Input and Output IO

1
Input and Output (IO)

• In addition to processing the job (using CPU and
  memory), IO is the main job of a computer system
• Input and output are typically carried out by
  peripheral devices, managed by the IO subsystem
  of the kernel
• Every device has a corresponding device driver
  that manages the interface (IF) between device
  and kernel
• All device drivers are typically implemented with
  a single interface that the kernel understands
  (syscalls?)
• Operations open, close, initialize, read, write
• Typically the device manufacturer provides the IF

2
Hardware
CPU
monitor
cache
cache
graphics controller
system bus
IDE Disk Controller
...
disk 1
disk n
3
Communicating with a Device

• The device driver (DD) has to communicate with
  the device to give it commands and receive
  feedback
• So-called IO-instructions read/write data to an
  IO-port
• usually a register communicates directly with the
  device (i.e., whenever a write is done on an IO
  port, the device subsequently receives the data)
• Another way of doing it is using memory-mapped IO
• same idea, but the communication is done through
  specific (predefined) memory locations
• Some systems use both (for example, a monitor has
  IO-ports for control and memory-mapped IO for
  large data transfers)

4
Communication (cont)

• Two ways of CPU-device communication
• Polling the device is asked (polled) whether it
  has data1- host checks the status register for
  that device2- host tells the device to go
  ahead3- device writes data/command to the
  IO-port
• allows CPU to control how/when it interacts with
  device, but large overhead if it has to poll
  repeatedly
• Interrupts the device takes the initiative1-
  the device interrupts CPU to pass on
  data/commands2- CPU does a context switch to
  process the interrupt3- CPU returns from
  interrupt and resumes process
• CPU doesnt waste time polling, but has no
  control of when device will interrupt

5
Real-Time Systems Device Communication

• In real-time systems (RTSs), due to deadlines,
  devices are usually not allowed to interrupt when
  they want
• a mal-functioning device could bring down the
  system
• In RTSs, many tasks are periodic (reading
  sensors, sending commands to actuators)
• If polling is used, need to guarantee that IO
  will be done in a timely fashion
• If interrupts are used, need to guarantee that
  they will not violate the deadline guarantees
  given to processes

6
Handling Interrupts

• Interrupt controller hardware provides ability to
  
• defer interrupts
• call the proper interrupt service routine (ISR)
• distinguish and prioritize between high- and
  low-priority interrupts
• In addition, processors can (and do!) mask
  interrupts
• disallow some interrupts to occur to process
  other stuff
• however, there is the non-maskable interrupt
  (emergency)

7
More on Interrupts

• Interrupt vectors are very common keep the
  address of the ISR in a fixed location, the
  hardware will read the number of the interrupt
  and jump to the appropriate location
• The interrupt handler will not have its own
  stack. However, it will execute on top of the
  kernel stack, not in the user stack
• This is because there is little control over the
  size of the user stack and the ISR may run out of
  memory

8
Interacting with Devices

• Sometimes a mixture of interrupts and IO using
  direct memory access (DMA) is beneficial.
• A disk that has to read large data block to
  position X
• receives the request, reads the data
• asks the DMA controller to put it in the memory
  location X
• and then, finally, interrupts the CPU
• The DMA controller and the disk exchange
  information (a protocol) to be able to do this
  transfer
• In some architectures this is called cycle
  stealing, since the bus is used to transfer data
  into the memory and thus the CPU cannot use the
  memory in those cycles

9
Kernel IO Subsystem

• The users want to use the devices. How to
  achieve it?
• The user uses a library that invokessystem calls
  in the OS.
• The system calls get translated intodevice
  driver requests
• The DDs request service from the IOcontrollers,
  who talk to the device
• When the service is done, the controller sends an
  interrupt, to signal CPU

10
Disk as a case study

• A disk drive has several physical components
• spindle
• surface (one side in the pack)
• read/write arm and head
• cylinder (or track)
• sector

11
Accessing the Disk

• A disk is accessed through the library, file
  system, DevDriver and disk controller
• The calls to the controller are called disk drive
  commands, such as drive select, head select,
  direction, read/write, data out, etc
• The disk controller does the synchronization
  between disk and OS, signaling/timing, some error
  control
• Several users may request data to/from the disk
  at once
• The file system is the first entity to recognize
  it and synchronize access to the disk

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Accessing the Disk (cont)

• The FS does the scheduling of the requests, that
  is, it determines the order in which the requests
  are serviced
• The FS and DD also use buffering for
  synchronization (support speed/data size mismatch
  between OS-device)
• The user, FS, and DD have different ideas
  (abstraction) of how the file looks like.
• user contiguous space, byte by byte
• file system blocks of data, with logical
  addresses
• DD sectors in disk, in specific hardware
  addresses (typically consists of
  sect

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Accessing the Disk (cont)

• The translations take place each time a new
  interface is crossed
• The file-relative logical address
  
• The volume-relative logical address
  
• The drive-relative physical address
  
• Some disks have also a zone, and the physical
  address becomes
• This is because the length of outer tracks
  /cylinders compared with inner tracks/cylinders
  (density)

