On Mon, Oct 3, 2016 at 6:54 AM Chris McGee <newton688@gmail.com> wrote:

Lately, I’m looking at the “iounit.” Initially, I had thought of it as a way to give the size of a file on Topen. After some testing I realized that the network subsystem in plan9 is expecting the iounit to be large enough to write commands into a /net/tcp ctl file. With a small iounit, the network subsystem truncates the commands, such as “connect xyz|1234” to match the size of the iounit.

So, what is this mysterious iounit? Is there another man page or plan9 document that describes it in more detail?

Given that there are comments like this:

/* ample room for Twrite/Rread header (iounit) */

paired with an old message from Mr. Quanstrom below, I suspect the

value is "as large as your network / disk can manage, but at least as large as the protocol overhead".  But that's just a WAG.

---------- Forwarded message ---------

From: erik quanstrom <quanstro@quanstro.net>

Date: Sat, Nov 16, 2013 at 6:45 PM

Subject: [9fans] simple gofast trick

To: <9fans@9fans.net>

this is a one-liner.  here's a little demo:

        chula; sed 's:8192:64*1024:g' </sys/src/cmd/cat.c > fatcat.c && tmk fatcat.c

        6c -FVTw fatcat.c

        6l -o 6.fatcat fatcat.6

        chula; cat /bin/gs>/dev/null;for(i in cat 6.fatcat)time $i /bin/gs > /dev/null

        0.00u 0.01s 0.41r        cat /bin/gs

        0.00u 0.00s 0.23r        6.fatcat /bin/gs

the trick is only to increase IOUNIT to 48k in both file server and cpu

server.

this works since the mtu of the network is 1500 bytes and the Rread messages

are up to 48k + 9p overhead, the network layer has multiple outstanding

messages and overlapping acks at the network layer much of the time, even

though the mount driver continues keeping a single outstanding message

per system call.

- erik