HP OpenVMS Systems Documentation
OpenVMS Programming Concepts Manual
12.4.2 Input Address Arrays and Return Address Arrays
When the SYS$EXPREG system service adds pages to a region, it adds them in the normal direction of growth for the region. The return address array, if requested, indicates the order in which the pages were added. For example:
The addresses returned indicate the first byte in the first page that was added or deleted and the last byte in the last page that was added or deleted, respectively.
When input address arrays are specified for the Create and Delete Virtual Address Space (SYS$CRETVA and SYS$DELTVA, respectively) system services, these services add or delete pages beginning with the address specified in the first longword and ending with the address specified in the second longword.
On Alpha systems, the order in which the pages are added or deleted does not have to be in the normal direction of growth for the region. Moreover, because these services add or delete only whole pages, they ignore the low-order bits of the specified virtual address (the low-order bits contain the byte offset within the page). Table 12-1 shows the page size and byte offset.
Table 12-2 shows some sample virtual addresses in hexadecimal that may be specified as input to SYS$CRETVA or SYS$DELTVA and shows the return address arrays if all pages are successfully added or deleted. Table 12-2 assumes a page size of 8 KB = 2000 hex.
For SYS$CRETVA and SYS$DELTVA, note that if the input virtual addresses are the same, as in the fourth and fifth items in Table 12-2, a single page is added or deleted. The return address array indicates that the page was added or deleted in the normal direction of growth for the region.
Note that for SYS$CRMPSC and SYS$MGBLSC, which are discussed in Section 12.4.7, the sample virtual address arrays in Table 12-2 do not apply. The reason is that the lower address value has to be an even multiple of the machine page size; that is, it must be rounded down to an even multiple page size. In addition, the higher address value must be one less than the even multiple page size, representing the last byte on the last page. That is, it must be rounded up to an even multiple page size, minus 1.
The procedure for determining start and end virtual addresses is as follows:
12.4.3 Allocating Memory in Existing Virtual Address Space on Alpha Systems (Alpha Only)
On Alpha systems, if you reallocate memory that is already in its virtual address space by using the SYS$CRETVA system service, you may need to modify the values of the following arguments to SYS$CRETVA:
To determine whether you must modify the address as specified in
inadr, specify the optional retadr
argument to determine the exact boundaries of the memory allocated by
the call to SYS$CRETVA.
Each page in the virtual address space of a process is owned by the access mode that created the page. For example, pages in the program region that initially provided for the execution of an image are owned by user mode. Pages that the image creates dynamically are also owned by user mode. Pages in the control region, except for the pages containing the user stack, are normally owned by more privileged access modes.
Only the owner access mode or a more privileged access mode can delete the page or otherwise affect it. The owner of a page can also indicate, by means of a protection code, the type of access that each access mode will be allowed.
The Set Protection on Pages (SYS$SETPRT) system service changes the protection assigned to a page or group of pages. The protection is expressed as a code that indicates the specific type of access (none, read-only, read/write) for each of the four access modes (kernel, executive, supervisor, user). Only the owner access mode or a more privileged access mode can change the protection for a page.
When an image attempts to access a page that is protected against the access attempted, a hardware exception called an access violation occurs. When an image calls a memory management system service, the service probes the pages to be used to determine whether an access violation would occur if the image attempts to read or write one of the pages. If an access violation occurs, the service exits with the status code SS$_ACCVIO.
Because the memory management services add, delete, or modify a single page at a time, one or more pages can be successfully changed before an access violation is detected. If the retadr argument is specified in the service call, the service returns the addresses of pages changed (added, deleted, or modified) before the error. If no pages are affected, that is, if an access violation occurs on the first page specified, the service returns a -1 in both longwords of the return address array.
If the retadr argument is not specified, no
information is returned.
On Alpha systems, when a process is executing an image, a subset of its pages resides in physical memory; these pages are called the working set of the process. The working set includes pages in both the program region and the control region. The initial size of a process's working set is defined by the process's working set default (WSDEFAULT) quota, which is specified in pagelets. When ample physical memory is available, a process's working-set upper growth limit can be expanded to its working set extent (WSEXTENT).
