Non-blocking version of recv (irecv)

[ClarkGuilty]

Hello.

I remember there being a non-blocking version of serialized Recv (irecv). Was this deprecated, if so, why? Is there an already established alternative?

Thank you.

[simonbyrne]

It still exists, but has been deprecated:

MPI.jl/src/deprecated.jl

Lines 239 to 246 in 2e94c5b

	@deprecate(irecv(src::Integer, tag::Integer, comm::Comm), begin
	(flag, stat) = Iprobe(src, tag, comm)
	flag || return (false, nothing, nothing)
	count = Get_count(stat, UInt8)
	buf = Array{UInt8}(undef, count)
	stat = Recv!(buf, Get_source(stat), Get_tag(stat), comm)
	(true, MPI.deserialize(buf), stat)
	end, false)

It was undocumented, and honestly it doesn't really make sense how its defined: it's not really analogous to Irecv since it didn't return a Request object (instead you have to keep calling it). Also in a multithreaded context, there is no guarantee that the Recv will match the same operation as the Iprobe. To be done properly:

• It should use MPI_Improbe and MPI_Mrecv
• It should be implemented via the MPI "Generalized Request" interface.

[simonbyrne]

If you do have need for it, we could add it back, but give it a more reasonable name (iprobe_recv?) A PR would also be welcome

[ClarkGuilty]

Thank you very much! I do need the function and have been trying to implement it (for now, only in the scope of my project but with the hope of eventually making a PR), but I am finding it hard to interact with MPI.API (this is the first time I use C bindings in Julia). I cannot use the deprecated version because the syntax has changed since it was deprecated (I believe) and the signature no longer works. Thus, I have a question regarding MPI.API.Improbe

Reading the Docs, the way to use MPI.API.Improbe is very similar to MPI.API.Iprobe, the only difference being the message pointer, However I keep getting:

MethodError: no method matching unsafe_convert(::MethodError: no method matching unsafe_convert(::Type{Ptr{Ptr{Nothing}}}, ::Bas
e.RefValue{Int32})Type{Ptr{Ptr{Nothing}}}, ::Base.RefValue{Int32})
Closest candidates are:
  unsafe_convert(::Type{Ptr{T}}, !Matched::Union{NNlib.BatchedAdjoint{T, S}, NNlib.BatchedTranspose{T, S}} where S) where T at ~/.julia/packages/NNlib/c0XLe/src/bat
ched/batchedadjtrans.jl:90
  unsafe_convert(::Type{Ptr{T}}, !Matched::Base.Threads.Atomic{T}) where T at atomics.jl:328
  unsafe_convert(::Type{Ptr{T}}, !Matched::LinearAlgebra.Adjoint{<:Real, <:AbstractVecOrMat}) where T at /usr/share/julia/stdlib/v1.8/LinearAlgebra/src/adjtrans.jl:
198
  ...
Stacktrace:
 [1] MPI_Improbe(source::Int64, tag::Int64, comm::MPI.Comm, flag::Base.RefValue{Int32}, message::Base.RefValue{Int32}, status::Base.RefValue{MPI.API.MPI_Status})
   @ MPI.API ~/.julia/packages/MPI/tJjHF/src/api/generated_api.jl:2106

This is my implementation so far

irecv(source::Integer, tag::Integer, comm::MPI.Comm)
    flag = Ref{Cint}()
    mess = Ref{MPI_Message}()
    status = Ref{MPI.Status}()
    MPI.API.MPI_Improbe(source,tag,comm,flag,mess,status)
    ...

How should I initialize the pointers?

[simonbyrne]

It's a bit hard to debug while inside mpiexec since the output gets messed up. However this should be possible to do in a single process, so you can just use the REPL. Firstly, check make sure you're on the master branch, and then can you try the following?:

using MPI, MPIPreferences
@show MPIPreferences.abi
MPI.Init()

source = 0
tag = 0
comm = MPI.COMM_WORLD
flag = Ref(Cint(0))
mess = Ref(MPI.API.MPI_MESSAGE_NULL[])
status = Ref(MPI.STATUS_ZERO)
MPI.API.MPI_Improbe(source,tag,comm,flag,mess,status)
mess[]

[ClarkGuilty]

Thank you. After cheking that I am on master:
MPI.Init() = MPI.ThreadLevel(2)
flag = Ref(Cint(0)) = Base.RefValue{Int32}(0)
mess = Ref(MPI.API.MPI_MESSAGE_NULL[]) = Base.RefValue{Ptr{Nothing}}(Ptr{Nothing} @0x00007f1ea5dfd2e0)
@show MPIPreferences.abi = "OpenMPI"
MPI.API.MPI_Improbe(source,tag,comm,flag,mess,status) returned true.
mess[] = Ptr{Nothing} @0x00007f1ea5dfd2e0

[simonbyrne]

Okay, that looks like it works!