Virtual storage

By careful design, you can minimize the amount of virtual storage used, and reduce your application's overhead.

A truly conversational CICS® task is one that converses with the terminal user for several or many interactions, by issuing a terminal read request after each write (for example, using either a SEND command followed by a RECEIVE command, or a CONVERSE command). This means that the task spends most of its extended life waiting for the next input from the terminal user.

Any CICS task requires some virtual storage throughout its life and, in a conversational task, some of this virtual storage is carried over the periods when the task is waiting for terminal I/O. The storage areas involved include the TCA and associated task control blocks (including EIS or EIB) and the storage required for all programs that are in use when any terminal read request is issued. Also included are the work areas (such as copies of COBOL working storage) associated with this task's use of those programs.

With careful design, you can sometimes arrange for only one very small program to be retained during the period of the conversation. The storage needed could be shared by other users. You must multiply the rest of the virtual storage requirement by the number of concurrent conversational sessions using that code.

By contrast, a pseudoconversational sequence of tasks requires almost all of its virtual storage only for the period spent processing message pairs. Typically, this takes a period of 1-3 seconds in each minute (the rest being time waiting for operator input). The overall requirement for multiple concurrent users is thus perhaps five percent of that needed for conversational tasks. However, you should allow for data areas that are passed from each task to the next. This may be a COMMAREA of a few bytes or a large area of temporary storage. If it is the latter, you are normally recommended to use temporary storage on disk rather than in main storage, but that means adding extra temporary storage I/O overhead in a pseudoconversational setup, which you do not need with conversational processing.

The extra virtual storage you need for conversational applications usually means that you need a correspondingly greater amount of real storage. The paging you need to control storage involves additional overhead and virtual storage. The adverse effects of paging increase as transaction rates go up, and so you should minimize its use as much as possible. See Reducing paging effects for information about doing so.