Re: [ns] ns scalability

[Aaron D Striegel <adstrieg@iastate.edu>]

Has there been any work on speeding up ns for multicast?  It seems to be exceptionally slow for multicast compared to unicasting which is probably due to how packets are copied at each node for multicasting.  If anyone has done any work on this, please let me know as I am extremely interested.  I've had exactly the opposite problem, I am running out of CPU speed far faster than I am running out of memory.

Aaron

You're correct that ns is packet-level, and your analysis is along the
right lines.

(Some cavetats: ns also includes optional abstraction approaches that
can speed things up, and we're working on an analytic-based
pre-filtering tool that is much faster but doesn't guarantee
accuracy.)

BUT I'd be careful projecting extrapolating from a short
(run-time and virtual-time) scenario, since you may be multipling
startup overhead that should be fixed.

I know that we've run some very long (virtual time) simulations in a
reasonable amount of real time.  I've asked several people for
specific data points.

The other thing to be VERY careful about in large simulations is
memory.  In every case I'm aware of with large simulations, we've run
out of memory before we've run out of CPU speed.  (And I've heard
similar things from people using other simulators, including
parallized ones.)

(Your best bet though is to try your experiment and see [and let us
know!].  It should be reasonable to do a simple version of your
scenario.)

   -John Heidemann