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Prestart Job Configuration Settings

There are two main considerations for how prestart jobs are managed by the system.

Dawn May i Can Blog

Prestart job entries are shipped with default configuration parameters. This basic configuration allows the operating system to manage the prestart jobs and generally works well on small systems or environments with limited use. But as your environment gets larger and the use of prestart jobs increases, you will need to review these parameters and customize them. You can view and change your presetart job entries via the command interface or with the Navigator graphical interface. 

There are two main considerations for how prestart jobs are managed by the system:

  • You want to ensure incoming work requests can be handled with a waiting prestart job. You don’t want an incoming work request to wait to start a job for that request.
  • You want to manage the workload on the system for starting additional jobs; that is, you don’t want to have too many additional jobs all starting at one time.

IBM has documented how to determine the best values for your environment. I’m not going to reproduce what IBM has already documented; see the resources at the end of this blog.

The parameters you need to be concerned with for tuning prestart jobs are below:

  • Initial number of jobs (INLJOBS)
    The initial number of jobs is the number of jobs that are started when the subsystem is started (although 7.3 changed this a bit). This value is also used for the THRESHOLD setting; when the initial number of jobs falls below the configured THRESHOLD value, the additional number of jobs are started.
    • Default is 1.
    • Generally, you want the INLJOBS value to be larger than the peak number of jobs that will be in use at any time. This is so you can have that number of jobs running, waiting for the work requests rather than starting jobs when the work requests are received by the system.
  • Threshold (THRESHOLD)
    The threshold parameter defines how many jobs must be active, waiting for work.  When the number of active jobs falls below this THRESHOLD value, then the additional number of jobs is started.
    • Default is 1.
    • You want the THRESHOLD value to be larger than the number of requests that can be received while additional prestart jobs are started.
  • Additional Jobs (ADLJOBS)
    As described above, this is the number of additional jobs that are started when the number of available jobs falls below the THRESHOLD setting.
    • Default is 2.
    • Generally, you’ll want the ADLJOBS value to be low to avoid starting a lot of additional jobs at one time. However, the value you select will also depend upon the INLJOBS and THRESHOLD parameters.

IBM has documented configuring and tuning prestart jobs. Read the following information for complete details on the above parameters and considerations for their settings. The experience report on tuning prestart job entries may have been written some time ago, but the material is still relevant.

Instead of starting all the jobs during subsystem start up some of the jobs are started after the prestart jobs start to be used