help, but single stream throughput is of interest only on benchmarks, FQing
as much as possible all the flows destined the station in each aggregate
masks loss and reduces the need to protect everything so much.

>
> Sorry if I offended you. I was just giving these as example combined with
effective rate usable on the medium to say that the bandwidth is more
dynamic in wireless and as such need dynamic change of queue depth. Now
this can be done by making the fraction size as used in your suggestion
adaptive to these conditions.

Well... see above. Maybe this technique will do more of the right thing,
but... go test.

>
>> 131072 bytes of queue on 40Gbit is not 1ms, but 26 usec of queueing, and
>> we get line rate nevertheless.
>
>
> I was saying it was about 1ms on *1Gbit* as the wireless TCP rates are
moving into that direction in 11ac.
>
>
>> We need this level of shallow queues (BQL, TSQ), to get very precise rtt
>> estimations so that TCP has good entropy for its pacing, even in the 50
>> usec rtt ranges.
>>
>> If we allowed 1ms of queueing, then a 40Gbit flow would queue 5 MBytes.
>>
>> This was terrible, because it increased cwnd and all sender queues to
>> insane levels.
>
>
> Indeed and that is what we would like to address in our wireless drivers.
I will setup some experiments using the fraction sizing and post my
findings. Again sorry if I offended you.

You really, really, really need to test at rates below 50mbit and with
other stations, also while doing this. It's not going to be a linear curve.

>
> Regards,
> Arend
>
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-- 
Dave Täht

thttp://www.bufferbloat.net/projects/bloat/wiki/Upcoming_Talks
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