14
Accessing the Disk (cont)

• When we need to access a sector, the disk head
  needs to be positioned over that sector (or more
  accurately, the disk must rotate until the sector
  is under the head)
• For that, several delays are involved in the disk
  operation, in decreasing time
• seek delay position HEAD on correct
  track/cylinder
• rotational delay position correct SECTOR under
  head
• transfer delay transfer data to/from memory
• Therefore, it is important to minimize the
  highest delay, namely, the seek delay

15
Improve Disk Speed

• Increase buffering in the device driver and/or
  disk controller (this is a type of cache)
• Reduce rotational delay
• place blocks of the same file in the same
  cylinder
• read the blocks in the appropriate order
• allow block interleaving (clearly, the number of
  sectors in the disk should be odd,unlike the
  drawing)

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4
2
3
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Interleaving

• Interleaving is done in the device driver, since
  it will have to tell the controller where to
  place sectors in the disk. Clearly, interleaving
  is tightly coupled with the device itself
• The device decides the interleaving degree (how
  many sectors to skip over), which is tightly
  coupled with the speeds of the disk
• Rotational
• Seek (or arm speed)
• Transfer

17
Improve Disk Speed (cont)

• To decrease seek delays one can
• increase the number of heads
• park the head in the middle track of the disk to
  decrease the average seek delay
• place the data in the appropriate locations
  (tracks)
• The organ pipe distribution does the last trick
• create a histogram of the disk block usage (count
  the number of times that disk blocks are used)
• place most used blocks in the middle track

spindle
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Improve Disk Speed (cont)

• RAID Redundant Array of Inexpensive Disks
• Allows for more parallelism when retrieving data
• store each part of a block in a separate disk,
  and allow all sub-blocks to be retrieved in
  parallel
• If disks are homogeneous and synchronized, even
  better performance can be achieved
• If disks are heterogeneous or not synchronized,
  performance is worse since it depends on the
  slowest of the disks, or the one off phase.
• can use slower, cheaper disks

19
Possible File Structures

• None - sequence of words or bytes
• Simple record structure
• Lines
• Fixed length
• Variable length
• Complex Structures
• Formatted document
• Relocatable load file
• Can simulate last two with first method by
  inserting appropriate control characters.
• Who decides
• Operating system
• Program

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File Attributes

• Name only data kept in human-readable form.
• Type needed for supporting different file
  types.
• Location pointer to file location on device.
• Size current file size.
• Protection controls who can do reading,
  writing, executing, access, etc.
• Time, date, and user identification data for
  protection, security, and usage monitoring.
• Information about files are kept in the directory
  structure, which is maintained on the disk.

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File Operations

• create
• write
• read
• file seek reposition within file
• delete
• truncate
• open(Fi) search the directory structure on disk
  for entry Fi, and move the content of entry to
  memory.
• close (Fi) move the content of entry Fi in
  memory to directory structure on disk.

22
Directory Structure

• A collection of nodes containing information
  about all files.

• Both the directory structure and the files reside
  on disk.
• Backups of these two structures are kept on tapes.

23
Information in a Device Directory

• Name
• Type
• Address
• Current length
• Maximum length
• Date last accessed (for archival)
• Date last updated
• Owner ID
• Protection information

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Operations Performed on a Directory

• Search for a file
• Create a file
• Delete a file
• List a directory
• Rename a file
• Traverse the file system

25
Logical Directory Organization

• Goals
• Efficiency locating a file quickly.
• Naming convenient to users.
• Two users can have same name for different files.
• The same file can have several different names.
• Grouping logical grouping of files by
  properties, (e.g., all Pascal programs, all
  games, )

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Single-Level Directory

• A single directory for all users.

• Naming problem
• Grouping problem

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Two-Level Directory

• Separate (flat) directory for each user.

• Path name
• Can have the same file name for different user
• Efficient searching
• No grouping capability

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Tree-Structured Directories

• Efficient searching
• Grouping Capability
• Current directory (working directory)
• cd /spell/mail/prog
• type list

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Tree-Structured Directories (Cont.)

• Absolute or relative path name
• Concept of current working directory
• Creating/deleting a new file/subdirectory is done
  in current directory.
• Example Deleting mail ? deleting the entire
  subtree rooted by mail.

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Acyclic-Graph Directories

• Have shared subdirectories and files.
• Two different names (aliasing)
• If dict deletes count ? dangling pointer.
• Solutions
• Backpointers, so we can delete all pointers.
• Entry-hold-count solution.

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Protection

• File owner/creator should be able to control
• what can be done
• by whom
• Types of access
• Read
• Write
• Execute
• Append
• Delete
• List

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Classical Unix Access Lists Groups

• Mode of access read, write, execute
• Three classes of users R W X
• a) owner access 7 ? 1 1 1 b) groups
  access 6 ? 1 1 0
• c) public access 1 ? 0 0 1
• SysAdmin creates a group (unique name), say G,
  and add users to the group.
• Attach a group to a file
• chgrp G game

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AFS (Andrew) Access Control Lists

• More details, more information
• For each file, maintain a list of users and their
  access capabilities
• More costly, due to more checking
• More flexibility, more degrees of sharing (read,
  list dir, admin, write, insert, delete)
• ACLs are easy to use, and render Unix protection
  bits useless
• dangers of different user interfaces (the unix
  bits are still listed in ls)