| SAS Companion for the OS/390 Environment |
In the OS/390
operating environment, you can use two options to limit the amount of memory
used by SAS. The MEMSIZE= option sets a hard limit on the amount of memory
used by SAS. The MEMLEAVE= option limits SAS memory relative to the user's
available region of memory. SAS Institute recommends that you specify a value
for the MEMLEAVE= option and let SAS determine the value of MEMSIZE= based
on your REGION size and on the value of MEMLEAVE=.
The following sections provide details on available memory management
techniques.
For most
programs and data, SAS uses memory (storage) above
the 16MB line. This is sometimes referred to as 31-bit addressing. The few
exceptions include data areas that require 24-bit addressability and a small
part of the host supervisor. Because the OS/390 JCL parameter REGION is ineffective
at controlling 31-bit memory usage, the SAS MEMSIZE= option has been implemented
for this purpose.
One way to limit the amount of virtual memory that SAS
can use is to specify a value for the MEMSIZE= system option when you invoke
SAS. Under OS/390, MEMSIZE= has a default value of 0, which means that SAS
can use memory up to the maximum amount that is available. This enables you
to invoke multiple procedures consecutively without reloading the program
each time. However, if there is no upper limit on virtual memory usage, then
memory is not freed for reuse even when the programs that are stored there
are no longer needed.
The default value for the MEMSIZE= option is calculated
internally using the following formula:
MEMSIZE_default = size_of_user_memory_region -- MEMLEAVE_value
For interactive applications that use multiple SAS System components
such as SAS/AF and SAS/GRAPH, specify MEMSIZE=24M. If you prefer to set the
value of the MEMSIZE= option yourself, a value of 12M is a reasonable choice
for most batch applications.
|
| Specify a value for MEMLEAVE= |
As with MEMSIZE=, the MEMLEAVE=
system option limits the amount
of memory used by SAS, but in a different way. Instead of setting an absolute
limit on the amount of memory SAS can use, MEMLEAVE= specifies a value that
is subtracted from the total amount of memory available in the user's REGION.
The amout of memory specified by MEMLEAVE= is reserved for the use of the
operating environment. The remainder of the user's REGION remains available
to SAS. The advantage provided by MEMLEAVE= is that it applies equally well
to all SAS sessions, regardless of the size of the REGION. Choosing an appropriate
value for MEMSIZE=, on the other hand, can be problematic in environments
where REGION sizes vary.
The default value of MEMLEAVE= is 512K. You might need
to increase this value depending on memory demands expected for host programs
running in the same REGION, to prevent SAS from using too much of that REGION.
For example, you might want to increase the value of MEMLEAVE= if you plan
to run a memory-intensive host sort routine while also running a large SAS
session.
If you specify a value for the MEMLEAVE= option, either
do not specify a value for the MEMSIZE= option or set the value of the MEMSIZE=
option to 0.
|
| Consider using superblocking options to control memory fragmentation |
Superblocking
options are SAS system options that set aside large blocks of memory for different
classes of use. In most cases, the default values for these options are appropriate
and should not be altered. However, if you receive a superblock-overflow warning
message in the SAS log, you may want to use these options to adjust the memory
allocation for your job.
For complete information on superblocking system options,
see the installation instructions for the SAS System in the OS/390 environment.
You can also submit the following SAS statement to list the superblocking
system options:
proc options group=memory;
run;
Sometimes
a job runs out of memory in spite of additional memory allocations. To ensure
that the job ends "gracefully" under that condition, you may want to increase
the values of the SAS system options SYSLEAVE= and PROCLEAVE=.
- The SYSLEAVE= option reserves a specified amount
of memory to ensure that, when a SAS task ends, enough memory is available
to close data sets and to "clean up" other resources. For details, including
the SAS default value, see SYSLEAVE=.
- The PROCLEAVE= option serves a similar function
for SAS procedures. For example, some procedures are designed to use memory
until no more is available; they then continue by opening and using work files.
PROCLEAVE= ensures that there will be enough memory left to open these work
files and to allocate I/O buffers for them so that the procedure can continue.
For details, including the SAS default value, see PROCLEAVE=.
Copyright 1999 by SAS Institute Inc., Cary, NC, USA. All rights reserved.
