PP-732: scheduler's attribute allows to control unset resources in placement sets

Overview:

Using placement sets it is created a pool of nodes with unset resource. When scheduler tries to run a job, it considers to fit a job into this pool. The value of unset resource is "\"\"". A new parameter allows to control whether or not this pool is created.

Interface 1: only_explicit_psets

Visibility: Public
Change Control: stable
Synopsis: scheduler attribute; attribute can control creation of pool of nodes with unset resource in placement sets
Details: Scheduler attribute. Specifies whether or not the scheduler creates a pool of nodes with unset resource in placement sets. Default value is False.
- Set to False means: The behavior is unchanged. The pool with unset resource is created. If you use placement sets, scheduler creates pools of nodes: one pool per value and one pool for nodes with unset value. The scheduler tries to fit a job into the pools from the smallest one to the largest one.
- Set to True means: The pool with unset resource is NOT created and the nodes with unset resources are skipped in this placement set.
Example:
- create resource infiniband
  set resource infiniband type = string
  set resource infiniband flag = h
  ...
  set node n1 infiniband = A
  set node n2 infiniband = B
  unset node n3 infiniband
  
  set sched do_not_span_psets = True
  set sched only_explicit_psets = True
  
  $ qsub -l place=group=infiniband
  The job can run on n1 or n2 but will never run on n3 with 'only_explicit_psets = True'.

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