From owner-freebsd-net@FreeBSD.ORG  Fri Feb 27 09:29:24 2009
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From: =?ISO-8859-1?Q?Michael_T=FCxen?= <Michael.Tuexen@lurchi.franken.de>
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Date: Fri, 27 Feb 2009 10:29:20 +0100
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Cc: Robin Seggelmann <seggelmann@fh-muenster.de>
Subject: TCP performance  question
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Dear all,

we have used a client to do
while (1) {
	send(fd, buffer, n);
}

against a discard server on a different machine. The
sender is running FreeBSD 7.1, for the receiver we
used two different machines, one running FreeBSD 7.1
another one Mac OS X 10.5.6. The machines are old,
so I do not care about the absolute numbers.

We measured the throughput for 60 seconds and different
values of n. Here is what we got:
  n    throughput
500, 77716.505790
501, 77490.917999
502, 77412.961710
503, 77333.560297
504, 77801.337017
505, 77427.814392
506, 77422.640826
507, 77360.185565
508, 77607.863180
509, 77445.288161
510, 77531.083699
511, 77486.518785
512, 77908.246985
513, 68175.140109
514, 68338.332801
515, 68359.883062
516, 68640.669589
517, 68314.060835
518, 68473.051618
519, 68507.277602
520, 68930.431180
521, 68590.979508
522, 68790.924360
523, 68848.960564
524, 69091.676020
525, 68720.795262
526, 68897.330715
527, 69070.935237
528, 69421.723656
529, 68908.417372
530, 69000.914392

The question I have is he following:
Why is the throughput smaller for n > 512
than for n <= 512?

Since the result is about the same for different
receivers, this seems to be related to the sender side.

Best regards
Michael