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When either of these JES systems runs on a single CPU, JES2 and JES3 perform the same functions. Both JES systems process jobs from the moment they are entered into the system to the point at which the jobs are purged from the system. However, if more than one CPU exists in the configuration, the differences between the two JES systems are more apparent. In a multiple‑processor configuration, each JES2 CPU is independent of the others, even though they can share a common spool file. JES3, however, controls all of its processors as a group. JES2 systems use multiple job queues. JES3 uses a single job queue. All of its processors are arranged in a ring structure, which is controlled by the JES3 global processor. Other processors in the complex are called local processors. Although all of the local processors share a common job queue, all jobs reach that queue only through the one central JES3 component, the global processor. The global processor selects and distributes work to the other processors.
Sometimes people ask which JES is best. Each system has its group of partisans. About 80 to 90 percent of all z/OS data centers use JES2. For smaller CPU installations, the numbers are even higher. However, the 10 - 20% of data centers that run JES3 are some of the largest data centers in the United States. During the last few years, a steady stream of enhancements (such as JES2 shared spool and network job entry (NJE)) reduced the major differences between the two systems. There is probably little reason to switch if a data center is happy with the JES system that it is currently using.
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