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Q: What can MP_EAGER_LIMIT do for me?

This parallel environment variable allows the user to select certain tuning values which might influence the performance of an application that has a significant amount of communication time. Some experimentation with these options might be necessary.

The default values for MP_EAGER_LIMIT, given below, are rather conservative for HPCx.

# of Tasks   MP_EAGER_LIMIT
==========   ==============
  1 - 16        4096
 17 - 32        2048
 33 - 64        1024
 65 - 128        512
129 - 256        256
     >256        128

We recommend putting this to the largest possible value, 65536, for codes which perform significant amounts of message passing, explicitly,

export MP_EAGER_LIMIT=65536

NB, if your code employs many processors, then MP_EAGER_LIMIT will be automatically capped by HPCx.

Debugging: If you set MP_EAGER_LIMIT to zero, then this will test the validity of your MPI calls, i.e. non-conforming MPI codes which ran on other machines, and may run on HPCx, may hang/crash when MP_EAGER_LIMIT is set to zero.

Further information

The communication subsystem of the IBM SP uses two methods for data transfer between tasks of a parallel application. Short messages (less than n bytes) are immediately sent to the receiver while long messages require a rendevouz protocol which increases the overhead for message transfer.

To ensure that at least 32 short messages can be outstanding between any two tasks, MP_EAGER_LIMIT will be adjusted based on the number of tasks according to the table given above, when the user has specified neither MP_BUFFER_MEM, nor MP_EAGER_LIMIT, nor MP_USE_FLOW_CONTROL.

If the program sends many short messages which are longer than the default value of MP_EAGER_LIMIT this value may be increased.

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