132IBM Power Systems 775 for AIX and Linux HPC Solution
Figure 2-13, Node 0 and Node 1 include a CAU and a processor. Node 2 does not have a
CAU and node 3 does not have a processor (uncommon).
Figure 2-13 CAU tree physical connectivity example
Specifying the number of CAU group
The performance of small broadcast and reduction operations is significantly enhanced with
the use of the CAU resources of the HFI. You specify the number of CAU groups for an
application by using the MP_COLLECTIVE_GROUPS environment variable or the
-collective_groups command line flag. Allowable values include any number that is greater
than zero and less than the n
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