How to benchmark communication cost in the beowulf (parallel system)

Quite long time ago, I've proposed one simple way of estimating
communication cost of an MPI program (or multiple-instruction)
appeared in the beowulf mailing list. Here is the outline of
the procedure.
[original post]

Definition Communication cost (Comm) is the ratio between the real
time used by all processors (processes actually if you use MPI)
and global real cpu time (GRU).

 Comm=GRU/GR = (GR-GU-GS)/GR 

Undefined symbols are as follows:

Comm= Communication cost
GR=global real time
LR=local worked real time 
LU=local user
LS=local system
GS=global system
GU=global user
n=# of procs
GU=n*LU
GS=n*LS
GR=n*LR 

In the actual MPI program this measure's implementation could be outlined
as follows

 BEGIN PROGRAM Myprogram
      begin distribution
       initial_timings=timings;
                ...do whatever is your distributed computing....
       end_timings=timings;
      end distribution
       if(myrank == 0) {compute communication cost}        
END Myprogram

BEGIN PROC timings
    for(n=0;n < numproc;numproc) {
       if(n==myrank) { measure timing of myrank and send to rank 0 }
     }
END PROC