Tuning main storage

Either the REGION value or the MAINSIZE/SIZE value can determine how much storage is available to DFSORT. See z/OS DFSORT Installation and Customization for details.

Generally, the most efficient way to allocate (virtual) main storage is to use MAINSIZE/SIZE=MAX explicitly or by default. However, problems can arise if the values for the TMAXLIM or MAXLIM installation options have been set excessively high (or low). Guidelines for setting these values are given in z/OS DFSORT Installation and Customization.

Note: Do not use SIZE/MAINSIZE=MAX with password-protected data sets if passwords are to be entered through a routine at a user exit, because DFSORT cannot then open the data sets during the initialization phase to make the necessary calculations.

If you specify MAINSIZE/SIZE=n and give n a value less than that specified for the MINLIM installation option, MINLIM is used.

When SIZE/MAINSIZE=MAX is in effect, DFSORT will use its Dynamic Storage Adjustment (DSA) feature, when appropriate, to improve performance.

If the MINLIM value is greater than that specified for REGION on the EXEC statement, DFSORT attempts to use the value specified for MINLIM; if it fails to get the amount specified by MINLIM, DFSORT still tries to run, provided at least 88KB (below 16MB virtual) are available to DFSORT.

Storage used for OUTFIL processing will be adjusted automatically, depending upon several factors, including:

Total available storage
Non-OUTFIL processing storage requirements
Number of OUTFIL data sets and their attributes (for example, block size).

OUTFIL processing is subject to the ODMAXBF limit and your system storage limits (for example, IEFUSI) but not to DFSORT storage limits, that is, SIZE/MAINSIZE, MAXLIM, and TMAXLIM. DFSORT attempts to use storage above 16MB virtual for OUTFIL processing whenever possible.

Note:

In some cases, this release of DFSORT may use more storage than prior releases for comparable applications. This might affect the operation of some applications. For example, some applications that run as in-storage sorts (with no SORTWKdd data sets) in previous releases might not run in-storage when using this release. The amount of storage allocated is normally controlled by TMAXLIM. A REGION size of at least 440KB must be available if DFSORT is to achieve acceptable performance. The allocation of storage can be adversely affected if you have a smaller region value or if DFSORT needs to allocate buffers below 16MB virtual.
For extremely large sorts (for example, 2GB or more of data), make sure that Hipersorting, memory object sorting and dataspace sorting are enabled, and that 32MB or more of main storage is available to DFSORT.

The relationship between TMAXLIM, MAXLIM, MINLIM, and REGION might be described as a series of checks and balances.

Your system programmer has set the default storage values according to your site's major sorting requirements. If you have an overnight or batch time window that must be met, increasing storage (using REGION or SIZE/MAINSIZE=n) can give you some relief from the time constraint. If you are concerned with processor time, decreasing storage (using REGION or SIZE/MAINSIZE=n) can reduce the processor time associated with sorting small files.

In general, when you vary the amount of storage available to DFSORT, several things occur:

If you increase the amount of storage:
- EXCPs are reduced.
- For larger files, processor time generally decreases; that is, overhead in managing the extra storage is offset by DFSORT having to make fewer passes over the data.
- For a very heavily loaded system, elapsed time might increase because DFSORT can be swapped out more often.
- For very small sorts, processor time might remain stable or increase because of the overhead in managing the extra storage. For larger files, processor time will usually decrease because the overhead in managing the extra storage would be less than the benefit gained by DFSORT making fewer passes over the data.
If you decrease the amount of storage:
- EXCPs increase.
- Elapsed time increases for most sorts.
- Processor time decreases for very small files, but increases for larger files.

Changing the main storage allocation can affect system efficiency. By reducing the amount of main storage allocated, you impair performance of DFSORT to allow other programs to have the storage they need to operate simultaneously; by increasing the allocation, you can run large DFSORT applications efficiently at the risk of decreasing the efficiency of other applications sharing the multiprogramming environment.