AS/400 and ISeries Job System Demystified

One of the most essential items every system administrator and operator must understand on the AS/400, iSeries and System i power system platforms is the job and batch systems. These two portions of the system are key for performing any task in a timely manner.

The job system on the midrange platform is both robust and elegant in design. At a very basic level jobs are first submitted into job queues for immediate or later processing by subsystems. By default the job queues are FIFO based, that is jobs are submitted to run on a first-in-first-out sequence.

You have the ability to re-sequence the jobs within a job queue by changing the job priority of a job waiting to run. The job priorities go from 1-9 and default to priority 5. So to change where at in the queue a job is and move it ahead of other jobs simply increase the job priority to a higher number.

When jobs actually process they run in an area of the system called a subsystem. All jobs currently operating on the AS/400 run in subsystems, this even includes non batch jobs like your green screen 5250 session. In many instances the system is configured so jobs submitted to batch run under the subsystem called QBATCH. However you do have the ability to add and change subsystems so you can finely control how jobs are processed on the system and allocate resources precisely where they are needed.

While running in batch a job is actively performing the task it was designed to do like generate a report, update database records and so on. It is also actively consuming processor and system resources. To view what jobs are active you use the command work with active jobs WRKACTJOB.

The WRKACTJOB command quickly lets you see all of the active jobs and some other important information like how much processor it is currently using, vital resource statistics, job logs and more. It is a very effective tool. And I often find the work with active jobs screen usefull for tracking down a job that has a message waiting for a reply.

Sometimes it is necessary to find out what other resources a job is consuming, that can be easily accomplished by taking option five which calls up the Work with Job command WRKJOB. From here you would want to check the option to display job run attributes. This will show you the jobs running priority (not to be confused with the job priority in the job queue), CPU time used and temporary storage, ie disk space, that the job is consuming.

A special thing to note from the Work With Job attribute screen is the time slice of the job. The time slice and the job priority values can have a major effect on the performance and time it takes for the job to complete as well as impacting the overall performance of your entire system. It is generally safe to judiciously increase the time slice of a job thereby increasing the amount of time the job can run in milliseconds before it is interrupted by the operating system. Just be careful or you can bring your entire system to a screeching halt.

John Andersen is an IT manager living in California. Be sure to check out his Midrange Jump Start web site for insider advice and tips on how to manage your AS/400 and iSeries systems.
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