HP OpenVMS Systems Documentation
OpenVMS Programming Concepts Manual
18.104.22.168 Mapping Global Sections
When a process maps a global section, it must specify the global section name assigned to the section when it was created, whether it is a group or system global section, and whether it desires read-only or read/write access. The process may also specify the following:
On Alpha systems, you should specify the retadr argument to determine the exact boundaries of the memory that was mapped by the call. If your application specifies the relpag argument, you must specify the retadr argument. In this case, it is not an optional argument.
Cooperating processes can both issue a SYS$CRMPSC system service to create and map the same global section. The first process to call the service actually creates the global section; subsequent attempts to create and map the section result only in mapping the section for the caller. The successful return status code SS$_CREATED indicates that the section did not already exist when the SYS$CRMPSC system service was called. If the section did exist, the status code SS$_NORMAL is returned.
Global page-file sections are used to store temporary data in a global section. A global page-file section is a section of virtual memory that is not mapped to a file. The section can be deleted when processes have finished with it. (Contrast this with demand-zero section file pages where no initialization is necessary, but the pages are saved in a file.) The system parameter GBLPAGFIL controls the total number of global page-file pages in the system.
To create a global page-file section, you must set the flag bits SEC$M_GBL and SEC$M_PAGFIL in the flags argument to the Create and Map Section (SYS$CRMPSC) system service. The channel (chan argument) must be 0.
You cannot specify the flag bit SEC$M_CRF with the flag bit
On Alpha systems, SYS$CRMPSC and SYS$MGBLSC interpret some of the arguments differently than on VAX systems if you are mapping a section into a defined area of virtual address space. The differences are as follows:
22.214.171.124 Mapping from an Offset into a Section File (Alpha Only)
On Alpha systems, you can map a portion of a section file by specifying the address at which to start the mapping as an offset from the beginning of the section file. You specify this offset by supplying a value to the relpag argument of SYS$CRMPSC. The value of the relpag argument specifies the pagelet number relative to the beginning of the file at which the mapping should begin.
To preserve compatibility, SYS$CRMPSC interprets the value of the relpag argument in 512-byte units on both VAX systems and Alpha systems. However, because the CPU-specific page size on the Alpha system is larger than 512 bytes, the address specified by the offset in the relpag argument probably does not fall on a CPU-specific page boundary on an Alpha system. SYS$CRMPSC can map virtual memory in CPU-specific page increments only. Therefore, on Alpha systems, the mapping of the section file will start at the beginning of the CPU-specific page that contains the offset address, not at the address specified by the offset.
If you map from an offset into a section file, you must still provide
an inadr argument that abides by the requirements
presented in Section 126.96.36.199 when mapping into a defined address range.
The first time an image executing in a process refers to a page that was created during the mapping of a disk file section, the page is copied into physical memory. The address of the page in the virtual address space of a process is mapped to the physical page. During the execution of the image, normal paging can occur; however, pages in sections are not written into the page file when they are paged out, as is the normal case. Rather, if they have been modified, they are written back into the section file on disk. The next time a page fault occurs for the page, the page is brought back from the section file.
If the pages in a section were defined as demand-zero pages or copy-on-reference pages when the section was created, the pages are treated differently, as follows:
In the case of global sections, more than one process can be mapped to the same physical pages. If these pages need to be paged out or written back to the disk file defined as the section, these operations are done only when the pages are not in the working set of any process.
In the following example for Alpha systems, process ORION creates a global section and process CYGNUS maps to that section:
188.8.131.52 Reading and Writing Data Sections
The sharing of global sections may involve application-dependent synchronization techniques. For example, one process can create and map to a global section in read/write fashion; other processes can map to it in read-only fashion and interpret data written by the first process. Alternatively, two or more processes can write to the section concurrently. (In this case, the application must provide the necessary synchronization and protection.)
After data in a process private section is modified, the process can release (or unmap) the section. The modified pages are then written back into the disk file defined as a section.
After data in a global section is modified, the process or processes can release (or unmap) the section. The modified pages are still maintained in memory until the section is deleted. The data is then written back into the disk file defined as a section. Applications relying on modified data to be in the file at a specific point in time must use the SYS$UPDSEC(W) system service to force the write action. See Section 184.108.40.206.
When the section is deleted, the revision number of the file is
incremented, and the version number of the file remains unchanged. A
full directory listing indicates the revision number of the file and
the date and time that the file was last updated.
A process unmaps a section by deleting the virtual addresses in its own virtual address space to which it has mapped the section. If a return address range was specified to receive the virtual addresses of the mapped pages, this address range can be used as input to the Delete Virtual Address Space (SYS$DELTVA) system service, as follows:
When a process unmaps a private section, the section is deleted; that is, all control information maintained by the system is deleted. A temporary global section is deleted when all processes that have mapped to it have unmapped it. Permanent global sections are not deleted until they are specifically marked for deletion with the Delete Global Section (SYS$DGBLSC) system service; they are then deleted when no more processes are mapped.
Note that deleting the pages occupied by a section does not delete the section file, but rather cancels the process's association with the file. Moreover, when a process deletes pages mapped to a process private read/write section, all modified pages are written back into the section file. For global sections, the system's modified page writer starts writing back modified pages when the section is deleted and all mapping processes have deleted their associated virtual address space. Applications relying on modified data to be in the file at a specific point in time must use the SYS$UPDSEC(W) system service to force the write action. See Section 220.127.116.11.
