Concurrency From last time

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Concurrency - From last time

• The theoretical core of OS
• An OS creates a set of processes that run
  concurrently
• A process is like a person - it has
• a stream of consciousness
• possessions - memory, files, data
• is created, reproduces, dies
• interacts with other processes
• In actuality, it is a program in execution
• Concurrent process misbehaviors
• Race conditions - unpredictable results because
  processes simultaneously modify data or devices
• Deadlocks - Each has what the other wants
• Starvation - some processes go hungry - others
  eat well

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Some operating system ideas - also review

• Everything is a file - even devices
• This allows a program to work with human input,
  or a device, or a temporary or permanent file
• Interrupt-driven behavior
• Example - windows - process is simultaneously
  sensitive to events from mouse, keyboard, process
  subsystem, and window manager
• Caching - Foreground and background copies
• Used in
• Cache memory
• Swapping
• The disk block cache - recent transactions are in
  memory
• Networking utilities

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Processes and their states

• The process state includes the following
• Running/ready/blocked (waiting on an
  event/events)
• Priority
• Kernel/User mode
• In-memory/swapped
• Process ID
• Priority
• The process owns
• U-area (but user cant examine/modify it)
• Entries in the system open-file table
• Its own file control blocks (linked to the system
  table)
• Its own memory space

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The system process table

• Used for scheduling and control of processes
• Scheduler acts on timer interrupt or a process
  state change
• It schedules the highest-priority process that is
  ready
• It causes a context switch (gives the process
  control)
• This mechanism
• Gives system and I/O processes high priority
• Keeps the CPU puercos from monopolizing
• Interrupts are outside process country
• This avoids make a context switch for each
  interrupt
• It means the interrupt service routines use only
  the kernels memory

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Mutual Exclusion - software solutions

• Unsuccessful solutions (on separate slides)
• Strict alternation
• Algorithms where mutual exclusion fails
• Petersons algorithm
• Description of Bakery Algorithm
• Just like Baskin-Robbins - incoming process takes
  a number
• Since two users can grab same number,
  lowest-number process gets priority

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Avoiding busy waiting - Semaphores and messages

• Classical semaphores
• The semaphore is an ADT with a counter, a process
  queue, and two operations
• The down operation sleeps enqueued if the counter
  is zero, and then decrements the counter
• The up operation increments the counter and
  awakens a sleeper if the counter was 0.
• The down operation enters a critical section, the
  up leaves it.
• Messages
• Reliable messages to an administrator work like a
  down or up
• Messages and semaphores are equivalent
• Thats why

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Semaphores and messages - continued

• Mistakes are easy
• Deadlock
• Geteway-controlling and resource-controlling
  semaphores

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Language Mechanisms - Monitors
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Classical IPC problems

• Producer-consumer
• Scenario
• Producer produces information
• Consumer consumes it
• Both must wait on buffer availability
• Problem resembler disk I/O and print servers
• Code
• Readers-Writers
• Scenario
• Readers can write whenever no writer is active
• Writers must have solitude
• Problem resembles database
• Code
• Dining Philosphers
• Scenario
• N philosophers think, wait for chopsticks, and
  eat
• Problem is useless but illustrative
• Code

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