Re: [9fans] Inducing artificial latency

[John Floren <slawmaster@gmail.com>]

On Tue, Jun 15, 2010 at 5:45 PM, Devon H. O'Dell <devon.odell@gmail.com> wrote:
> 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.
>

My plan was to have each side running the fs. I had thought about
sticking a sleep in the kernel's ip code right before it writes to the
device, but since I planned on netbooting the test systems (so much
easier to bring in a new kernel), I thought it might be advantageous
to only slow down for one process. I suppose I could make it sleep
conditionally, like make the process write to a ctl file if it wants
latency, then sleep only for that pid and its children.


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