Caché Installation Guide - InterSystems Documentation

Calculating System Parameters for OpenVMS
Working Set Control
Type of Process
Users
Interactive Processes
Batch Processes
Batch Queues
Method of Working Set Control
The AUTHORIZE utility lets you change the default working set
on a user-by-user basis.
The DCL command SET WORKING_SET allows interactive
processes to change working set parameters.
The SUBMIT command qualifiers for working sets allow batch
processes to alter physical memory allocations.
You can use the DCL command INITIALIZE/QUEUE to alter
working set parameters for batch queues.
A.3.3 Keeping Memory Free for New Processes
Memory is finite. Every time a new process begins under OpenVMS, it takes WSDEFAULT pages
away from the total number of available pages. The remaining memory is available for the free page
list and for WSINC additions to each process's allocation.
Heavily loaded systems can run so many concurrent processes that the number of pages remaining for
use in the free page list becomes very small. OpenVMS includes a parameter called FREELIM that
sets a lower limit for the number of pages in the free page list.
The size of the free and modified page lists directly affects the ratio of hard page faults to soft page
faults. Hard page faults cause a process to experience a resource wait state, resulting in slow execution.
For better performance, it is important to tune the system to minimize hard page faults. While soft
faults are less expensive, an excessive soft page fault rate can also lead to poor performance.
Whenever OpenVMS detects that the size of the free page list falls below FREELIM, OpenVMS starts
a mechanism to bring the free list size up to FREEGOAL. To achieve this, if all other mechanisms are
inadequate, OpenVMS may move the entire working set of an inactive process out to disk. This procedure
is called swapping. The process placed on disk is swapped out. When the process is brought back
into memory, it is swapped in. Swapping has a detrimental effect on overall system performance.
A.3.4 Allocation for Shared Sections
OpenVMS uses a slightly different approach for allocating memory for shared code and data. The
GBLSECTIONS parameter sets the number of shared sections to be allocated when the system is started.
The GBLPAGES parameter sets the number of global page table entries. Every group of 128 page
entries requires 4 bytes of resident memory. In addition, OpenVMS uses the GBLPAGFIL parameter
to set the maximum number of page file blocks available for global pages.
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