10.1 Fast I/O
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Fast I/O is a set of three system services that were developed as a $QIO alternative built for speed. 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 and Integrity servers. 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 VAX, Alpha, and Integrity server 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:
For direct I/O, avoid per-I/O buffer lockdown or unlocking.
For buffered I/O, avoid allocation and deallocation of a separate system buffer, because the user buffer is always addressable.
Complete Fast I/O operations at IPL 8, thereby avoiding the interrupt chaining usually required by the more general-purpose $QIO system service. For each I/O, this eliminates the IPL 4 IOPOST interrupt and a kernel AST.
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 percent for uniprocessor systems and 10 percent for multiprocessor systems.
The lockdown of user-process data structures is accomplished by buffer objects. A “buffer object” is process memory whose physical pages have been locked in memory and double-mapped into system space. 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, double-mapping into system space allows the OpenVMS system direct access to the process memory from system context.
To date, only the $QIO system service and the Fast I/O services have been changed to accept 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.
Two system services can be used to create and delete buffer objects, respectively, and can be called from any access mode. To create a buffer object, the $CREATE_BUFOBJ system service is called. This service expects as inputs an existing process memory range and returns a buffer handle for the buffer object. The buffer handle is an opaque identifier used to identify the buffer object on future I/O requests. The $DELETE_BUFOBJ system service is used to delete the buffer object and accepts as input the buffer handle. Although image rundown deletes all existing buffer objects, it is good form for the application to clean up properly.
A 64-bit equivalent version of the $CREATE_BUFOBJ system service ($CREATE_BUFOBJ_64) can be used to create buffer objects from the new 64-bit P2 or S2 regions. The $DELETE_BUFOBJ system service can be used to delete 32-bit or 64-bit buffer objects.
Buffer objects require system management. Because buffer objects tie up physical memory, extensive use of buffer objects requires system management planning. All the bytes of memory in the buffer object are deducted from a systemwide system parameter called MAXBOBMEM (maximum buffer object memory). System managers must set this parameter correctly for the application loads that run on their systems.
The MAXBOBMEM parameter defaults to 100 Alpha pages, but for applications with large buffer pools it will likely be set much larger. To prevent user-mode code from tying up excessive physical memory, user-mode callers of $CREATE_BUFOBJ must have a new system identifier, VMS$BUFFER_OBJECT_USER, assigned. This new identifier is automatically created in an OpenVMS Version 7.0 upgrade if the file SYS$SYSTEM:RIGHTSLIST.DAT is present. 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 is currently a restriction on the type of process memory that can be used for buffer objects. Global section memory cannot be made into a buffer object.
The precise definition of high-volume I/O operations optimized by Fast I/O services is important. I/O that does not comply with this definition either is not possible with the Fast I/O services or is not optimized. The characteristics of the high-volume I/O optimized by Fast I/O services can be seen by contrasting the operation of Fast I/O system services to the $QIO system service as follows:
The $QIO system service I/O status block (IOSB) is replaced by an I/O status area (IOSA) that is larger and quadword aligned. The transfer byte count returned in IOSA is 64 bits, and the field is aligned on a quadword boundary. Unlike the IOSB, which is optional, the IOSA is required.
User data buffers must be aligned to a 512-byte boundary.
All user process structures passed to the Fast I/O system services must reside in buffer objects. This includes the user data buffer and the IOSA.
Only transfers that are multiples of 512 bytes are supported.
Only the following function codes are supported: IO$_READVBLK, IO$_READLBLK, IO$_WRITEVBLK, and IO$_WRITELBLK.
Only I/O to disk and tape devices is optimized for performance.
No event flags are used with Fast I/O services. If application code must use an event flag in relation to a specific I/O, then the Event No Flag EFN (EFN$C_ENF) can be used. This event flag is a no-overhead EFN that can be used in situations when an EFN is required by a system service interface but has no meaning to an application.
