Caché Installation Guide - InterSystems Documentation

How Caché Uses OpenVMS Memory
A.4 How Caché Uses OpenVMS Memory
Caché uses both shared memory and memory private to each process when running on OpenVMS. It
uses the following techniques to increase performance:
• Balancing Memory Locking and Paging
• Using Process-Private Space to Reduce Paging
• Using Resident Memory
A.4.1 Balancing Memory Locking and Paging
All Caché code is shared, and can be physically locked in memory. The same is true for the global and
routine buffer pools.
If a routine is larger than the locked portion of a routine buffer, only a portion of the routine in the
routine buffer is locked in memory. The remainder of the routine is loaded into unlocked physical
memory.
Locking shared data and routines in memory allows better response time since memory access is
quicker than disk access. As a result, the more globals and routines that are kept in memory, the better.
Memory, however, is a finite resource. The more global and routine buffers that are allocated, the less
memory is available for OpenVMS processes. When less memory is available, more OpenVMS paging
occurs.
The goal, therefore, is to choose a number of global and routine buffers that lets you keep enough
globals and routines in memory without negatively affecting OpenVMS paging. This document provides
guidelines for selecting these values.
A.4.2 Using Process-Private Space to Reduce Paging
Every Caché process maps to the shared memory sections, but it also has access to a private area of
memory called process-private space. This private area includes variables, arrays, stacks, and other
data structures that belong to a particular process.
A portion of this private area of memory may be locked into the process working set to reduce paging.
None of the private area, however, is physically locked in memory.
A.4.3 Using Resident Memory
The OpenVMS platform makes use of the Reserved Memory Registry, a mechanism for reserving
memory within memory-resident sections. The two features that Caché supports, memory-resident
global sections and shared page tables, are always used as a pair.
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