Device Independent I/O Software(see Fig. 3)

Figure 3: (a) Without a standard driver interface (b) With a standard driver interface.

• There is commonality between drivers of similar classes
• Divide I/O software into device-dependent and device-independent I/O software
• Device independent software includes
  • Buffer or Buffer-cache management
  • Managing access to dedicated devices
  • Error reporting
• Driver $\Leftrightarrow$ Kernel Interface; Major Issue is uniform interfaces to devices and kernel
  • Uniform device interface for kernel code
    • Allows differents devices to be used the same way
      • No need to rewrite filesystem to switch between SCSI, IDE or RAM disk
    • Allows internal changes to device driver with fear of breaking kernel code
  • Uniform kernel interface for device code
    • Drivers use a defined interface to kernel services (e.g. kmalloc, install IRQ handler, etc.)
    • Allows kernel to evolve without breaking existing drivers
  • Together both uniform interfaces avoid a lot of programming implementing new interfaces

  Figure 4: (a) Unbuffered input (b) Buffering in user space (c) Single buffering in the kernel followed by copying to user space (d) Double buffering in the kernel.

  
• No Buffering (see Fig. 4)
  • Process must read/write a device a byte/word at a time
  • Each individual system call adds significant overhead
  • Process must what until each I/O is complete
    • Blocking/interrupt/waking adds to overhead.
    • Many short runs of a process is inefficient (poor CPU cache temporal locality)
• User-level Buffering (see Fig. 4)
  • Process specifies a memory buffer that incoming data is placed in until it fills
    • Filling can be done by interrupt service routine
    • Only a single system call, and block/wakeup per data buffer; Much more efficient
  • Issues
    • What happens if buffer is paged out to disk
      • Could lose data while buffer is paged in
      • Could lock buffer in memory (needed for DMA), however many processes doing I/O reduce RAM available for paging. Can cause deadlock as RAM is limited resource
    • Consider write case, When is buffer available for re-use?
      • Either process must block until potential slow device drains buffer
      • or deal with asynchronous signals indicating buffer drained
• Single Buffer (see Fig. 4)
  • Operating system assigns a buffer in main memory for an I/O request
  • Stream-oriented
    • Used a line at time
    • User input from a terminal is one line at a time with carriage return signaling the end of the line
    • Output to the terminal is one line at a time
  • Block-oriented
    • Input transfers made to buffer
    • Block moved to user space when needed
    • Another block is moved into the buffer; Read ahead
    • User process can process one block of data while next block is read in
    • Swapping can occur since input is taking place in system memory, not user memory
    • Operating system keeps track of assignment of system buffers to user processes
  • What happens if kernel buffer is full, the user buffer is swapped out, and more data is received??? We start to lose characters or drop network packets
• Double Buffer (see Fig. 4)
  • Use two system buffers instead of one
  • A process can transfer data to or from one buffer while the operating system empties or fills the other buffer
  • May be insufficient for really bursty traffic
    • Lots of application writes between long periods of computation
    • Long periods of application computation while receiving data
    • Might want to read-ahead more than a single block for disk
• Notice that buffering, double buffering are all Bounded-Buffer Producer-Consumer Problems
• Buffering in Fast Networks (see Fig. 5)

  Figure 5: Networking may involve many copies.

  
  • Copying reduces performance; Especially if copy costs are similar to or greater than computation or transfer costs
  • Super-fast networks put significant effort into achieving zero-copy
  • Buffering also increases latency