Task Placement

Default Placement

By default, task placement is performed according to the group round-robin policy: each compute node gets as many MPI tasks as specified as number of tasks per node, one compute node after the other.

The actual task placement can be checked through reading the rankfile of the job. The absolute path of this rankfile is provided by the LSB_DJOB_RANKFILE environment variable.

User-Defined Placement

An alternate task placement can be requested by using the following environment variable: LSB_TASK_GEOMETRY. This environment variable allows the user to specify the group of MPI tasks that will coexist on the same compute node.

Example : Specifying an Alternate Task Placement through LSB_TASK_GEOMETRY Environment Variable

export LSB_TASK_GEOMETRY="{(0,3)(1,4)(2,5)}"

The result of such a geometry would be the following:

• MPI Tasks #0 and #3 on node #1
• MPI Tasks #1 and #4 on node #2
• MPI Tasks #2 and #5 on node #3

The geometry does not define the identify for nodes #1, #2 and #3. This assignment remains under the responsibility of the workload manager.