MPI Programming and Subroutine Reference

Appendix C. MPI Size Limits

MPI Tunables and Limits

The following is a list of MPI size limits. This list includes system limits on the size of various MPI elements and the relevant environment variable or tunable parameter.

• Number of tasks: MP_PROCS

• Maximum number of tasks: 1024 (2048 for IP library)

• Maximum message size for Point-to-Point communication: No specific limit

• Default receive buffer size: (MP_BUFFER_MEM)

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

  When using User Space (US): 64MB

• Maximum receive buffer size: 64MB

• Default eager limit: See Table 15

• Maximum eager limit: 64K bytes

• To ensure that at least 32 messages can be outstanding between any two tasks, MP_EAGER_LIMIT is adjusted according to Table 15 (and: when MP_USE_FLOW_CONTROL=YES and MP_EAGER_LIMIT and MP_BUFFER_MEM have not been set by the user):
  

  Table 15. MPI Eager Limits
  

  Number of Tasks	MP_EAGER_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 given by the 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: approximately 2000

• Maximum number of 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