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Kernel I/O Subsystem
I/O Scheduling
* Scheduling I/O requests can greatly increase overall efficiency. Importance can also play a part in request scheduling.
* The basic example is the scheduling of disk accesses
* Buffering and caching can also help, and can permit for more adaptable scheduling options.
* On systems with many devices, single request queues are often kept for each device:
Buffering
* Buffering of I/O is performed for (atleast) 3 major reasons:
1. Speed differences between two devices. A slow device may write data into a buffer, and when the buffer is complete, the entire buffer is sent to the fast device all at once. So that the slow device still has some place to write while this is going on, a second buffer is used, and the two buffers another as each becomes full. This is known as double buffering. (Double buffering is randomly used in (animated ) graphics, so that one screen image can be generated in a buffer while the other ( completed ) buffer is displayed on the screen. This avoids the user from ever seeing any half-finished screen images)
2. Data transfer size differences. Buffers are used in specific in networking systems to break messages up into smaller packets for transfer, and then for re-assembly at the receiving side.
3. To support copy semantics. For example, when an application makes a ask for a disk write, the data is copied from the user's memory area into a kernel buffer. Now the application can modify their copy of the data, but the data which eventually gets written out to disk is the
version of the data at the time the write
request was made.
Caching
* Caching includes keeping a duplicate of data in a faster-access location than where the data is normally stored.
* Buffering and caching are very equal, other than that a buffer may hold the only copy of a given data item, whereas a cache is just a xerox copy of some other data stored elsewhere.
* Buffering and caching go hand-in-hand, and random the same storage space may be used for both purposes. For example, after a buffer is written to disk, then the copy in memory can be used as a cached copy, (until that buffer is required for other purposes. )
Spooling and Device Reservation
* A spool ( Simultaneous Peripheral Operations On-Line ) buffers data for (peripheral ) devices such as printers that cannot support intermixed data streams.
* If many processes want to print at the similar time, they each send their print data to files stored in the spool directory. When each file is finished, then the application sees that print job as complete, and the print scheduler sends each file to the appropriate printer one at a time.
* Support is given for viewing the spool queues, removing jobs from the queues,
transferring jobs from one queue to another queue, and in some cases changing the priorities of jobs in the queues.
* Spool queues can be basic ( any laser printer ) or specific ( printer number 42. )
* OSes can also gives support for processes to request / get exclusive access to a particular device, and/or to wait until a device becomes available.
Error Handling
* I/O requests can fail for many reasons, either temporary ( buffers overflow ) or permanent ( disk crash ).
* I/O requests usually return an error bit (or more ) implicating the problem. UNIX systems also set the global variable errno to one of a hundred or so well-defined values to implies the specific error that has occurred.
* Some devices, such as SCSI devices, are capable of giving much more detailed
information about errors, and even keep an on-board error log that can be appeal by the host.
I/O Protection
* The I/O system must guard against either accidental or deliberate erroneous I/O.
* User applications are not permited to perform I/O in user mode - All I/O requests are handled through system calls that must be performed in kernel mode.
* Memory mapped areas and I/O ports must be kept safed by the memory management system, but access to these areas cannot be totally denied to user programs.Instead the memory protection system restricts access so that only one process at a time can access particular parts of memory, such as the portion of the screen memory corresponding to a particular window.
Kernel Data Structures
* The kernel keeps a number of important data structures pertaining to the I/O system, such as the open file table.
* These structures are object-oriented, and adaptable to allow access to a wide variety of I/O devices through a common interface.
* Windows NT takes the object-orientation one step further, executing I/O as a message-passing system from the source through various intermediaries to the device.
