[9fans] Inducing artificial latency

[John Floren <slawmaster@gmail.com>]

I'm going to be doing some work with 9P and high-latency links this
summer and fall. I need to be able to test things over a high-latency
network, but since I may be modifying the kernel, running stuff on
e.g. mordor is not the best option. I have enough systems here to do
the tests, I just need to artificially add latency between them.

I've come up with a basic idea, but before I go diving in I want to
run it by 9fans and get opinions. What I'm thinking is writing a
synthetic file system that will collect writes to /net; to simulate a
high-latency file copy, you would run this synthetic fs, then do "9fs
remote; cp /n/remote/somefile .". If it's a control message, that gets
sent to the file immediately, but if it's a data write, that data
actually gets held in a queue until some amount of time (say 50ms) has
passed, to simulate network lag. After that time is up, the fs writes
to the underlying file, the data goes out, etc.

I have not really done any networking stuff or filesystems work on
Plan 9; thus far, I've confined myself to kernel work. I could be
completely mistaken about how 9P and networking behave, so I'm asking
for opinions and suggestions. For my part, I'm trying to find good
example filesystems to read (I've been looking at gpsfs, for
instance).


John
--
"With MPI, familiarity breeds contempt. Contempt and nausea. Contempt,
nausea, and fear. Contempt, nausea, fear, and .." -- Ron Minnich