When installation option SIZE=MAX, EXEC PARM option SIZE=MAX, or run-time option MAINSIZE=MAX is in effect, the TMAXLIM, RESALL, and RESINV options are used. These options are not used when SIZE=n or MAINSIZE=n is in effect.

You should also be aware of how the JCL REGION parameter can affect DFSORT virtual storage allocation. While subject to the constraints of your site's IEFUSI and IEALIMIT exits, the JCL REGION value limits the amount of below 16 MB virtual storage.

DFSORT attempts to place as much of its storage as possible above 16 MB virtual. DFSORT, however, still needs sufficient storage below 16 MB virtual to run effectively. In general, a REGION value of at least 512 KB is best. If DFSORT is called by a program, the REGION value should be large enough to allow sufficient storage for DFSORT and the program. If E15 or E35 user exits are used, a larger REGION value might improve performance because these functions use more storage below 16 MB virtual.

If the available storage below 16 MB virtual is insufficient, DFSORT issues message ICE039A, which indicates the minimum additional storage required below 16 MB virtual. Although the application might run if you add the minimum storage indicated, it is recommended that you increase either the REGION value, or the SIZE or MAINSIZE value (or both) to provide sufficient virtual storage for the application to run efficiently.

PARM parameter SIZE=n or OPTION parameter MAINSIZE=n can be used to allow more (or less) storage for specific applications for which the installation default is not appropriate. For example, you might want to specify MAINSIZE=8M to improve performance for a critical large application when your installation default is 6 MB. Note that DSA is not used when SIZE=n or MAINSIZE=n is specified.

When SIZE=n or MAINSIZE=n is in effect, RESALL and RESINV are not used. Generally, this does not cause a problem, but if it does you should ensure that the user exit size values in your MODS statement, if any, are correct. Because the user exit size in the MODS statement is only an estimate, you can raise it if necessary to allow more reserved storage. Alternatively, you could raise the REGION value or go back to using SIZE=MAX or MAINSIZE=MAX to resolve the problem. See z/OS DFSORT Installation and Customization for details of the relationships between SIZE, MAINSIZE, RESALL, RESINV, REGION, and other storage parameters.

Alternatively, you could specify SIZE=n or MAINSIZE=n at run-time to provide more (or less) storage for specific DFSORT applications.

The MINLIM value is important only for jobs which specify a SIZE or MAINSIZE value that is less than the MINLIM value. By not using a value of MAX, these jobs become locked into a specific virtual storage limit. They must also reserve storage without using RESALL and RESINV, usually by making the SIZE or MAINSIZE value less than the REGION value or by coding user exit sizes on a MODS control statement. Such applications are prime candidates for your tuning efforts.

One way to improve the performance of these applications is to raise the MINLIM value (for example, to the MAXLIM value). This enables such applications to run with a larger amount of virtual storage. This strategy, however, might cause some of these applications to fail due to insufficient reserved storage. Using run-time options, you should change the failing applications to use SIZE=MAX or MAINSIZE=MAX and set appropriate values for user exit sizes (on the MODS control statement), RESALL, and RESINV.