immediate_receive and count?

[sbromberger]

Hi all,

First off, thanks for maintaining this package!

I'm trying to understand why immediate_receive() is hardcoded with a count of 1 - I think this makes it impossible to receive a vector of, say, u8.

a) is there a reason for the restriction?
b) is there another way to do an MPI_IRecv with a count?

If the answer to a) is "There's no particular reason", then I might just try my hand at a PR (I'll probably need some help).

Cheers!

[jtronge]

That seems like it could be a good idea. Something like immediate_receive(count)?

There's also the immediate_receive_into_*() set of functions, which would allow you to do an MPI_IRecv with a count. These take a mutable reference to some buffer of a fixed size and then return a request. To ensure that this buffer isn't dropped while MPI is still using it, you have to wrap your code in a closure that takes a Scope object and ensure that you allocate any buffers outside the closure. Before exiting the closure, all requests must be waited on, or else it will panic.

examples/immediate.rs is a good example that demonstrates this.

For the current version of immediate_receive(), I think you might be able to work around this if you derived Equivalence on a struct containing a buffer. Something like this:

#[derive(Equivalence)]
struct Wrapper {
    buf: [u8; 128],
}

Then you should be able to do an immediate_receive::<Wrapper>() to pass around a buffer. The count will then be fixed at compile time, however.