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#### A Quick Example (NOT Hellow World!)
This is a standard example coming with <code>Rmpi</code> to approximate
$\pi \approx 3.1415926$
---
#### SPMD Basic Steps:
- Step:
0. Initialize. (<code>init</code>)
1. Read data partially by workers.
2. Compute.
(<code>send</code>, <code>recv</code>, <code>barrier</code>, ...)
3. Receive results among workers.
(<code>gather</code>, <code>allgather</code>, <code>reduce</code>,
<code>allreduce</code>, ...)
4. Finalize (<code>finalize</code>).
---
#### Parallel (SPMD) code:
See <a href="./cookbook/ex_pi_spmd.r" target="_blank">ex_pi_spmd.r</a>
for ultra-large/unlimited $N$
```
# File name: ex_pi_spmd.r
# Run: mpiexec -np 2 Rscript --vanilla ex_pi_spmd.r
### Load pbdMPI and initial the communicator.
library(pbdMPI, quiet = TRUE)
init()
### Compute pi.
n <- 1000
totalcpu <- .comm.size
id <- .comm.rank + 1
mypi <- 4*sum(1/(1+((seq(id,n,totalcpu)-.5)/n)^2))/n # The example from Rmpi.
mypi <- reduce(mypi, op = "sum")
### Output from RANK 0 since mpi.reduce(...) will dump only to 0 by default.
comm.print(mypi)
finalize()
```
---
#### Run SPMD code in a command mode: (Batch Job)
Note that the possible commands to invoke MPI could be <code>mpiexec</code>,
<code>mpiexec</code>, <code>orterun</code>, or <code>mpiexec.exe</code>
which are totally dependent on the operating systems and the MPI systems.
Also, see <a href="./rscript.html">Rscript</a> for the usages.
```
SHELL> mpiexec -np 2 Rscript --vanilla ex_pi_spmd.r
```
---
#### Run SPMD code in an interactive mode: (Master/Worker)
For OpenMPI, you need a file <font
color="red"><b><code>.Rprofile</code></b></font> in the working directory
or your home directory. This file can be copied from the installed
directory of <code>Rmpi</code>. By default, it is located at <font
color="red"><b><code>$R_HOME/library/Rmpi/Rprofile</code></b></font>.
Then, you can have an interactive mode
by initial <code>mpirun</code> and <code>R</code>
(NOT <code>Rscript</code>) at which <code>.Rprofile</code> is located.
```
SHELL> mpirun -np 2 R --no-save -q
###
### Some messages will show the workers are running.
### The "spawn" is no needed for OpenMPI anymore.
###
R> # library(Rmpi)
R> # mpi.spawn.Rslaves() # Require for LAM/MPI.
R> mpi.bcast.cmd(source("ex_pi_spmd.r")) # Worker go first and wait Manager.
R> source("ex_pi_spmd.r") # Manager runs and collects results.
### Remark mpi.quit() in "ex_pi_spmd.r" to avoid terminate mpirun and R.
```
---
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