MVS- System Internals

System internals explains what happens behind the scenes: how a job moves through JES, what an initiator does, how paging works, and how SMF records everything.

This page is written at intermediate depth - enough to read job output, understand waits, and talk to operations and systems programmers.

JES2 and JES3

• JES = Job Entry Subsystem. It handles batch jobs from submission to output.
• JES receives the job, queues it, presents it to MVS for execution, and routes the spooled output.
• There are two versions: JES2 and JES3. They are incompatible with each other.
• A shop installs only one of them, so learn which one your site uses.

How a job enters the system

• You write JCL in an editor and issue the SUBMIT (SUB) command.
• JES reads the job stream from the data set and copies it to the job queue on the JES spool.
• From then on, the job is tracked by its job name and job number.

Job scheduling: classes and priorities

• Jobs are not run in submission order. JES picks the most important jobs first.
• Job class (one character, A-Z, 0-9) groups jobs by their processing needs. Class is more important than priority.
• Priority is a number within the class; higher-priority jobs run first.
• Each shop defines its own classes, for example by expected CPU time and output size.

Initiators

• An initiator is a program that runs in an address space eligible for batch processing.
• Each initiator handles one job at a time.
• The initiator looks at the JES spool, selects a job of its classes, runs it in its address space, then takes the next job.
• Each initiator is assigned one or more job classes and only runs jobs from those classes.
• Within a class, jobs run in priority order.

How a job is executed

• The JCL interpreter (converter) checks the JCL and builds control blocks.
• The initiator allocates the data sets, devices, and storage the job needs.
• The user program runs in the private area of the initiator's address space.
• When the job ends, the initiator deallocates everything and picks up the next job.

SYSIN and SYSOUT data sets

• SYSIN (in-stream data): input data coded inside the job stream after a DD * statement. JES stores it on the spool.
• SYSOUT: program output written as if to a printer. JES holds it on the spool until it is printed or viewed.
• SYSOUT=* means: use the MSGCLASS of the JOB statement.

The SYSOUT data sets of a job

• JESMSGLG: the JES message log - messages from JES as the job ran.
• JESJCL: the listing of the JCL after the converter processed it.
• JESYSMSG: system messages produced by MVS during the job.
• Plus one SYSOUT data set per program output DD statement.
• Output class A is normal printer output; B is special output; X is held output that stays on the queue until released or deleted.
• View all of these with SDSF (System Display and Search Facility).

Virtual storage and paging

• Virtual storage is divided into 4 KB pages.
• A block of real storage is a frame. A block of auxiliary storage (on disk) is a slot.
• A page, a frame, and a slot are all the same size: 4096 bytes.
• Page-in moves a page from a slot to a frame; page-out moves it back.
• Dynamic Address Translation (DAT) converts virtual addresses to real addresses using hardware tables.
• When frames run low, the system steals pages that have not been used recently (page stealing).

Swapping

• Swapping moves a whole address space in or out of real storage.
• A swapped-in address space is active and can run. A swapped-out address space waits on auxiliary storage.
• Special swap data sets hold the pages of swapped-out address spaces.
• Swapping balances the workload when memory is tight.
• Programs that must always stay in memory are called resident.

SMF: System Management Facilities

• SMF records what the system does: job accounting, performance, and resource usage.
• Each event writes an SMF record with a type number, for example:
• Type 4 / 5: job step end / job end (CPU time, start and end times).
• Type 14 / 15: data set open and close activity.
• Type 30: common address space work (the modern replacement for types 4, 5, 34, 35).
• Installations use SMF data for chargeback (billing departments for usage) and performance tuning.
• SMF records are usually dumped to data sets and processed by reporting tools.

Address space layout

• Each address space has a private area (the user's program and data) and a common area (shared system code).
• The common area holds the nucleus, SQA, CSA, and the link pack area (LPA).
• The private area holds the user region, LSQA, and SWA.
• System address spaces are started at IPL for the master scheduler and subsystems.
• TSO/E creates one address space per logged-on user; batch creates one per job.

Key subsystems and facilities

• TSO/E: lets terminal users work interactively, each in their own address space.
• ISPF: full-screen editor and utilities running under TSO/E.
• VTAM: controls terminal and network communication (part of SNA).
• CICS: runs online transaction programs in its own address space.
• DB2: relational database manager, accessed with SQL.
• RACF: security - controls who can access data sets and facilities.
• SMS: automates storage management (data class, storage class, management class).
• WLM: sets performance goals and gives resources to the most important work first.