# How do I stop Julia optimising out a variable? (or how to use \`MPI.Ibarrier()\`?)
**URL:** https://discourse.julialang.org/t/how-do-i-stop-julia-optimising-out-a-variable-or-how-to-use-mpi-ibarrier/121579
**Category:** General Usage
**Tags:** compilation, mpi
**Created:** [October 22, 2024, 9:41am UTC](https://discourse.julialang.org/t/how-do-i-stop-julia-optimising-out-a-variable-or-how-to-use-mpi-ibarrier/121579 "2024-10-22T09:41:01Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@johnomotani](https://discourse.julialang.org/u/johnomotani)
#### Post date: [October 22, 2024, 9:41am UTC](https://discourse.julialang.org/t/how-do-i-stop-julia-optimising-out-a-variable-or-how-to-use-mpi-ibarrier/121579/1 "2024-10-22T09:41:01Z")
I want to use `MPI.Ibarrier()` to synchronize some processes where one needs to complete some work before the others can start, so that the first process does not need to wait at the barrier. Pseudo-code:
```julia
using MPI
comm = MPI.COMM_WORLD
comm_rank = MPI.Comm_rank(comm)
for i in 1:100
if comm_rank == 0
... do some work ...
dummy_req = MPI.Ibarrier(comm)
else
req = MPI.Ibarrier(comm)
MPI.Wait(req)
... do some other work ...
end
end
```
I get a weird error like
```julia
error in running finalizer: MPI.MPIError(code=7)
MPI_Request_free at /home/.../.julia/packages/MPI/TKXAj/src/api/generated_api.jl:2331 [inlined]
free at /.../.julia/packages/MPI/TKXAj/src/nonblocking.jl:161
...
```
which I think is due to the finalizer instantly running on the ‘Request’ object returned from `MPI.Ibarrier()` on rank-0. The error goes away if I print `dummy_req`! So I think MPI needs the ‘Request’ to exist for a little while. I think the compiler is removing `dummy_req` instantly because it’s never used, and causing the problem (the same as if I didn’t even store the return value from `MPI.Ibarrier()` in a variable). I think I only need it to persist until the end of the `if`-statement, or one iteration of the loop, but don’t know how to do that without actually doing something with `dummy_req`.
---
### Author:  [@jipolanco](https://discourse.julialang.org/u/jipolanco)
#### Post date: [October 22, 2024, 10:45am UTC](https://discourse.julialang.org/t/how-do-i-stop-julia-optimising-out-a-variable-or-how-to-use-mpi-ibarrier/121579/2 "2024-10-22T10:45:55Z")
You can use `GC.@preserve dummy_req` to make sure that `dummy_req` is not finalised within a block of code.
Your particular case is a bit tricky because you want all the 100 requests to be preserved. One solution would be to use an [`MPI.MultiRequest`](https://juliaparallel.org/MPI.jl/v0.20/reference/pointtopoint/#MPI.MultiRequest) to store all of them, which could look like:
```julia
using MPI
MPI.Init()
comm = MPI.COMM_WORLD
comm_rank = MPI.Comm_rank(comm)
dummy_reqs = MPI.MultiRequest(100)
GC.@preserve dummy_reqs begin
for i in 1:100
if comm_rank == 0
# ... do some work ...
MPI.Ibarrier(comm, dummy_reqs[i])
else
req = MPI.Ibarrier(comm)
MPI.Wait(req)
# ... do some other work ...
end
end
MPI.Barrier(comm) # make sure dummy_reqs hasn't been finalised up to this point
end
```
---
### Author:  [@johnomotani](https://discourse.julialang.org/u/johnomotani)
#### Post date: [October 22, 2024, 1:37pm UTC](https://discourse.julialang.org/t/how-do-i-stop-julia-optimising-out-a-variable-or-how-to-use-mpi-ibarrier/121579/3 "2024-10-22T13:37:48Z")
Thanks @jipolanco, the MPI.MultiRequest was what I needed! For anyone else who comes across a similar problem - if you want to re-use the MultiRequest, you need to free it first, e.g. something like
```julia
request_store = MPI.MultiRequest(8)
for i in 1:100
for j in 1:8
... do some stuff ...
MPI.Ibarrier(comm, request_store[j])
end
MPI.Barrier(comm)
MPI.Free(request_store)
end
```