that has the fixed head feature installed. The moveable-head paging area is paging
space on a DASD volume that is accessed by a moveable arm. Normally, CP dynamically invokes page migration, based on calculated load levels, once every ten
minutes.
Inactive pages in the fixed-head preferred paging area are migrated every time CP invokes migration. For pages in the moveable head preferred paging area, the
installation can decide at what point inactive pages are selected for migration. The
system programmer can use the SET SRM MHFULL command to set moveable
head page migration limits.
If a percentage for MHFULL has been specified, CP migrates pages from move­
able head preferred paging areas only when that percentage is reached and the time
interval has elapsed, rather than whenever fixed head areas are full. Thus,
migration from moveable head preferred paging areas and fixed head preferred
paging areas can take place separately.
In addition, the system operator can use the MIGRATE command to immediately
invoke page and swap table migration. Page migration can also be invoked only for
a specific virtual machine.
The format of the MIGRATE command is described in the VM/SP Operator's
Guide.
Querying and Setting the System Resource Management Variables
The QUERY SRM and SET SRM commands allow the system analyst to query
and/ or change internal system activity counters or parameters. Formats for the
QUERY SRM and SET SRM commands are contained in the VM/SP Operator's
Guide.
The system analyst can use the Class E QUERY SRM command to display the fol­
lowing information: • Current number of pageable pages Size of the dispatching time slice Setting of the maximum working set estimate
Maximum drum page allocation limit
Current page migration counters
Unused segment elapsed time as criteria for page migration
Current PCI flag setting mode for 2305 page requests
Maximum page bias value
Current interactive shift bias value
Moveable head page migration limit
The class E SET SRM command allows the system analyst to set some of the sys­
tem variables that can affect the values displayed by the QUERY SRM command.
Queryillg and Setting the Pagillg Variable
The paging variable is used in the working set size algorithm. The current paging,
load is constantly compared with the paging variable. Adjustments are then made
in the working set size estimates, based on how well the actual load compares with
the paging load variable.
Performance Observation and Analysis 51

The MONITOR Command
The QUERY PAGING command displays the paging variable used in the working
set size estimate control algorithm. Information on the paging rate per second is
available as a response to the INDICATE LOAD command.
The SET PAGING command is used to change the paging variable used in the
working set size estimate.
Information about the formats of the QUERY PAGING and SET PAGING com­
mands is contained in the VM / SP Operator's Guide. VM/SP Monitor collects data in two ways:
1. By handling interruptions caused by executing MONITOR CALL (MC)
instructions.
2. By using timer interruptions to give control periodically to sampling routines. MONITOR CALL instructions with appropriate classes and codes are presently
embedded in strategic places throughout the main body of VM/SP code (CP). When a MONITOR CALL instruction executes, a program interruption occurs if
the particular class of MONITOR CALL is enabled. The classes of MONITOR CALL that are enabled are determined by the mask in control register 8. For the
format and function of the MONITOR CALL instruction, refer to the System/3 70 Principles of Operation. The format of control register 8 is as follows: I xxxx I xxxx I xxxx I xxxx I 0123 I 4567 I 89AB I CDEF I where:
x O-F (hexadecimal)
indicates unassigned bits.
indicates the bit associated with each class of the MONI­ TOR CALL.
When a MONITOR CALL interruption occurs, the CP program interruption han­
dler (DMKPRG) transfers control to the VM/SP Monitor interruption handler (DMKMON) where data collection takes place. Sixteen classes of separately enabled MONITOR CALL instructions are possible,
but only eight are implemented in the VM/SP Monitor.
Monitor output consists of event data and sampled data. Event data is obtained via MONITOR CALL instructions placed within the VM/SP code. Sampled data is
collected following timer interruptions. All data is recorded as though it were
obtained through a MONITOR CALL instruction. This simplifies the identifica­
tion of the records.
The following table indicates the type of collection mechanism for each Monitor
class:
2
There is no class name for monitor class 3, but it is reserved.
52 VM/SP System Programmer's Guide