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Job Slicing Overview
Ansible, by default, runs jobs from a single control instance. At best a single Ansible job can be sliced up on a single system via forks but this doesn't fully take advantage of AWX's ability to distribute work to multiple nodes in a cluster.
Job Slicing solves this by adding a Job Template field job_slice_count. This field specifies the number of Jobs to slice the Ansible run into. When this number is greater than 1 AWX will generate a Workflow from a JobTemplate instead of a Job. The Inventory will be distributed evenly amongst the slice jobs. The workflow job is then started and proceeds as though it were a normal workflow. The API will return either a Job resource (if job_slice_count < 2) or a WorkflowJob resource otherwise. Likewise, the UI will redirect to the appropriate screen to display the status of the run.
Implications for Job execution
When jobs are sliced they can run on any Tower node and some may not run at the same time. Because of this, anything that relies on setting/sliced state (using modules such as set_fact) will not work as expected. It's reasonable to expect that not all jobs will actually run at the same time (if there is not enough capacity in the system for example)
Simultaneous Execution Behavior
By default Job Templates aren't normally configured to execute simultaneously (allow_simultaneous must be checked). Slicing overrides this behavior and implies allow_simultaneous even if that setting is unchecked.