# Improving MPI Collective I/O Performance With Intra-node Request Aggregation

## TL;DR

This paper introduces an intra-node request aggregation method for MPI-IO collective operations, significantly reducing communication costs and improving performance on large-scale parallel systems.

## Contribution

It proposes a new intra-node request aggregation layer to enhance MPI-IO collective I/O performance by reducing inter-node communication overhead.

## Key findings

- Up to 29x performance improvement on large-scale systems
- Effective reduction of inter-node communication congestion
- Validated with real application and benchmark workloads

## Abstract

Two-phase I/O is a well-known strategy for implementing collective MPI-IO functions. It redistributes I/O requests among the calling processes into a form that minimizes the file access costs. As modern parallel computers continue to grow into the exascale era, the communication cost of such request redistribution can quickly overwhelm collective I/O performance. This effect has been observed from parallel jobs that run on multiple compute nodes with a high count of MPI processes on each node. To reduce the communication cost, we present a new design for collective I/O by adding an extra communication layer that performs request aggregation among processes within the same compute nodes. This approach can significantly reduce inter-node communication congestion when redistributing the I/O requests. We evaluate the performance and compare with the original two-phase I/O on a Cray XC40 parallel computer with Intel KNL processors. Using I/O patterns from two large-scale production applications and an I/O benchmark, we show the performance improvement of up to 29 times when running 16384 MPI processes on 256 compute nodes.