After a process private section is deleted, the channel assigned to it can be deassigned. The process that created the section can deassign the channel with the Deassign I/O Channel (SYS$DASSGN) system service, as follows:
For global sections, the channel is only used to identify the file to
the system. The system then assigns a different channel to use for
future paging I/O to the file. The used assigned channel can be deleted
immediately after the global section is created.
Because read/write sections are not normally updated on disk until either the physical pages they occupy are paged out, or until the section is deleted, a process should ensure that all modified pages are successfully written back into the section file at regular intervals.
The Update Section File on Disk (SYS$UPDSEC) system service writes the
modified pages in a section into the disk file. The SYS$UPDSEC system
service is described in the OpenVMS System Services Reference Manual.
Memory-resident global sections allow a database server to keep larger amounts of currently used data cached in physical memory. The database server then accesses the data directly from physical memory without performing I/O read operations from the database files on disk. With faster access to the data in physical memory, runtime performance increases dramatically.
Memory-resident global sections are non-file-backed global sections. Pages within a memory-resident global section are not backed by the pagefile or by any other file on disk. Thus, no pagefile quota is charged to any process or to the system. When a process maps to a memory-resident global section and references the pages, working set list entries are not created for the pages. No working set quota is charged to the process.
For details on how to create and identify shareable images and how to
link them with private object modules, see the OpenVMS Linker Utility Manual. For
information about how to install shareable images and make them
available for sharing as global sections, see the OpenVMS System Manager's Manual.
A page frame section is one or more contiguous pages of physical memory or I/O space that have been mapped as a section. One use of page frame sections is to map to an I/O page, thus allowing a process to read device registers.
A page frame section differs from a disk file section in that it is not associated with a particular disk file and is not paged. However, it is similar to a disk file section in most other respects: you create, map, and define the extent and characteristics of a page frame section in essentially the same manner as you do for a disk file section.
To create a page frame section, you must specify page frame number (PFN) mapping by setting the SEC$M_PFNMAP flag bit in the flags argument to the Create and Map Section (SYS$CRMPSC) system service. The vbn argument is now used to specify that the first page frame is to be mapped instead of the first virtual block. You must have the user privilege PFNMAP to either create or delete a page frame section but not to map to an existing one.
Because a page frame section is not associated with a disk file, you do not use the chan, and pfc arguments to the SYS$CRMPSC service to create or map this type of section. For the same reason, the SEC$M_CRF (copy-on-reference) and SEC$M_DZRO (demand-zero) bit settings in the flags argument do not apply. Pages in page frame sections are not written back to any disk file (including the paging file). The pagcnt and relpag arguments are in units of CPU-specific pages for page frame sections.
18.104.22.168 Partial Sections (Alpha Only)
On Alpha systems, a partial section is one where not all of the defined section, whether private or global, is entirely backed up by disk blocks. In other words, a partial section is where a disk file does not map completely onto an Alpha system page.
For example, suppose a file for which you wish to create a section consists of 17 virtual blocks on disk. To map this section, you would need two whole Alpha 8 KB pages, the smallest size Alpha page available. The first Alpha page would map the first 16 blocks of the section, and the second Alpha page would map the 17th block of the section. (A block on disk is 512 bytes, same as on OpenVMS VAX.) This results in 15/16ths of the second Alpha page not being backed up by the section file. This is called a partial section because the second Alpha page of the section is only partially backed up.
When the partial page is faulted in, a disk read is issued for only as many blocks as actually back up that page, which in this case is 1. When that page is written back, only the one block is actually written.
If the upper portion of the second Alpha page is used, it is done so at
some risk, because only the first block of that page is saved on a
write-back operation. This upper portion of the second Alpha page is
not really useful space to the programmer, because it is discarded
during page faulting.
In the following example, two programs are communicating through a global section. The first program creates and maps a global section (by using SYS$CRMPSC) and then writes a device name to the section. This program also defines the device terminal and process names and sets the event flags that synchronize the processes.
The second program maps the section (by using SYS$MGBLSC) and then reads the device name and the process that allocated the device and any terminal allocated to that process. This program also writes the process named to the terminal global section where the process name can be read by the first program.
The common event cluster is used to synchronize access to the global section. The first program sets REQ_FLAG to indicate that the device name is in the section. The second program sets INFO_FLAG to indicate that the process and terminal names are available.
Data in a section must be page aligned. The following is the option file used at link time that causes the data in the common area named DATA to be page aligned: PSECT_ATTR = DATA, PAGE
For high-level language usage, use the solitary attribute of the linker. See the OpenVMS Linker Utility Manual for an explanation of how to use the solitary attribute. The address range requested for a section must end on a page boundary, so SYS$GETSYI is used to obtain the system page size.
Before executing the first program, you need to write a user-open routine that sets the user-open bit (FAB$V_UFO) of the FAB options longword (FAB$L_FOP). Because the Fortran OPEN statement specifies that the file is new, you should use $CREATE to open it rather than $OPEN. No $CONNECT should be issued. The user-open routine reads the channel number that the file is opened on from the status longword (FAB$L_STV) and returns that channel number to the main program by using a common block (CHANNEL in this example).