# MPI and println inside imported modules
**URL:** https://discourse.julialang.org/t/mpi-and-println-inside-imported-modules/95026
**Category:** General Usage
**Tags:** question, package, optim, mpi, pretty-printing
**Created:** [February 22, 2023, 3:02pm UTC](https://discourse.julialang.org/t/mpi-and-println-inside-imported-modules/95026 "2023-02-22T15:02:45Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@mesonepigreco](https://discourse.julialang.org/u/mesonepigreco)
#### Post date: [February 22, 2023, 3:02pm UTC](https://discourse.julialang.org/t/mpi-and-println-inside-imported-modules/95026/1 "2023-02-22T15:02:45Z")
Dear Juliawers,
I’m using optimizing a function in julia which is parallelized using MPI.
Each time I have to print something, I check if the rank is zero to avoid printing the text as many times as the number of processors.
To optimize I’m using the Optim module.
To print in stdout the progress of the optimization as it goes, I pass to the optimize function the show\_trace and show\_every variables.
However, if I’m running on several processors, the status of the minimization is printed many times. How can I override the print statements in the module imported? Is there any way to do so without modifying the source code of Optim?
Example:
```plaintext
V(r, θ) = a (r - r0)² + b cos(θ - θ0)
Parameters:
r0 = 2.0
theta0 = 0.0
a = 6.0
b = 1.0
n_configs = 10000
n_layers = 1
[ Info: Optimizing the free energy using 4 processor(s)
x0 = [1.0, 0.0, 1.0, 2.0, 0.0, 0.0, 1.0, 0.0, 1.0, 2.0, 0.0]
Iter Function value Gradient norm
Iter Function value Gradient norm
Iter Function value Gradient norm
Iter Function value Gradient norm
0 3.493721e+00 1.949049e+03
0 3.493721e+00 1.949049e+03
0 3.493721e+00 1.949049e+03
0 3.493721e+00 1.949049e+03
* time: 0.011278867721557617
* time: 0.01034092903137207
* time: 0.010904073715209961
* time: 0.01090693473815918
1 3.090287e+00 2.541144e+00
1 3.090287e+00 2.541144e+00
* time: 24.226807117462158
1 3.090287e+00 2.541144e+00
* time: 24.24724793434143
1 3.090287e+00 2.541144e+00
* time: 24.238826990127563
* time: 24.223412036895752
2 2.670622e+00 1.909469e-01
2 2.670622e+00 1.909469e-01
* time: 30.47301197052002
2 2.670622e+00 1.909469e-01
* time: 30.493449926376343
2 2.670622e+00 1.909469e-01
* time: 30.48503279685974
* time: 30.46961498260498
3 2.647692e+00 5.272250e-01
3 2.647692e+00 5.272250e-01
* time: 34.207842111587524
3 2.647692e+00 5.272250e-01
* time: 34.22828006744385
3 2.647692e+00 5.272250e-01
```
As you can see, the optimization trace is a mess.
This specific example regards Optim, however, if there is a general solution that can be implemented for any module loaded, it would be nice!
Do you have a solution?
---
### Author:  [@fredrikekre](https://discourse.julialang.org/u/fredrikekre)
#### Post date: [February 22, 2023, 3:39pm UTC](https://discourse.julialang.org/t/mpi-and-println-inside-imported-modules/95026/2 "2023-02-22T15:39:40Z")
You can [`redirect_stdout`](https://docs.julialang.org/en/v1/base/io-network/#Base.redirect_stdout) for every process to it’s own file. For example:
```julia
using MPI
MPI.Init()
const comm = MPI.COMM_WORLD
const rank = MPI.Comm_rank(comm)
redirect_stdout(open("rank-$(rank).stdout", "a")) do
print("Hello from rank $(rank)\n")
end
```
results in
```plaintext
$ mpiexecjl -n 2 julia main.jl
$ cat rank-0.stdout
Hello from rank 0
$ cat rank-1.stdout
Hello from rank 1
```
If you want to to it for the entire process (instead of locally as in the example above) just put
```julia
redirect_stdout(open("rank-$(rank).stdout", "a"))
```
or similar at the top of the script.
---
### Author:  [@mesonepigreco](https://discourse.julialang.org/u/mesonepigreco)
#### Post date: [February 22, 2023, 3:51pm UTC](https://discourse.julialang.org/t/mpi-and-println-inside-imported-modules/95026/3 "2023-02-22T15:51:26Z")
Thanks a lot!
This goes toward the direction I’m looking for and it could be already the solution.
Is it possible to hack this function to redirect the stdout of the rank 0 to the actual stdout while all other ranks to /dev/null or something similar?
---
### Author:  [@mesonepigreco](https://discourse.julialang.org/u/mesonepigreco)
#### Post date: [February 22, 2023, 3:55pm UTC](https://discourse.julialang.org/t/mpi-and-println-inside-imported-modules/95026/4 "2023-02-22T15:55:31Z")
@fredrikekre
> Thanks a lot!
> This goes toward the direction I’m looking for and it could be already the solution.
> Is it possible to hack this function to redirect the stdout of the rank 0 to the actual stdout while all other ranks to /dev/null or something similar?
Yes, I found the solution
```julia
using MPI
MPI.Init()
comm = MPI.COMM_WORLD
rank = MPI.Comm_rank(comm)
if rank != 0
redirect_stdout(devnull)
end
println("Hello from rank $rank")
```
In this way all the print not from rank zero is redirected to /dev/null while the master can print on standard stdout. Thanks a lot!
---
### Author:  [@fredrikekre](https://discourse.julialang.org/u/fredrikekre)
#### Post date: [February 22, 2023, 3:57pm UTC](https://discourse.julialang.org/t/mpi-and-println-inside-imported-modules/95026/5 "2023-02-22T15:57:48Z")
```julia
redirect_stdout(rank == 0 ? stdout : devnull) do
print("Hello from rank $(rank)\n")
end
```
Edit: Didn’t see your second comment, looks like you already got it.