HP OpenVMS Systems Documentation
OpenVMS Programming Concepts Manual
12.3.6 Process Swapping
The operating system balances the needs of all the processes currently executing, providing each with the system resources it requires on an as-needed basis. The memory management routines balance the memory requirements of the process. Thus, the sum of the working sets for all processes currently in physical memory is called the balance set.
When a process whose working set is in memory becomes inactive---for example, to wait for an I/O request or to hibernate---the entire working set or part of it may be removed from memory to provide space for another process's working set to be brought in for execution. This removal from memory is called swapping.
The working set may be removed in two ways:
A privileged process may lock itself in the balance set. While pages can still be paged in and out of the working set, the process remains in memory even when it is inactive. To lock itself in the balance set, the process issues the Set Process Swap Mode (SYS$SETSWM) system service, as follows:
This call to SYS$SETSWM disables process swap mode. You can also disable swap mode by setting the appropriate bit in the STSFLG argument to the Create Process (SYS$CREPRC) system service; however, you need the PSWAPM privilege to alter process swap mode.
A process can also lock particular pages in memory with the Lock Pages in Memory (SYS$LCKPAG) system service. These pages are forced into the process's working set if they are not already there. When pages are locked in memory with this service, the pages remain in memory even when the remainder of the process's working set is swapped out of the balance set. These remaining pages stay in memory until they are unlocked with SYS$ULKPAG. The SYS$LCKPAG system service can be useful in special circumstances, for example, for routines that perform I/O operations to devices without using the operating system's I/O system.
You need the PSWAPM privilege to issue the SYS$LCKPAG or SYS$ULKPAG
A section is a disk file or a portion of a disk file containing data or instructions that can be brought into memory and made available to a process for manipulation and execution. A section can also be one or more consecutive page frames in physical memory or I/O space; such sections, which require you to specify page frame number (PFN) mapping, are discussed in Chapter 13, Section 22.214.171.124.
When modified pages in writable disk file sections are paged out of memory during image execution, they are written back into the section file rather than into the paging file, as is the normal case with files. (However, copy-on-reference sections are not written back into the section file.)
The use of disk file sections involves these two distinct operations:
The Create and Map Section (SYS$CRMPSC) system service creates and maps a private section or a global section. Because a private section is used only by a single process, creation and mapping are simultaneous operations. In the case of a global section, one process can create a permanent global section and not map to it; other processes can map to it. A process can also create and map a global section in one operation.
The following sections describe the creation, mapping, and use of disk
file sections. In each case, operations and requirements that are
common to both private sections and global sections are described
first, followed by additional notes and requirements for the use of
global sections. Section 126.96.36.199 discusses global page-file sections.
188.8.131.52 Opening the Disk File
Before you can use a file as a section, you must open it using OpenVMS RMS. The following example shows the OpenVMS RMS file access block ($FAB) and $OPEN macros used to open the file and the channel specification to the SYS$CRMPSC system service necessary for reading an existing file:
The file options parameter (FOP) indicates that the file is to be opened for user I/O; this option is required so that OpenVMS RMS assigns the channel using the access mode of the caller. OpenVMS RMS returns the channel number on which the file is accessed; this channel number is specified as input to the SYS$CRMPSC system service (chan argument). The same channel number can be used for multiple create and map section operations.
The option RTV= -1 tells the file system to keep all of the pointers to be mapped in memory at all times. If this option is omitted, the SYS$CRMPSC service requests the file system to expand the pointer areas if necessary. Storage for these pointers is charged to the BYTLM quota, which means that opening a badly fragmented file can fail with an EXBYTLM failure status. Too many fragmented sections may cause the byte limit to be exceeded.
The file may be a new file that is to be created while it is in use as a section. In this case, use the $CREATE macro to open the file. If you are creating a new file, the file access block (FAB) for the file must specify an allocation quantity (ALQ parameter).
