MPI Send Modes

Overview Table

Mode	Blocking	Non-blocking
Standard	MPI_Send	MPI_Isend
Buffered	MPI_Bsend	MPI_Ibsend
Synchronous	MPI_Ssend	MPI_Issend
Ready	MPI_Rsend	MPI_Irsend

Note: “I” = immediate (non-blocking)

Communication Modes

Standard Mode

• Behavior: May buffer (implementation-dependent)
• Returns: Immediately for small messages, blocks for large
• Use: Default choice

Buffered Mode

• Behavior: Always uses user-provided buffer
• Returns: Immediately after buffer copy
• Setup: Needs MPI_Buffer_attach/detach

Synchronous Mode

• Behavior: Waits for receiver handshake
• Returns: Only after receiver starts receiving
• Warning: Causes deadlock in ring patterns

Ready Mode

• Behavior: Assumes receiver is waiting
• Returns: Immediately (no checks)
• Risk: Crashes if receiver not ready

Quick Examples

// Deadlock with synchronous
P0: MPI_Ssend → P1; MPI_Recv ← P1
P1: MPI_Ssend → P0; MPI_Recv ← P0
 
// Safe with non-blocking
MPI_Isend(..., &req1);
MPI_Irecv(..., &req2);
MPI_Waitall(2, requests, statuses);

Key Points

• Use MPI_Send unless you need specific behavior
• Use MPI_Ssend for debugging/testing
• Use non-blocking to prevent deadlocks
• Ready mode is fastest but dangerous