operating systems cmpsc 473
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Operating Systems CMPSC 473. I/O Management (1) November 30 2010 - Lecture 23 Instructor: Bhuvan Urgaonkar. Announcements. Project 3 due today Please indicate time to meet TA on sheet Project 4 out and due on Dec 10 You need to study material on content addressable storage - PowerPoint PPT PresentationTRANSCRIPT
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Operating SystemsCMPSC 473I/O Management (1)
November 30 2010 - Lecture 23Instructor: Bhuvan Urgaonkar
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Announcements• Project 3 due today• Please indicate time to meet TA on sheet• Project 4 out and due on Dec 10
– You need to study material on content addressable storage
– Will release slides on this later today• Will release practice question set to
help you prepare for the final exam (sometime this week)– Exam is on Dec 16
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I/O Management: Topics
• General issues in I/O management
• Secondary storage management– Properties of media
• Magnetic disks and Flash– Systems software for using these media
• File Systems
• Note: Would like you to read ahead for P4– Material on content addressable storage:
will post slides
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I/O Hardware• Incredible variety of I/O devices• Two ways of communicating
– Port – Bus
• Plus … interrupts – Unidirectional: From device to CPU– Special CPU input lines/pins for
these
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Some familiar ports
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I/O Hardware• Incredible variety of I/O devices• Two ways of communicating
– Port– Bus
• Controller: Electronics to operate a port/bus or a device – More complex controller: host adapter
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Some examples
PCI slots and card
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Some examplesSATA cable
SATA ports on a motherboard
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A Typical PC Bus Structure
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I/O Hardware
• I/O instructions control devices• Devices have addresses, used by
– Direct I/O instructions• These are part of the ISA• E.g., IN and OUT on Intel
– Memory-mapped I/O• Not to be confused with memory mapped file IO!
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Direct V. Memory Mapped IO
• Direct I/O• Uses special instructions for accessing the
I/O devices• i.e., different from loads/stores
• IO devices have separate address space from general memory• Either accomplished by an extra IO pin in
CPU’s physical interface• Or, separate bus for IO devices• Also called isolated IO
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Direct V. Memory Mapped IO
• Memory Mapped I/O• Uses loads and stores
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Device I/O Port Locations on PCs
(partial)
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Communication between host and
device• Employs registers on the port/controller
• Typically four important registers– Data-in: read by the host to get input– Data-out: written by the host to send output– Status: Bits readable by the host that
indicate state of port• Has the current command been completed?• Has a device error occurred?• Is there data available to be read from Data-in
register?– Control: Writable by the host to start a
command, select certain properties of the port (e.g., speed)
• Interaction between host and controller– Polling– Interrupts
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Polling• Determines state of device
– command-ready– busy– Error
• Busy-wait cycle to wait for I/O from device
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Interrupts• CPU Interrupt-request line triggered by
I/O device• Interrupt handler receives interrupts• Maskable to ignore or delay some interrupts• Interrupt vector to dispatch interrupt to
correct handler– Based on priority– Some non-maskable
• Interrupt mechanism also used for exceptions
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Intel Pentium Processor Event-Vector Table
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Direct Memory Access
• Used to avoid programmed I/O for large data movement
• Requires DMA controller
• Bypasses CPU to transfer data directly between I/O device and memory
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Six Step Process to Perform DMA Transfer
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Application I/O Interface
• I/O system calls encapsulate device behaviors in generic classes
• Device-driver layer hides differences among I/O controllers from kernel
• Devices vary in many dimensions– Character-stream or block– Sequential or random-access– Sharable or dedicated– Speed of operation– read-write, read only, or write only
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A Kernel I/O Structure
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Characteristics of I/O Devices
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Block and Character Devices
• Block devices include disk drives– Commands include read, write, seek – Raw I/O or file-system access– Memory-mapped file access possible
• Character devices include keyboards, mice, serial ports– Commands include get, put– Libraries layered on top allow line editing
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Network Devices• Varying enough from block and
character to have own interface• Unix and Windows NT/9x/2000 include
socket interface– Separates network protocol from network
operation– Includes select functionality
• Approaches vary widely (pipes, FIFOs, streams, queues, mailboxes)
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Clocks and Timers• Provide current time, elapsed time,
timer
• Programmable interval timer used for timings, periodic interrupts
• ioctl (on UNIX) covers odd aspects of I/O such as clocks and timers
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Synchronous I/O
• Blocking - process suspended until I/O completed– Easy to use and understand– Likely to be inefficient
• Quiz: When and why?
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Asynchronous I/O
• Process runs while I/O executes– Returns immediately
• “Event-driven” programming– Difficult to use– I/O subsystem signals process when I/O
completed
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Two I/O Methods
Synchronous Asynchronous
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• Nonblocking - I/O call returns as much as available– One approach: Implemented via multi-threading
• Some threads do blocking I/O, while others continue executing
– Other approach: OS provides non-blocking call• Does not halt execution of application for an
extended time, returns quickly with count of bytes read or written
• Good e.g., select () system call for network I/O
“Hybrid” IO
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Kernel I/O Subsystem
• Scheduling– Some I/O request ordering via per-device
queue– Some OSs try fairness
• Buffering - store data in memory while transferring between devices– To cope with device speed mismatch– To cope with device transfer size mismatch– To maintain “copy semantics”
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Kernel I/O Subsystem• Caching - fast memory holding copy of data
– Always just a copy– Key to performance
• Spooling - hold output for a device– If device can serve only one request at a time – i.e., Printing
• Device reservation - provides exclusive access to a device– System calls for allocation and deallocation– Watch out for deadlock
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Error Handling• OS can recover from disk read, device
unavailable, transient write failures
• Most return an error number or code when I/O request fails
• System error logs hold problem reports
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I/O Protection• User process may accidentally or
purposefully attempt to disrupt normal operation via illegal I/O instructions– All I/O instructions defined to be
privileged– I/O must be performed via system calls
• Memory-mapped and I/O port memory locations must be protected too
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Use of a System Call to Perform I/O
Monitor = privileged/kernel mode
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Life Cycle of An I/O Request
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Performance• I/O a major factor in system
performance:
– Demands CPU to execute device driver, kernel I/O code
– Context switches due to interrupts– Data copying
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Improving Performance
• Reduce number of context switches• Reduce data copying • Reduce interrupts by using large
transfers, smart controllers, polling • Use DMA• Balance CPU, memory, bus, and I/O
performance for highest throughput
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I/O W/O a Unified Buffer Cache
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Unified Buffer Cache
• A unified buffer cache uses the same page cache to cache both memory-mapped pages and ordinary file system I/O
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I/O Using a Unified Buffer Cache
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Recovery• Consistency checking – compares data in
directory structure with data blocks on disk, and tries to fix inconsistencies
• Use system programs to back up data from disk to another storage device (floppy disk, magnetic tape, other magnetic disk, optical)
• Recover lost file or disk by restoring data from backup