[Cake] [Bloat] Little's Law mea culpa, but not invalidating my main point

Bob McMahon bob.mcmahon at broadcom.com
Mon Jul 12 14:38:24 EDT 2021


To be clear, it's a OS write() using a socket opened with TCP and the final
OS read() of that write. The write size is set using -l or --length. OWD
requires --trip-times option.

Bob

On Mon, Jul 12, 2021 at 11:21 AM Bob McMahon <bob.mcmahon at broadcom.com>
wrote:

> iperf 2 supports OWD and gives full histograms for TCP write to read, TCP
> connect times, latency of packets (with UDP), latency of "frames" with
> simulated video traffic (TCP and UDP), xfer times of bursts with low duty
> cycle traffic, and TCP RTT (sampling based.) It also has support for
> sampling (per interval reports) down to 100 usecs if configured with
> --enable-fastsampling, otherwise the fastest sampling is 5 ms. We've
> released all this as open source.
>
> OWD only works if the end realtime clocks are synchronized using a
> "machine level" protocol such as IEEE 1588 or PTP. Sadly, *most data
> centers don't provide sufficient level of clock accuracy and the GPS pulse
> per second * to colo and vm customers.
>
> https://iperf2.sourceforge.io/iperf-manpage.html
>
> Bob
>
> On Mon, Jul 12, 2021 at 10:40 AM David P. Reed <dpreed at deepplum.com>
> wrote:
>
>>
>> On Monday, July 12, 2021 9:46am, "Livingood, Jason" <
>> Jason_Livingood at comcast.com> said:
>>
>> > I think latency/delay is becoming seen to be as important certainly, if
>> not a more direct proxy for end user QoE. This is all still evolving and I
>> have to say is a super interesting & fun thing to work on. :-)
>>
>> If I could manage to sell one idea to the management hierarchy of
>> communications industry CEOs (operators, vendors, ...) it is this one:
>>
>> "It's the end-to-end latency, stupid!"
>>
>> And I mean, by end-to-end, latency to complete a task at a relevant layer
>> of abstraction.
>>
>> At the link level, it's packet send to packet receive completion.
>>
>> But at the transport level including retransmission buffers, it's
>> datagram (or message) origination until the acknowledgement arrives for
>> that message being delivered after whatever number of retransmissions,
>> freeing the retransmission buffer.
>>
>> At the WWW level, it's mouse click to display update corresponding to
>> completion of the request.
>>
>> What should be noted is that lower level latencies don't directly predict
>> the magnitude of higher-level latencies. But longer lower level latencies
>> almost always amplfify higher level latencies. Often non-linearly.
>>
>> Throughput is very, very weakly related to these latencies, in contrast.
>>
>> The amplification process has to do with the presence of queueing.
>> Queueing is ALWAYS bad for latency, and throughput only helps if it is in
>> exactly the right place (the so-called input queue of the bottleneck
>> process, which is often a link, but not always).
>>
>> Can we get that slogan into Harvard Business Review? Can we get it taught
>> in Managerial Accounting at HBS? (which does address logistics/supply chain
>> queueing).
>>
>>
>>
>>
>>
>>
>>

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