IBM® provides
two kinds of job entry subsystems: JES2 and JES3. In many cases, JES2 and
JES3 perform similar functions, but most installations use JES2.
Both JES2 and JES3 read jobs into the system, convert them to internal
machine-readable form, select them for processing, process their output, and
purge them from the system.
Some principle differences between the two JES systems include:
- In a mainframe installation that has only one processor, JES3 provides
tape setup, dependent job control, and deadline scheduling for users of the
system, while JES2 in the same system would require its users to manage these
activities through other means. In an installation with a multiprocessor configuration,
there are noticeable differences between the two, mainly in how JES2 exercises
independent control over its job processing functions. That is, within the
configuration, each JES2 processor controls its own job input, job scheduling,
and job output processing.
- In cases where multiple z/OS® systems are clustered (a sysplex), it is
possible to configure JES2 to share spool and checkpoint data sets with other
JES2 systems in the same sysplex. This configuration is called Multi-Access
Spool (MAS). In contrast, JES3 exercises centralized control over its processing
functions through a single global JES3 processor. This global processor provides
all job selection, scheduling, and device allocation functions for all of
the other JES3 systems.
- With JES3, installations may decide whether the global JES3 or z/OS base control
program will handle device allocation. With JES2, only the z/OS base control
program handles device allocation.