For example, Fast I/O services do not use EFNs, so the application cannot specify a valid EFN associated with the I/O to the $SYNCH system service with which to synchronize I/O completion. To resolve this issue, the application can call the $SYNCH system service passing as arguments: EFN$C_ENF and the address of the appropriate IOSA. Specifying EFN$C_ENF signifies to $SYNCH that no EFN is involved in the synchronization of the I/O. Once the IOSA has been written with a status and byte count, return from the $SYNCH call occurs. The IOSA is now the central point of synchronization for a given Fast I/O (and is the only way to determine whether the asynchronous I/O is complete).
To minimize arguments passing overhead to these services, the $QIO parameters P3 through P6 are replaced by a single argument that is passed directly by the Fast I/O system services to device drivers. For disk-like devices, this argument is the media address (VBN or LBN) of the transfer. For drivers with complex parameters, this argument is the address of a descriptor or of a buffer specific to the device and function.
Segmented transfers are supported by Fast I/O but are not fully optimized. There are two major causes of segmented transfers. The first is disk fragmenting. While this can be an issue, it is assumed that sites seeking maximum performance have eliminated the overhead of segmenting I/O due to fragmentation.
A second cause of segmenting is issuing an I/O that exceeds the port's maximum limit for a single transfer. Transfers beyond the port maximum limit are segmented into several smaller transfers. Some ports limit transfers to 64KB. If the application limits its transfers to less than 64KB, this type of segmentation should not be a concern.
The three Fast I/O system services are:
$IO_SETUP—-Sets up an I/O
$IO_PERFORM[W]—-Performs an I/O request
$IO_CLEANUP—Cleans up an I/O request
A key concept behind the operation of the Fast I/O services is the file handle or fandle. A fandle is an opaque token that represents a “setup” I/O. A fandle is needed for each I/O outstanding from a process.
All possible setup, probing, and validation of arguments is performed off the mainline code path during application startup with calls to the $IO_SETUP system service. The I/O function, the AST address, the buffer object for the data buffer, and the IOSA buffer object are specified on input to $IO_SETUP service, and a fandle representing this setup is returned to the application.
To perform an I/O, the $IO_PERFORM system service is called, specifying the fandle, the channel, the data buffer address, the IOSA address, the length of the transfer, and the media address (VBN or LBN) of the transfer.
If the asynchronous version of this system service, $IO_PERFORM, is used to issue the I/O, then the application can wait for I/O completion using a $SYNCH specifying EFN$C_ENF and the appropriate IOSA. The synchronous form of the system service, $IO_PERFORMW, is used to issue an I/O and wait for it to complete. Optimum performance comes when the application uses AST completion; that is, the application does not issue an explicit wait for I/O completion.
To clean up a fandle, the fandle can be passed to the $IO_CLEANUP system service.
Modifying an application to use the Fast I/O services requires a few source-code changes. For example:
A programmer adds code to create buffer objects for the IOSAs and data buffers.
The programmer changes the application to use the Fast I/O services. Not all $QIOs need to be converted. Only high-volume read/write I/O requests should be changed.
A simple example is a “database writer” program, which writes modified pages back to the database. Suppose the writer can handle up to 16 simultaneous writes. At application startup, the programmer would add code to create 16 fandles by 16 $IO_SETUP system service calls.
In the main processing loop within the database writer program, the programmer replaces the $QIO calls with $IO_PERFORM calls. Each $IO_PERFORM call uses one of the 16 available fandles. While the I/O is in progress, the selected fandle is unavailable for use with other I/O requests. The database writer is probably using AST completion and recycling fandle, data buffer, and IOSA once the completion AST arrives.
If the database writer routine cannot return until all dirty buffers are written (that is, it must wait for all I/O completions), then $IO_PERFORMW can be used. Alternatively $IO_PERFORM calls can be followed by $SYNCH system service calls passing the EFN$C_ENF argument to await I/O completions.
