[Starlink] [Make-wifi-fast] TCP_NOTSENT_LOWAT applied to e2e TCP msg latency

[Bob McMahon]

Thanks Stuart this is helpful. I'm measuring the time just before the first
write() (of potentially a burst of writes to achieve a burst size) per a
socket fd's select event occurring when TCP_NOT_SENT_LOWAT being set to a
small value, then sampling the RTT and CWND and providing histograms for
all three, all on that event. I'm not sure the correctness of RTT and CWND
at this sample point. This is a controlled test over 802.11ax and OFDMA
where the TCP acks per the WiFi clients are being scheduled by the AP using
802.11ax trigger frames so the AP is affecting the end/end BDP per
scheduling the transmits and the acks. The AP can grow the BDP or shrink it
based on these scheduling decisions.  From there we're trying to maximize
network power (throughput/delay) for elephant flows and just latency for
mouse flows. (We also plan some RF frequency stuff to per OFDMA) Anyway,
the AP based scheduling along with aggregation and OFDMA makes WiFi
scheduling optimums non-obvious - at least to me - and I'm trying to
provide insights into how an AP is affecting end/end performance.

The more direct approach for e2e TCP latency and network power has been to
measure first write() to final read() and compute the e2e delay. This
requires clock sync on the ends. (We're using ptp4l with GPS OCXO
atomic references for that but this is typically only available in some
labs.)

Bob


On Mon, Oct 25, 2021 at 8:11 PM Stuart Cheshire <cheshire at apple.com> wrote:

> On 21 Oct 2021, at 17:51, Bob McMahon via Make-wifi-fast <
> make-wifi-fast at lists.bufferbloat.net> wrote:
>
> > Hi All,
> >
> > Sorry for the spam. I'm trying to support a meaningful TCP message
> latency w/iperf 2 from the sender side w/o requiring e2e clock
> synchronization. I thought I'd try to use the TCP_NOTSENT_LOWAT event to
> help with this. It seems that this event goes off when the bytes are in
> flight vs have reached the destination network stack. If that's the case,
> then iperf 2 client (sender) may be able to produce the message latency by
> adding the drain time (write start to TCP_NOTSENT_LOWAT) and the sampled
> RTT.
> >
> > Does this seem reasonable?
>
> I’m not 100% sure what you’re asking, but I will try to help.
>
> When you set TCP_NOTSENT_LOWAT, the TCP implementation won’t report your
> endpoint as writable (e.g., via kqueue or epoll) until less than that
> threshold of data remains unsent. It won’t stop you writing more bytes if
> you want to, up to the socket send buffer size, but it won’t *ask* you for
> more data until the TCP_NOTSENT_LOWAT threshold is reached. In other words,
> the TCP implementation attempts to keep BDP bytes in flight +
> TCP_NOTSENT_LOWAT bytes buffered and ready to go. The BDP of bytes in
> flight is necessary to fill the network pipe and get good throughput. The
> TCP_NOTSENT_LOWAT of bytes buffered and ready to go is provided to give the
> source software some advance notice that the TCP implementation will soon
> be looking for more bytes to send, so that the buffer doesn’t run dry,
> thereby lowering throughput. (The old SO_SNDBUF option conflates both
> “bytes in flight” and “bytes buffered and ready to go” into the same
> number.)
>
> If you wait for the TCP_NOTSENT_LOWAT notification, write a chunk of n
> bytes of data, and then wait for the next TCP_NOTSENT_LOWAT notification,
> that will tell you roughly how long it took n bytes to depart the machine.
> You won’t know why, though. The bytes could depart the machine in response
> for acks indicating that the same number of bytes have been accepted at the
> receiver. But the bytes can also depart the machine because CWND is
> growing. Of course, both of those things are usually happening at the same
> time.
>
> How to use TCP_NOTSENT_LOWAT is explained in this video:
>
> <https://developer.apple.com/videos/play/wwdc2015/719/?time=2199>
>
> Later in the same video is a two-minute demo (time offset 42:00 to time
> offset 44:00) showing a “before and after” demo illustrating the dramatic
> difference this makes for screen sharing responsiveness.
>
> <https://developer.apple.com/videos/play/wwdc2015/719/?time=2520>
>
> Stuart Cheshire

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