You can also use SYS$CREATE to open an existing file; if the file does not exist, it is created. The following example shows the required fields in the FAB for the conditional creation of a file:
When the $CREATE macro is invoked, it creates the file GLOBAL.TST if the file does not currently exist. The CBT (contiguous best try) option requests that, if possible, the file be contiguous. Although section files are not required to be contiguous, better performance can result if they are.
184.108.40.206 Defining the Section Characteristics
Table 12-3 shows the flag bits that must be set for specific characteristics on Alpha systems.
When you specify section characteristics, the following restrictions apply:
220.127.116.11 Defining Global Section Characteristics
If the section is a global section, you must assign a character string name (gsdnam argument) to it so that other processes can identify it when they map it. The format of this character string name is explained in Section 18.104.22.168.
Group global sections can be shared only by processes executing with the same group number. The name of a group global section is implicitly qualified by the group number of the process that created it. When other processes map it, their group numbers must match.
A temporary global section is automatically deleted when no processes are mapped to it, but a permanent global section remains in existence even when no processes are mapped to it. A permanent global section must be explicitly marked for deletion with the Delete Global Section (SYS$DGBLSC) system service.
You need the user privileges PRMGBL and SYSGBL to create permanent group global sections or system global sections (temporary or permanent), respectively.
A system global section is available to all processes in the system.
Optionally, a process creating a global section can specify a
protection mask (prot argument) to restrict all access
or a type of access (read, write, execute, delete) to other processes.
Translation of the gsdnam argument proceeds in the following manner:
For example, assume that you have made the following logical name assignment:
Your program contains the following statements:
The following logical name translation takes place:
There are three exceptions to the logical name translation method discussed in this section:
22.214.171.124 Mapping Sections
When you call the SYS$CRMPSC system service to create or map a section, or both, you must provide the service with a range of virtual addresses (inadr argument) into which the section is to be mapped.
On Alpha systems, the inadr argument specifies the size and location of the section by its start and end addresses. SYS$CRMPSC interprets the inadr argument in the following ways:
On Alpha systems, if you know specifically which pages the section should be mapped into, you provide these addresses in a 2-longword array. For example, to map a private section of 10 pages into virtual pages 10 through 19 of the program region, specify the input address array as follows:
On Alpha systems, the inadr argument range must have a lower address on an even page boundary and a higher address exactly one less than a page boundary. You do this to avoid programming errors that might arise because of incorrect programming assumptions about page sizes. For example, the range can be expressed as the following on an 8 KB page system:
0 ----> 1FFF
If the range is not expressed in terms of page-inclusive boundaries, then an SS$_INVARG condition value is returned.
You do not need to know the explicit addresses to provide an input address range. If you want the section mapped into the first available virtual address range in the program region (P0) or control region (P1), you can specify the SEC$M_EXPREG flag bit in the flags argument. In this case, the addresses specified by the inadr argument control whether the service finds the first available space in the P0 or P1. The value specified or defaulted for the pagcnt argument determines the amount of space mapped.
On Alpha systems, the relpag argument specifies the location in the section file at which you want mapping to begin.
On Alpha systems, the SYS$CRMPSC and SYS$MGBLSC system services map a minimum of one CPU-specific page. If the section file does not fill a single page, the remainder of the page is filled with zeros after faulting the page into memory. The extra space on the page should not be used by your application because only the data that fits into the section file will be written back to the disk.
The following example shows part of a program used to map a section at the current end of the program region:
The addresses specified do not have to be currently in the virtual address space of the process. The SYS$CRMPSC system service creates the required virtual address space during the mapping of the section. If you specify the retadr argument, the service returns the range of addresses actually mapped.
On Alpha systems, the starting retadr address should match inadr, plus relpag if specified. The ending (higher) address will be limited by the lower of:
After a section is mapped successfully, the image can refer to the pages using one of the following:
The following example shows part of a program used to create and map a process section on Alpha systems:
Notes on Example