# Non-blocking collectives (MPI)
Non-blocking point-to-point communication lets a process post a send or receive and continue computing while the transfer happens in the background (see [[blocking-mpi|blocking and non-blocking]]). **Non-blocking collectives** extend the same idea to collective operations. MPI-3 introduced variants that return immediately with a request handle rather than blocking until the operation completes. `MPI_Ibcast`, `MPI_Ireduce`, `MPI_Iallreduce`, `MPI_Iscatter`, and others follow the same request/wait pattern. The send and receive buffers must not be touched between the call and `MPI_Wait`.

```c
double local = compute_first_part();
double global;
MPI_Request req;
MPI_Iallreduce(&local, &global, 1, MPI_DOUBLE, MPI_SUM, MPI_COMM_WORLD, &req);
do_independent_work();
MPI_Wait(&req, MPI_STATUS_IGNORE);
use(global);
```

Not every implementation overlaps the collective with the intervening computation; some simply defer the work to `MPI_Wait`. On implementations that do use hardware-assisted collectives (common in high-end interconnects like InfiniBand), the overlap can hide significant latency in reduction-heavy algorithms.