System control and performance EXEC parameters for the IMS control region

Identifying the nucleus

The SUF parameter is generated as a null value for the IMS procedure, indicating that the default nucleus name has a one-character suffix of 0. If you are using an alternative control program, you must specify this parameter.

Overriding the number of active regions

Use the PST parameter to override the expected number of regions that are to be in operation during the online execution. Additional regions can be dynamically allocated, up to the maximum allowable number permitted by your operating system. If you do not specify the PST parameter, the default number of regions is the number specified for the MAXREGN keyword in the IMSCTRL macro.

Specifying performance options

Three parameters contribute to general performance strategy. The FIX= parameter specifies a two-character suffix for the member DFSFIXxx in the IMS PROCLIB data set. This suffix indicates which member should describe all modules and control blocks that are to be page fixed. The EXVR= parameter allows you to page-fix buffers used for the management of message queues. The parameter is generated with a null value; you must specify EXVR=1 to indicate the page-fixing action. Similarly, the PRLD= parameter specifies a two-character suffix for the member DFSMPLxx, where all preloaded modules are listed.

Specifying z/OS options

Several parameters apply only to the z/OS operating system. The SRCH parameter allows you to take advantage of any special library structure to optimize the search for loaded modules. You can override the default value of 0 with a value of 1 if you want the JPA and LPA to be searched before IMS program libraries.

To reduce the amount of z/OS CSA (common storage area) that IMS uses you can use one of the following techniques:

One technique is to specify LSO=Y on the IMS procedure to cause some control blocks and some IMS modules that are used for DL/I processing to be loaded into the control program's private storage. To allow for this transfer, use the following sum to increase the size of the control region:

220K + OSAM buffer pool + VSAM buffers + enqueue/dequeue tables

Specifying DL/I separate address space options

An additional variation of the local storage option is to use the DL/I separate address space. You do this by specifying LSO=S. This address space contains most of the DL/I code, control blocks, and database buffers for full-function databases. Again, z/OS cross memory services are used.

Specifying dump formatting options

In a DC environment, you can request the following types of dump outputs for errors that terminate IMS: SDUMP, SYSMDUMP, SYSABEND, or SYSUDUMP. To do this, specify the FMTO startup parameter in combination with z/OS dump DD statements.

For SYSMDUMP, you should provide operational procedures for saving and formatting dumps; otherwise, you can overlay a SYSMDUMP if you must restart IMS before the previous SYSMDUMP is transferred.

You can also request dump outputs for some errors that do not terminate IMS. Your choice of dump depends on several factors: the type of failure, the FMTO parameter option, and the IMS spin-off and z/OS dump DD statements that have been selected. For more information about the FMTO parameter and about using dumps in these situations, see IMS Version 15.2 System Utilities.

Specifying IRLM options

If an execution of the IMS online system is to use IRLM as lock manager or participate in block-level sharing, specify IRLM=YES. Specify the z/OS subsystem name for the IRLM associated with this control region on the IRLMNM parameter.

You cannot use the UHASH parameter with the IRLM to specify the name of an alternative Fast Path hashing module. The UHASH parameter is ignored, and DBFLHSH0 is always used as the hashing module.

Enabling Fast Path

Use FP= to enable or disable Fast Path.

The FPCTRL macro, although allowed for compatibility, is ignored by system definition. Fast Path is enabled by setting FP=Y in the DFSPBxxx member of the IMS PROCLIB data set or by specifying FP=Y as a startup parameter. The default is FP=N, so you must explicitly specify FP=Y to enable Fast Path.

If you specify FP=N and you attempt to use a Fast Path resource or command, results are unpredictable.

To override the initial value coded on DFSPBxxx, use the FP= parameter at execution time.

Specifying subsystem identification parameters

Use the SSM parameter to reference a member in the IMS PROCLIB data set which identifies the Db2 for z/OS or IBM® MQ subsystems that can be accessed from application programs executing in dependent regions. The one- to four-character suffix that you specify, together with the currently assigned name for IMSID, forms the member name. The member contains entries, each identifying a Db2 for z/OS or IBM MQ subsystem (by its z/OS subsystem name). All subsystems to be accessed from programs executing in dependent regions must be identified by an entry.

Depending on its entry, a region can access all, some, or none of these subsystems. To allow a dependent region to access all subsystems identified to the IMS control region, do not specify the SSM parameter for the dependent region or have the SSM entry specify the same member as the IMS procedure. To allow a dependent region to access only selected Db2 for z/OS or IBM MQ subsystems, specify an SSM parameter on the dependent region procedure that points to a member containing only those specific subsystems. To prevent a dependent region from having access to any subsystem identified to the IMS control region, specify a member containing no entries.

See IMS Version 15.2 System Administration for details about how to specify the individual entries and construct the member of the IMS PROCLIB data set.

Modifying storage pool definitions for the storage manager

You can set an upper expansion limit for the AOIP, HIOP, CIOP, CMDP, SPAP, DYNP, LUMP, LUMC, FPWP, and EMHB storage manager pools by using the appropriate parameters at execution time. IMS establishes these storage pool definitions without an upper expansion limit, because they are dynamic storage pools that expand and contract as needed during execution.

Use caution in specifying upper expansion limits. If an upper limit is too low, IMS might abend. Under normal circumstances, a pool should never reach its upper limit. The intent of the upper limit is to keep pools from consuming so much storage that an out-of-storage condition occurs.

Use the SPM=nn parameter to specify the suffix for DFSPMnn. DFSPMnn identifies the member of the IMS PROCLIB data set that overrides the storage pool definitions established by IMS.