The database writer runs faster and scale better because I/O requests now use less CPU time.
When the application exits, an $IO_CLEANUP system service call is done for each fandle returned by a prior $IO_SETUP system service call. Then the buffer objects are deleted. Image rundown performs fandle and buffer object cleanup on behalf of the application, but it is good form for the application to clean up properly.
The central point of synchronization for a given Fast I/O is its IOSA. The IOSA replaces the $QIO system service's IOSB argument. Larger than the IOSB argument, the byte count field in the IOSA is 64 bits and quadword aligned. Unlike the $QIO system service, Fast I/O services require the caller to supply an IOSA and require the IOSA to be part of a buffer object.
The IOSA context field can be used in place of the $QIO system service ASTPRM argument. The $QIO ASTPRM argument is typically used to pass a pointer back to the application on the completion AST to locate the user context needed for resuming a stalled user-thread; however, for the $IO_PERFORM system service, the ASTPRM on the completion AST is always the IOSA. Because there is no user-settable ASTPRM, an application can store a pointer to the user-thread context for this I/O in the IOSA context field and retrieve the pointer from the IOSA in the completion AST. )
The $IO_SETUP system service performs the setup of an I/O and returns a unique identifier for this setup I/O, called a fandle, to be used on future I/Os. The $IO_SETUP arguments used to create a given fandle remain fixed throughout the life of the fandle. This has implications for the number of fandles needed in an application. For example, a single fandle can be used only for reads or only for writes. If an application module has up to 16 simultaneous reads or writes pending, then potentially 32 fandles are needed to avoid any $IO_SETUP calls during mainline processing.
The $IO_SETUP system service supports an expedite flag, which is available to boost the priority of an I/O among the other I/O requests that have been handed off to the controller. Unrestrained use of this argument is useless, because if all I/O is expedited, nothing is expedited. Note that this flag requires the use of ALTPRI and PHY_IO privilege.
The $IO_PERFORM[W] system service accepts a fandle and five other variable I/O parameters for the high-performance I/O operation. The fandle remains in use to the application until the $IO_PERFORMW returns or if $IO_PERFORM is used until a completion AST arrives.
The CHAN argument to the fandle contains the data channel returned to the application by a previous file operation. This argument allows the application the flexibility of using the same fandle for different open files on successive I/Os; however, if the fandle is used repeatedly for the same file or channel, then an internal optimization with $IO_PERFORM is taken.
Note that $IO_PERFORM was designed to have no more than six arguments to take advantage of the HP OpenVMS Calling Standard, which specifies that calls with up to six arguments can be passed entirely in registers.
Because $IO_PERFORM supports only four function codes, this system service does not use the generalized function decision table (FDT) dispatching that is contained in the $QIO system service. Instead, $IO_PERFORM uses a single vector in the driver dispatch table called DDT$PS_FAST_FDT for the four supported functions. The DDT$PS_FAST_FDT field is a FDT routine vector that indicates whether the device driver called by $IO_PERFORM is set up to handle Fast I/O operations. A nonzero value for this field indicates that the device driver supports Fast I/O operations and that the I/O can be fully optimized.
If the DDT$PS_FAST_FDT field is zero, then the driver is not set up to handle Fast I/O operations. The $IO_PERFORM system service tolerates such device drivers, but the I/O is only slightly optimized in this circumstance.
The OpenVMS disk and tape drivers that ship as part of OpenVMS Version 7.0 have added the following line to their driver dispatch table (DDTAB) macro:
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This line initializes the DDT$PS_FAST_FDT field to the address of the standard Fast I/O FDT routine, ACP_STD$FASTIO_BLOCK.
If you have a disk or tape device driver that can handle Fast I/O operations, you can add this DDTAB macro line to your driver. If you cannot use the standard Fast I/O FDT routine, ACP_STD$FASTIO_BLOCK, you can develop your own based on the model presented in this routine.