[Beowulf] PVM on wireless...

Jim Lux James.P.Lux at jpl.nasa.gov
Wed Feb 6 14:26:35 EST 2008


At 07:40 AM 2/6/2008, Robert G. Brown wrote:
>Anybody on list have any idea why PVM fails to add hosts over a wireless
>link?  I've now tried this over multiple distro version and at least one
>PVM update, and it just doesn't work.  Works fine over a wire, fails on
>wireless, and as far as I know wire and wireless are both "identical"
>at the kernel interface layer so that any e.g. socket one might open is
>absolutely ecumenical about what the underlying hardware is (good old
>ISO/OSI layering, right?).
>
>And yes, I'm well aware that from a latency/bw point of view this
>arrangement isn't going to be a speed demon or scale terribly well, but
>for testing PVM from a laptop or writing code from a laptop or just
>playing with PVM itself for fun or profit from a laptop it would
>certainly be lovely if it WORKED, however poorly as far as IPCs are
>concerned.


You brave man.. trying to do what is trivial in a wired network with 
wireless stuff.

I would look for timing assumptions that aren't met in the wireless 
environment. There's a channel capacity issue, of course, but there's 
also some constraints on round trip messages, particularly if you've 
got a "infrastructure" network as opposed to "ad-hoc".  A packet from 
A to B has to go from A to Access Point( AP), which takes some back 
and forth handshaking and protocol overhead.  Then, it gets sent from 
AP to B, with more back and forth.  Don't expect 1 ms ping times...

I spent quite a while getting NTP (which I thought would be trivial.. 
it explicitly handles long delays and intermittent connections) to 
work in a 802.11a network, complicated by the fact that I was using 
Access Points (in a "point to multipoint" configuration) as the 
interfaces, so the computers actually had a wired ethernet connection 
through a dumb 5 port switch, to the wireless AP. Getting PXE and 
DHCP to work was trivial by comparison

Lots of weird things happen in these systems because there are hidden 
assumptions about timing and whether a path exists between two points.

Jim


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