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Re: Why is my gigabit ethernet so slow?

Thor Lancelot Simon <> writes:

> Why the automatic socket buffer sizing code isn't bringing the socket
> buffer size and thus the window size up to 128K by itself over the course
> of a 10GB test is not obvious to me.  Either it's buggy, or it isn't
> causing the window scaling option to be used (in other words, it's buggy),
> I think has to be the answer.

I have been checking TCP throughput on 15+ Mb/s links with ~35ms delay
and it was immediately apparent that the default window needed to be
bigger to max out the link.  I use tcpdump to record the traffic and
then tcpdump2xplot (in xplot, which is in pkgsrc) to visualize the TCP
connection.  It's obvious from the sender-side trace that the sender
becomes limited by its notion of the receiver's available window.

Then, I turned on automatic buffer tuning, and it seemed that it took a
long time for the receiver buffer to open up.  Looking at receiver-side
plots, I found it very unobvious how this ought to work - at the
receiver, the window is almost always close to empty.  I didn't get to
looking at the code, but I would expect that it would have to measure
receive data rate and RTT and try to adapt to that - the key point is
that the receiver has to open up the window so the transmitter isn't
constrained, which I think means more than data_in_flight * RTT.

It would be nice if there were an explanation of how this is supposed to
work in the man page, or a pointer to a paper.

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