Allow setting thread level for tests

[giordano]

MVAPICH on Ookami doesn't support threading, but we default MPI.Init to threadlevel=:serialized

MPI.jl/src/environment.jl

Line 81 in a179cf8

function Init(;threadlevel=:serialized, finalize_atexit=true, errors_return=true)

and this creates a lot of noise when running the tests because each test would print a series of

┌ Warning: MPI thread level requested = MPI.ThreadLevel(2), provided = MPI.ThreadLevel(0)                                                                                                                         
└ @ MPI ~/.julia/dev/MPI/src/environment.jl:96                                                                                                                                                                    
┌ Warning: MPI thread level requested = MPI.ThreadLevel(2), provided = MPI.ThreadLevel(0)                                                                                                                         
└ @ MPI ~/.julia/dev/MPI/src/environment.jl:96                                                                                                                                                                    
┌ Warning: MPI thread level requested = MPI.ThreadLevel(2), provided = MPI.ThreadLevel(0)                                                                                                                         
└ @ MPI ~/.julia/dev/MPI/src/environment.jl:96                                                                                                                                                                    
┌ Warning: MPI thread level requested = MPI.ThreadLevel(2), provided = MPI.ThreadLevel(0)                                                                                                                         
└ @ MPI ~/.julia/dev/MPI/src/environment.jl:96                                                                                                                                                                    

The solution should be as easy as setting the default value of threadlevel in tests/common.jl and always initialise with MPI.Init(; threadlevel) in all tests, the only question is how to let the user choose the threadlevel for the tests. Another environment variable?

Alternatively, have a smarter threadlevel default value, but can we query whether the MPI library supports threading before Init?

[simonbyrne]

I think as part of #627 we should have an environment variable that allows you to set the default threadlevel (JULIA_MPI_THREADLEVEL?)

[sloede]

Alternatively, have a smarter threadlevel default value, but can we query whether the MPI library supports threading before Init?

As far as I know, no. MPI_Query_thread only returns the provided threadlevel after initialization, thus it does not help here. Further, threading support is both implementation- and system-dependent, thus it is not possible to store it as a constant either.