Re: [9fans] Inducing artificial latency

["Devon H. O'Dell" <devon.odell@gmail.com>]

2010/6/15 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).

This seems reasonable, but remember that you're only going to be able
to simulate latency in one direction as far as the machine is
concerned. So you'll want to have the same configuration on both
machines, otherwise things will probably look weird. I'd recommend
doing this at a lower level, perhaps introducing a short sleep in IP
output so that the latency is consistent across all requests. And of
course both machines would need to have this configuration. You could
have the stack export another file that you can tune if you want to
maintain a file-system approach. But I think a synthetic FS for this
is overkill.

If you're on a local network, this should do the trick, otherwise you
may need to do some adaptive latency.

--dho

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