FastSort Manual

Optimizing Sort Performance
FastSort Manual—429834-003
9-11
Using VLM
extended memory, if available. FastSort uses the additional extended memory either to
complete the sort in a single pass or to store partial information until the sort is
complete.
Without VLM, the maximum number of records that FastSort can sort in memory is
32,767. This limit applies regardless of the amount of memory available. With VLM,
available memory and extended segment size determine the number of records that
can be sorted in memory.
Turning On VLM
Depending on your system configuration, memory usage, and the interface to FastSort
you use, the VLM option can help improve sort performance. VLM is off by default
because it can use more physical memory and does not always improve performance.
You can turn on VLM from:

A SORT DEFINE, including the =_SORT_DEFAULTS DEFINE

The SORTMERGESTART procedure
For information on which method takes precedence, see Determining the Precedence
of DEFINEs on page 7-1.
When VLM is on, the maximum amount of extended memory for sorting is 127.5 MB,
or 62,255 pages. This memory limit overrides any value you otherwise specify for
segment size. When VLM is off, the maximum extended memory FastSort can use is
67 MB, or 32,767 pages.
Do not use VLM for parallel sort or load operations.
How VLM Affects Swap Files
A larger extended memory segment requires a larger swap file. Using VLM can cause
an increase in data swapping if SORTPROG competes with other processes for
memory. If increased swapping impacts performance, use one of these strategies:

Use the SEGMENT attribute or parameter to specify a smaller extended segment

Use the CPU attribute to specify a less-busy CPU

Use the SWAP attribute to move the swap file to a different disk volume

Turn VLM off to use the default extended memory and disk space utilization
For more information about FastSort swap files, see Using Swap Files on page 9-10.
How VLM Affects Scratch Files
VLM can help reduce the disk space FastSort uses for scratch files. With a larger
extended memory segment, FastSort can perform some sort operations entirely in
memory. Sorts performed in memory do not require scratch files.