MPI Programming and Subroutine Reference

Appendix J. MPI Environment Variables Quick Reference

Table 18 summarizes the environment variables and flags for the Message Passing Interface. These environment variables and flags allow you to change message and memory sizes, as well as other message passing information.

Table 18. POE Environment Variables and Command-Line Flags for MPI

Environment Variable Command-Line Flag Set: Possible Values: Default:
MP_BUFFER_MEM
-buffer_mem
To change the maximum size of memory used by the communication subsystem to buffer early arrivals. An integer less than or equal to 64MB

nnnn nnnnK nnM

2800000 bytes (IP) 64MB (US)

MP_CLOCK_SOURCE
-clock_source
To use the SP switch clock as a time source. See "Using the SP Switch Clock as a Time Source".
AIX SWITCH

MP_CSS_INTERRUPT
-css_interrupt
Whether or not arriving packets generate interrupts. This may provide better performance for certain applications. Setting this explicitly will suppress the MPI-directed switching of interrupt mode, leaving the user in control for the rest of the run. See MPI_FILE_OPEN.
yes no

no
MP_EAGER_LIMIT
-eager_limit
To change the threshold value for message size, above which rendezvous protocol is used.
To ensure that at least 32 messages can be outstanding between any 2 tasks, MP_EAGER_LIMIT will be adjusted based on the number of tasks according to the following table (and: when MP_USE_FLOW_CONTROL=YES and MP_EAGER_LIMIT and MP_BUFFER_MEM have not been set by the user):
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 128 number of tasks supported by the implementation
An integer less than or equal to 65536
nnnK
4KB
MP_INTRDELAY
-intrdelay
To tune the delay parameter without having to recompile existing applications. An integer greater than 0
35 &usec. (TB2) 1 &usec. (TB3)

MP_MAX_TYPEDEPTH
-max_typedepth
To change the maximum depth of message derived data types. An integer greater than or equal to 1 5
MP_SINGLE_THREAD
-single_thread
To avoid lock overheads in a program that is known to be single-threaded. Note that MPI-IO cannot be used if this variable is set to yes. Results are undefined if this variable is set to yes with multiple message threads in use.
no yes

no
MP_THREAD_STACKSIZE
-thread_stacksize
To specify the additional stacksize allocated for user programs executing on an MPI service thread. If you allocate insufficient space, the program may encounter a SIGSEGV exception.
nnnnn nnnK nnM
(where K=1024 bytes and M=1024*1024 bytes)
None
MP_TIMEOUT
(no associated command-line flag)
To change the length of time the communication subsystem will wait for a connection to be established during message passing initialization. An integer greater than 0 150 seconds
MP_USE_FLOW_CONTROL
-use_flow_control
To limit the maximum number of outstanding messages posted by a sender.
yes no

no
MP_WAIT_MODE
-wait_mode
To specify how a thread or task behaves when it discovers it is blocked, waiting for a message to arrive.
poll yield sleep

poll (US) yield (IP)

Environment Variable Command-Line Flag	Set:	Possible Values:	Default:
MP_BUFFER_MEM -buffer_mem	To change the maximum size of memory used by the communication subsystem to buffer early arrivals.	An integer less than or equal to 64MB `nnnn` `nnnn`K `nn`M	2800000 bytes (IP) 64MB (US)
MP_CLOCK_SOURCE -clock_source	To use the SP switch clock as a time source. See "Using the SP Switch Clock as a Time Source".	AIX SWITCH
MP_CSS_INTERRUPT -css_interrupt	Whether or not arriving packets generate interrupts. This may provide better performance for certain applications. Setting this explicitly will suppress the MPI-directed switching of interrupt mode, leaving the user in control for the rest of the run. See MPI_FILE_OPEN.	yes no	no
MP_EAGER_LIMIT -eager_limit	To change the threshold value for message size, above which rendezvous protocol is used. To ensure that at least 32 messages can be outstanding between any 2 tasks, MP_EAGER_LIMIT will be adjusted based on the number of tasks according to the following table (and: when MP_USE_FLOW_CONTROL=YES and MP_EAGER_LIMIT and MP_BUFFER_MEM have not been set by the user): 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 128 number of tasks supported by the implementation	An integer less than or equal to 65536 `nnn`K	4KB
MP_INTRDELAY -intrdelay	To tune the delay parameter without having to recompile existing applications.	An integer greater than 0	35 &usec. (TB2) 1 &usec. (TB3)
MP_MAX_TYPEDEPTH -max_typedepth	To change the maximum depth of message derived data types.	An integer greater than or equal to 1	5
MP_SINGLE_THREAD -single_thread	To avoid lock overheads in a program that is known to be single-threaded. Note that MPI-IO cannot be used if this variable is set to yes. Results are undefined if this variable is set to yes with multiple message threads in use.	no yes	no
MP_THREAD_STACKSIZE -thread_stacksize	To specify the additional stacksize allocated for user programs executing on an MPI service thread. If you allocate insufficient space, the program may encounter a SIGSEGV exception.	`nnnnn` `nnn`K `nn`M (where K=1024 bytes and M=1024*1024 bytes)	None
MP_TIMEOUT (no associated command-line flag)	To change the length of time the communication subsystem will wait for a connection to be established during message passing initialization.	An integer greater than 0	150 seconds
MP_USE_FLOW_CONTROL -use_flow_control	To limit the maximum number of outstanding messages posted by a sender.	yes no	no
MP_WAIT_MODE -wait_mode	To specify how a thread or task behaves when it discovers it is blocked, waiting for a message to arrive.	poll yield sleep	poll (US) yield (IP)

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