Caché Installation Guide - InterSystems Documentation

OpenVMS Physical Memory Allocation
Parameter
BORROWLIM
PFRATH (Page Fault Rate
High)
PFRATL (Page Fault Rate
Low)
Definition
Minimum number of pagelets that must be in the free
page list before a process can borrow pages in WSINC
amounts up to WSEXTENT pages.
Indicates the number of page faults that must occur
within the period specified by AWSTIME, before
OpenVMS makes an “automatic working set
adjustment” to increase the working set by WSINC.
If the number of page faults is less than this value,
OpenVMS makes an “automatic working set
adjustment” to decrease the working set by WSDEC,
but not below the point set by AWSMIN.
Note:
While parameters have units of pagelets, OpenVMS allocates memory in pages. For
this reason, the parameter values you choose should be even multiples of the number
of pagelets per page. OpenVMS automatically rounds these parameters if you do
not follow this guideline.
A.3.1 Default Memory Allocation
The following steps summarize the algorithm OpenVMS uses to determine the amount of
physical memory available to each process. Each capitalized term represents a system
parameter that OpenVMS uses in allocating physical memory.
1. The process begins executing an image. OpenVMS allocates to it the number of pagelets
specified by the quota parameter WSDEFAULT.
Page Allocation:
0--->WSDEFAULT
2. If the process has a higher page fault rate than PFRATH, OpenVMS allocates more
pagelets to it. OpenVMS allocates these pagelets in increments of WSINC until the process
has WSQUOTA total pagelets.
Page Allocation:
0--->WSDEFAULT-->WSINC-->WSINC-->WSQUOTA
3. If the process continues to have a high page fault rate and the free list size exceeds
BORROWLIM, OpenVMS will continue to allocate pagelets in WSINC blocks until the
free list size is inadequate or WSEXTENT is reached.
Page Allocation:
0-->WSDEFAULT-->WSINC-->WSQUOTA-->WSINC-->WSEXTENT
Caché Installation Guide 55