In particular for TP operation, a paging rate that is too high is critical (see the maximum recommended number of paging I/O operations in the section "Standard values for BS2000 servers"). Too high a paging rate always results in unsatisfactory response times. If the critical number of paging I/O operations is repeatedly reached or even exceed, the main memory must be expanded.
In VM2000 operation the main memory should be distributed as efficiently as possible over the guest systems before a general upgrade of the main memory configuration.
Monitoring main memory utilization
The utilization of the main memory should be continuously monitored by systems support using SM2 in order to detect an impending main memory bottleneck in good time.
The following key figures are of significance for calculating and evaluating the memory utilization (see also section "Control functions within the framework of main memory management" (Activating/deactivating tasks)):
NPP (Number of Pageable Pages): Corresponds to the size of main memory which is available minus the number of resident pages.
SWS (System Working Set): Number of used and accessible pageable pages (corresponds to the NPP minus the number of freely usable pages in the so-called free pool).
The utilization level of main memory (in percent) is obtained using the following formula:
N = SWS * 100 / NPP %
Measurements for N up to 75% are regarded as uncritical.
When the values for N exceed 90% a main memory upgrade should be considered. This is particularly the case when the new version of a software product is to be introduced and this software is used by several tasks.