HP OpenVMS Systems Documentation
OpenVMS Programming Concepts Manual
23.20.1 Mailbox Name
Translation of the lognam argument proceeds as follows:
For example, assume that you have made the following logical name assignment:
Assume also that your program contains the following statements:
The following logical name translation takes place:
Because further translation is unsuccessful, the logical name CHKPNT_001 is created with the equivalence name MBAn (n is a number assigned by the system).
There are two exceptions to the logical name translation method discussed in this section:
23.20.2 System Mailboxes
The system uses mailboxes for communication among system processes. All system mailbox messages contain, in the first word of the message, a constant that identifies the sender of the message. These constants have symbolic names (defined in the $MSGDEF macro) in the following format:
The remainder of the message contains variable information, depending on the system component that is sending the message.
The format of the variable information for each message type is
documented with the system function that uses the mailbox.
When a process creates another process, it can specify the unit number of a mailbox as an argument to the Create Process ($CREPRC) system service. When you delete the created process, the system sends a message to the specified termination mailbox.
You cannot use a mailbox in memory shared by multiple processors as a
process termination mailbox.
In the following Fortran example, the first program, SEND.FOR, creates a mailbox named MAIL_BOX, writes data to it, and then indicates the end of the data by writing an end-of-file message.
The second program, RECEIVE.FOR, creates a mailbox with the same logical name, MAIL_BOX. It reads the messages from the mailbox into an array. It stops the read operations when a read operation generates an end-of-file message and the second longword of the I/O status block is nonzero. By checking that the I/O status block is nonzero, the second program confirms that the writing process sent the end-of-file message.
The processes use common event flag number 64 to ensure that SEND.FOR does not exit until RECEIVE.FOR has established a channel to the mailbox. (If RECEIVE.FOR executes first, an error occurs because SYS$ASSIGN cannot find the mailbox.)
23.22 Fast I/O and Fast Path Features (Alpha Only)
Fast I/O and Fast Path are two optional features that can provide improved I/O performance. Performance improvement is achieved by reducing the CPU cost per I/O request, and improving symmetric multiprocessing (SMP) scaling of I/O operations. The CPU cost per I/O is reduced by optimizing code for high-volume I/O and by using better SMP CPU memory cache. SMP scaling of I/O is increased by reducing the number of spinlocks taken per I/O and by substituting finer-granularity spinlocks for global spinlocks.
The improvements follow a division that already exists between the device-independent and device-dependent layers in the OpenVMS I/O subsystem. The device-independent overhead is addressed by Fast I/O, which is a set of system services that can substitute for certain $QIO operations. Using these services requires some coding changes in existing applications, but the changes are usually modest and well contained. The device-dependent overhead is addressed by Fast Path, which is an optional performance feature that creates a "fast path" to the device. It requires no application changes.
Fast I/O and Fast Path can be used independently. However, together
they can provide a reduction in CPU cost per I/O on uniprocessor and on
Fast I/O is a set of three system services, SYS$IO_SETUP, SYS$IO_PERFORM, and SYS$IO_CLEANUP, that were developed as an alternative to $QIO. These services are not a $QIO replacement; $QIO is unchanged, and $QIO interoperation with these services is fully supported. Rather, the services substitute for a subset of $QIO operations, namely, only the high-volume read/write I/O requests.
The Fast I/O services support 64-bit addresses for data transfers to and from disk and tape devices.
While Fast I/O services are available on OpenVMS VAX, the performance
advantage applies only to OpenVMS Alpha. OpenVMS VAX has a run-time
library (RTL) compatibility package that translates the Fast I/O
service requests to $QIO system service requests, so one set of source
code can be used on both VAX and Alpha systems.
The performance benefits of Fast I/O result from streamlining high-volume I/O requests. The Fast I/O system service interfaces are optimized to avoid the overhead of general-purpose services. For example, I/O request packets (IRPs) are now permanently allocated and used repeatedly for I/O rather than allocated and deallocated anew for each I/O.
The greatest benefits stem from having user data buffers and user I/O status structures permanently locked down and mapped using system space. This allows Fast I/O to do the following:
In total, Fast I/O services eliminate four spinlock acquisitions per
I/O (two for the MMG spinlock and two for the SCHED spinlock). The
reduction in CPU cost per I/O is 20% for uniprocessor systems and 10%
for multiprocessor systems.