When the image refers to a page that is not in memory, a page fault occurs, and the page is brought into memory, possibly replacing an existing page in the working set. If the page that is going to be replaced is modified during the execution of the image, that page is written into a paging file on disk. When this page is needed again, it is brought back into memory, again replacing a current page from the working set. This exchange of pages between physical memory and secondary storage is called paging.
The paging of a process's working set is transparent to the process. However, if a program is very large or if pages in the program image that are used often are being paged in and out frequently, the overhead required for paging may decrease the program's efficiency. The SYS$ADJWSL, SYS$PURGWS, and SYS$LKWSET system services allow a process, within limits, to counteract these potential problems.
The Adjust Working Set Limit (SYS$ADJWSL) system service increases or decreases the maximum number of pages that a process can have in its working set. The format for this routine is as follows:
On Alpha systems, use the pagcnt argument to specify the number of pagelets to add or subtract from the current working set size. The Alpha system rounds the specified number of pagelets to a multiple of the system's page size. The new working set size is returned in wsetlm in units of pagelets.
The Purge Working Set (SYS$PURGWS) system service removes one or more pages from the working set.
The Lock Pages in Working Set (SYS$LKWSET) system service makes one or more pages in the working set ineligible for paging by locking them in the working set. Once locked into the working set, those pages remain in the working set until they are unlocked explicitly with the Unlock Pages in Working Set (SYS$ULWSET) system service, or program execution ends. The format is as follows:
On Alpha systems, use the inadr argument to specify the range of addresses to be locked. SYS$LKWSET rounds the addresses to CPU-specific page boundaries, if necessary. The range of addresses of the pages actually locked are returned in the retadr argument.
However, because the Alpha system's instructions cannot contain full virtual addresses, the Alpha system's images must reference procedures and data indirectly through a pointer to a procedure descriptor. The procedure descriptor contains information about the procedure, including the actual code address. These pointers to procedure descriptors and data are collected into a program section called a linkage section. Therefore, it is not sufficient simply to lock a section of code into memory to improve performance. You must also lock the associated linkage section into the working set.
To lock the linkage section into memory, you must determine the start and end addresses that encompass the linkage section and pass these addresses as values in the inadr argument to a call to SYS$LKWSET. For more information about linking, see Migrating to an OpenVMS AXP System: Recompiling and Relinking Applications. Note that this manual has been archived but is available on the OpenVMS Documentation CD-ROM.
Use the acmode argument to specify the access mode to
be associated with the pages you want locked.
The operating system balances the needs of all the processes currently executing, providing each with the system resources it requires on an as-needed basis. The memory management routines balance the memory requirements of the process. Thus, the sum of the working sets for all processes currently in physical memory is called the balance set.
When a process whose working set is in memory becomes inactive---for example, to wait for an I/O request or to hibernate---the entire working set or part of it may be removed from memory to provide space for another process's working set to be brought in for execution. This removal from memory is called swapping.
The working set may be removed in two ways:
A privileged process may lock itself in the balance set. While pages can still be paged in and out of the working set, the process remains in memory even when it is inactive. To lock itself in the balance set, the process issues the Set Process Swap Mode (SYS$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$ULKPAG
A section is a disk file or a portion of a disk file containing data or instructions that can be brought into memory and made available to a process for manipulation and execution. A section can also be one or more consecutive page frames in physical memory or I/O space; such sections, which require you to specify page frame number (PFN) mapping, are discussed in Chapter 13, Section 184.108.40.206.
When modified pages in writable disk file sections are paged out of memory during image execution, they are written back into the section file rather than into the paging file, as is the normal case with files. (However, copy-on-reference sections are not written back into the section file.)
The use of disk file sections involves these two distinct operations:
The Create and Map Section (SYS$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 220.127.116.11 discusses global page-file sections.
18.104.22.168 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.