Optimizing noncontiguous accesses in MPI-IO

The I/O access patterns of many parallel applications consist of accesses to a large number of small, noncontiguous pieces of data. If an application's I/O needs are met by making many small, distinct I/O requests, however, the I/O performance degrades drastically. To avoid this problem, MPI-IO allows users to access noncontiguous data with a single I/O function call, unlike in Unix I/O. In this paper, we explain how critical this feature of MPI-IO is for high performance and how it enables implementations to perform optimizations. We first provide a classification of the different ways of expressing an application's I/O needs in MPI-IO--we classify them into four levels, called level 0 through level 3. We demonstrate that, for applications with noncontiguous access patterns, the I/O performance improves dramatically if users write their applications to make level-3 requests (noncontiguous, collective) rather than level-0 requests (Unix style). We then describe how our MPI-I...

• Type: Journal
• Year: 2002
• Where: PC
• Authors: Rajeev Thakur, William Gropp, Ewing L. Lusk
• Keywords: Access Patterns, Management, Noncontiguous Requests, PC 2002, Performance