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@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@deepplum.com> wrote:
 
On Monday, July 12, 2021 9:46am, "Livingood, Jason" <Jason_Livingood@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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