Buffer objects accomplish the lockdown of user-process data structures. Buffer objects are process entities that are associated with a process's virtual address range. When a buffer object is created, all its physical pages in its address range are locked in memory and can be double-mapped into system space. These locked pages in a process's address range cannot be freed until the buffer object has been deleted. The Fast I/O environment uses this feature by locking the buffer object itself during $IO_SETUP. This prevents the buffer object and its associated pages from being deleted. The buffer object is unlocked during $IO_CLEANUP, or at image rundown. After creating a buffer object, the process remains fully pageable and swappable and the process retains normal virtual memory access to its pages in the buffer object.
If the buffer object contains process data structures to be passed to an OpenVMS system service, the OpenVMS system can use the buffer object to avoid any probing, lockdown, and unlocking overhead associated with these process data structures. Additionally, if the buffer object has performed double-mapping into system space, this allows the OpenVMS system direct access to the process memory from system context.
To date, only the Fast I/O services are supported with buffer objects. For example, a buffer object allows a programmer to eliminate I/O memory management overhead. On each I/O, each page of a user data buffer is probed and then locked down on I/O initiation and unlocked on I/O completion. Instead of incurring this overhead for each I/O, it can be done once at buffer object creation time. Subsequent I/O operations involving the buffer object can completely avoid this memory management overhead.
The system space window buffer object allows several I/O related tasks to be performed entirely from system context at high IPL, without having to assume process context. When a buffer object is created, the system maps by default a section of system space (S2) to process pages associated with the buffer object. This protected system space window allows read and write access only from kernel mode. Because all of system space is equally accessible from within any context, it is now possible to avoid the context switch to assume the original user's process context. Optionally, the system space window can be in S0/S1 space, or it can be suppressed.
Two system services are used to create and delete buffer objects: SYS$CREATE_BUFOBJ_64 and SYS$DELETE_BUFOBJ. Both services can be called from any access mode. To create a buffer object, the SYS$CREATE_BUFOBJ_64 system service is called. This service expects as inputs an existing process memory range and returns a handle for the buffer object. The handle is an opaque identifier used to identify the buffer object on future requests. The SYS$DELETE_BUFOBJ system service is used to delete the buffer object and accepts as input the handle. Although image rundown deletes all existing buffer objects, it is good practice for the application to clean up properly.
Buffer objects require system management. Because buffer objects tie up physical memory, extensive use of buffer objects require system management planning. All the bytes of memory in the buffer object are deducted from the systemwide SYSGEN parameter MAXBOBMEM (maximum buffer object memory). System managers must set this parameter correctly for the application loads that run on their systems. Additionally, two other SYSGEN parameters MAXBOBS0S1 and MAXBOBS2 are available for system managers. MAXBOBS0S1 and MAXBOBS2, however, are now regarded as obsolete system parameters. Initially, the MAXBOBS0S1 and MAXBOBS2 parameters were intended to ensure that users could not adversely affect the system by creating hugh buffer objects. But as users began to use buffer objects more widely, managing the combination of these parameters proved to be too complex.
Now, users who want to create buffer objects must either hold the VMS$BUFFER_OBJECT_USER identifier or execute in executive or kernel mode. Therefore, these users are considered privileged applications, and the additional safeguard that these parameters provided is unnecessary.
To determine current usage of system memory resources, enter the following command:
Table 23-5 shows these three parameters and their meanings.
The MAXBOBMEM, MAXBOBS0S1, and MAXBOBS2 parameters default to 100 Alpha pages, but for applications with large buffer pools it can be set much larger. To prevent user-mode code from tying up excessive physical memory, user-mode callers of $CREATE_BUFOBJ_64 must have a new system identifier, VMS$BUFFER_OBJECT_USER, assigned. The system manager can assign this identifier with the DCL command SET ACL command to a protected subsystem or application that creates buffer objects from user mode. It may also be appropriate to grant the identifier to a particular user with the Authorize utility command GRANT/IDENTIFIER, for example, to a programmer who is working on a development system.
There are several buffer object restrictions which are listed as follows:
For complete information about using Fast I/O, the Fast I/O system services, and the buffer objects system services that are in the following list, see the OpenVMS I/O User's Reference Manual, and the OpenVMS System Services Reference Manual: A--GETUAI and the OpenVMS System Services Reference Manual: GETUTC--Z: