MPI Programming Guide

Chapter 10. MPI size limits

The following list includes system limits on the

size of various MPI elements and the relevant environment variable or tunable parameter. The MPI standard identifies several values that have limits in any MPI implementation. For these values, the standard indicates a named constant to express the limit. See mpi.h for these constants and their values.

• Number of tasks: MP_PROCS
• Maximum number of tasks: 4096 (2048 for IP library)
• Maximum buffer size for any MPI communication (for 32-bit applications only): 2GB
• Default early arrival buffer size: (MP_BUFFER_MEM)

  When using Internet Protocol (IP): 2,800,000 bytes

  When using user space (US): 64MB

• Maximum early arrival buffer size: 64MB
• Default eager limit: See Table 6
• Maximum eager limit: 64KB
• To ensure that at least 32 messages can be outstanding between any two tasks, MP_EAGER_LIMIT is adjusted according to Table 6, unless the user has specified MP_EAGER_LIMIT and MP_BUFFER_MEM and has set MP_USE_FLOW_CONTROL=no.
  

  Table 6. MPI eager limits
  

  Number of tasks	Default limit
  1 to 16	4096
  17 to 32	2048
  33 to 64	1024
  65 to 128	512
  129 to 256	256
  257 to the maximum number of tasks supported by the implementation	128

• The maximum number of outstanding unmatched send requests (smaller than MP_EAGER_LIMIT) per node for any single destination node is at least 1, and is specified by this formula:

  (0.75*MP_BUFFER_MEM)/(MP_PROCS*(max(MP_EAGER_LIMIT,64)))
  

• Maximum aggregate unsent data, per task: no specific limit
• Maximum number of communicators, file handles, and windows: approximately 2000
• Maximum number of committed data types: depends on MP_BUFFER_MEM
• Maximum data type depth: default is 5 (MP_MAX_TYPEDEPTH)
• Maximum number of distinct tags: all non-negative integers less than 2**32-1