[Bloat] DSLReports Speed Test has latency measurement built-in

Mon Apr 20 21:15:12 PDT 2015

I've discovered something perhaps you guys can explain it better or shed
some light.
It isn't specifically to do with buffer bloat but it is to do with TCP
tuning.

Attached is two pictures of my upload to New York speed test server with 1
stream.
It doesn't make any difference if it is 1 stream or 8 streams, the picture
and behaviour remains the same.
I am 200ms from new york so it qualifies as a fairly long (but not very
fat) pipe.

The nice smooth one is with linux tcp_rmem set to '4096 32768 65535' (on
the server)
The ugly bumpy one is with linux tcp_rmem set to '4096 65535 67108864' (on
the server)

It actually doesn't matter what that last huge number is, once it goes much
about 65k, e.g. 128k or 256k or beyond things get bumpy and ugly on the
upload speed.

Now as I understand this setting, it is the tcp receive window that Linux
advertises, and the last number sets the maximum size it can get to (for
one TCP stream).

For users with very fast upload speeds, they do not see an ugly bumpy
upload graph, it is smooth and sustained.
But for the majority of users (like me) with uploads less than 5 to 10mbit,
we frequently see the ugly graph.

The second tcp_rmem setting is how I have been running the speed test
servers.

Up to now I thought this was just the distance of the speedtest from the
interface: perhaps the browser was buffering a lot, and didn't feed back
progress but now I realise the bumpy one is actually being influenced by
the server receive window.

I guess my question is this: Why does ALLOWING a large receive window
appear to encourage problems with upload smoothness??

This implies that setting the receive window should be done on a connection
by connection basis: small for slow connections, large, for high speed,
long distance connections.

In addition, if I cap it to 65k, for reasons of smoothness,
that means the bandwidth delay product will keep maximum speed per upload
stream quite low. So a symmetric or gigabit connection is going to need a
ton of parallel streams to see full speed.

Most puzzling is why would anything special be required on the Client -->
Server side of the equation
but nothing much appears wrong with the Server --> Client side, whether
speeds are very low (GPRS) or very high (gigabit).

Note that also I am not yet sure if smoothness == better throughput. I have
noticed upload speeds for some people often being under their claimed sync
rate by 10 or 20% but I've no logs that show the bumpy graph is showing
inefficiency. Maybe.

help!

On Tue, Apr 21, 2015 at 12:56 PM, Simon Barber <simon at superduper.net> wrote:

> One thing users understand is slow web access.  Perhaps translating the
> latency measurement into 'a typical web page will take X seconds longer to
> load', or even stating the impact as 'this latency causes a typical web
> page to load slower, as if your connection was only YY% of the measured
> speed.'
>
> Simon
>
> Sent with AquaMail for Android
> http://www.aqua-mail.com
>
>
>
> On April 19, 2015 1:54:19 PM Jonathan Morton <chromatix99 at gmail.com>
> wrote:
>
>  >>>> Frequency readouts are probably more accessible to the latter.
>> >>>
>> >>>     The frequency domain more accessible to laypersons? I have my
>> doubts ;)
>> >>
>> >> Gamers, at least, are familiar with “frames per second” and how that
>> corresponds to their monitor’s refresh rate.
>> >
>> >       I am sure they can easily transform back into time domain to get
>> the frame period ;) .  I am partly kidding, I think your idea is great in
>> that it is a truly positive value which could lend itself to being used in
>> ISP/router manufacturer advertising, and hence might work in the real work;
>> on the other hand I like to keep data as “raw” as possible (not that ^(-1)
>> is a transformation worthy of being called data massage).
>> >
>> >> The desirable range of latencies, when converted to Hz, happens to be
>> roughly the same as the range of desirable frame rates.
>> >
>> >       Just to play devils advocate, the interesting part is time or
>> saving time so seconds or milliseconds are also intuitively understandable
>> and can be easily added ;)
>>
>> Such readouts are certainly interesting to people like us.  I have no
>> objection to them being reported alongside a frequency readout.  But I
>> think most people are not interested in “time savings” measured in
>> milliseconds; they’re much more aware of the minute- and hour-level time
>> savings associated with greater bandwidth.
>>
>>  - Jonathan Morton
>>
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>
>
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