Slow FreeBSD -> Windows performance with inflight enabled

[Steven Hartland]

I've just been looking at an issue reported by some
of our users that downloads from our one of our sites
run on FreeBSD 6.1 and Apache 1.3 where strangely
slow.

After doing some digging around I found that two remote
machines on the same network had wildly different results.
The difference being one was Windows (slow) and one was
FreeBSD 6.1 (fast).

The results where 380Kb/s download speeds on Windows vs
500Kb/s on FreeBSD. I played around and Googled to find
that this appears to caused by inflight. Disabling it
and restarting apache cures the problem.

This has been mentioned a few times in the past e.g.
http://lists.freebsd.org/pipermail/freebsd-stable/2006-February/022622.html
http://lists.freebsd.org/pipermail/freebsd-net/2005-November/008989.html

But I cant find any real conclusive results.

For reference the connection between the test machines
is initially GE on both ends but at one point goes via
DSL but is still relatively low latency the trace being:

 Host       Loss%   Snt   Last   Avg  Best  Wrst StDev
 1. x.x.x.x  0.0%    12    0.3   0.3   0.3   0.3   0.0
 2. x.x.x.x  0.0%    12    0.4   0.4   0.4   0.4   0.0
 3. x.x.x.x  8.3%    12    1.4   1.7   1.3   3.1   0.6
 4. x.x.x.x  0.0%    11    5.5  10.1   5.5  21.6   5.9
 5. x.x.x.x  0.0%    11    7.6   6.8   6.3   7.6   0.3
 6. x.x.x.x  0.0%    11    7.7   7.2   6.7   8.1   0.4

Looking at the before and after traces using wireshark
on the windows box there are no notable changes just
an increased throughput in exchanges.

Possibly of note is that the server in question is
running a 200HZ kernel.

With the common client being Windows I'd say it would
be good to get the default improved either by disabling
inflight or changing it so that it better detects this
sort of common connection arrangement.

Does anyone have any ideas why inflight is causing such
poor performance?

    Steve


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