netdev - Little's law byte wait time and iperf 2

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Message-ID: <CAEBrVk5BNZmJWAK0q+eX0zxEq+VJtf0C7pK-xvCMGTo6Ct6jMQ@mail.gmail.com>
Date: Fri, 28 Jun 2024 10:33:59 -0700
From: Robert McMahon <rjmcmahon@...cmahon.com>
To: netdev@...r.kernel.org
Subject: Little's law byte wait time and iperf 2

Hi All,

I have two questions with respect to adding support for TCP byte wait
times based on Little's law into iperf 2.

1) Is this avg byte wait time field useful
2) Should the queue depth be based on the sampled CWND or the bytes inflight

Thanks in advance for comments on this (including a better column header name)

Example output:

rjmcmahon@...ora:~/Code/pyflows/iperf2-code$ src/iperf -c 192.168.1.77
-i 1 -e --fq-rate 100m -w 4M --tcp-write-prefetch 256K
------------------------------------------------------------
Client connecting to 192.168.1.77, TCP port 5001 with pid 60030 (1/0 flows/load)
Write buffer size: 131072 Byte
fair-queue socket pacing set to  100 Mbit/s
TCP congestion control using cubic
TOS defaults to 0x0 (dscp=0,ecn=0) (Nagle on)
TCP window size: 8.00 MByte (WARNING: requested 4.00 MByte)
Event based writes (pending queue watermark at 262144 bytes)
------------------------------------------------------------
[  1] local 192.168.1.103%enp4s0 port 55468 connected with
192.168.1.77 port 5001 (prefetch=262144) (icwnd/mss/irtt=14/1448/627)
(ct=0.69 ms) on 2024-06-28 10:21:14.724 (PDT)
[ ID] Interval        Transfer    Bandwidth      Wait        Write/Err
 Rtry     InF(pkts)/Cwnd(pkts)/RTT(var)  fq-rate  NetPwr
[  1] 0.00-1.00 sec  12.3 MBytes   103 Mbits/sec  15.147 ms    99/0
   0      190K(135)/207K(147)/403(25) us  100 Mbit/sec 31874
[  1] 1.00-2.00 sec  11.9 MBytes  99.6 Mbits/sec  10.444 ms    95/0
   0      127K(90)/207K(147)/384(21) us  100 Mbit/sec 32427
[  1] 2.00-3.00 sec  12.0 MBytes   101 Mbits/sec  15.462 ms    96/0
   0      190K(135)/207K(147)/366(17) us  100 Mbit/sec 34380
[  1] 3.00-4.00 sec  11.9 MBytes  99.6 Mbits/sec  10.444 ms    95/0
   0      127K(90)/207K(147)/385(13) us  100 Mbit/sec 32342
[  1] 4.00-5.00 sec  12.0 MBytes   101 Mbits/sec  15.462 ms    96/0
   0      190K(135)/207K(147)/378(26) us  100 Mbit/sec 33288
[  1] 5.00-6.00 sec  11.9 MBytes  99.6 Mbits/sec  10.444 ms    95/0
   0      127K(90)/207K(147)/394(30) us  100 Mbit/sec 31604
[  1] 6.00-7.00 sec  11.9 MBytes  99.6 Mbits/sec  10.444 ms    95/0
   0      127K(90)/207K(147)/385(19) us  100 Mbit/sec 32342
[  1] 7.00-8.00 sec  12.0 MBytes   101 Mbits/sec  15.462 ms    96/0
   0      190K(135)/207K(147)/384(19) us  100 Mbit/sec 32768
[  1] 8.00-9.00 sec  11.9 MBytes  99.6 Mbits/sec  10.444 ms    95/0
   0      127K(90)/207K(147)/349(17) us  100 Mbit/sec 35679
[  1] 9.00-10.00 sec  12.0 MBytes   101 Mbits/sec  15.462 ms    96/0
    0      190K(135)/207K(20) us 32113

Bob

https://en.wikipedia.org/wiki/Little%27s_law