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If the job requests per-chunk grouping, and any chunk can not fit, the entire job will span. This is regardless if other chunks can fit in their placement sets.
Interaction with only_explicit_psets
The only_explicit_psets scheduler attribute tune the placement set creation. If any node does not have a grouping resource set on it, it is usually added to a resource="" (e.g. color="") placement set. If only_explicit_psets is true, then the resource="" placement set is not created, and those nodes are not available for placement.
There is no change in this behavior with per-chunk grouping. If only_explicit_psets is true, then the resource="" placement set is not created for that chunk complex's placement pool.
Clarifications
The nodes used to create placement sets:
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