240323, Part II Processes

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240-323, Part II Processes
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Process Concept
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Process State Transitions
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Process State
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Process control block (PCB)

• Stores all of information about a process
• Processes (PCBs) are manipulated by two main
• components of the process subsystem in order
  to
• achieve the effects of multiprogramming
• Scheduler determines the order by which
  processes will
• gain access to the CPU. Efficiency and fair-play
  are issues here.
• Dispatcher actually allocates CPU to process
  next in line as
• determined by the scheduler.

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Process control block (PCB)
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Process control block (PCB)

• Process status (or state) new, ready to
  run, user running,
• kernel running, waiting, halted
• Program counter where in program the process
  is executing
• CPU registers contents of general-purpose
  register stack pointer,
• PSW, index registers
• Memory Management info segment base and limit
  registers,
• page table, location of pages on disk, process
  size
• User ID, Group ID, Process ID, Parent PID, ...
• Event Descriptor when process is in the
  sleep or waiting state
• Scheduling info process priority, size of CPU
  quantum, length
• of current CPU burst

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Process scheduling
Scheduling queues
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Process scheduling
Schedulers

• select process
• long-term scheduler (job scheduler) selects
  processes
• from the pool and loads them into memory for
  execution.
• Short-term scheduler(cpu scheduler) selects
  processes
• from among the processes that are ready to
  execute and
• allocates the CPU to one of them.

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Process Priority
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Context Switch

• Switch the CPU to another process
• speed varied from machine to machine
• context-switch times are depended on hardware
  support

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Multiprogramming to Context Switch
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Forking a new process
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Process terminate
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Process terminate
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Concurrent Process

• Implementing a multiprogramming OS requires
  programming to accommodate a number of
  simultaneously executing processes
• Multiple-process paradigm also useful for
  applications (e.g., parallel processing,
  background processing)
• Two kinds of parallelism in today's computer
  systems
• Pseudo-parallelism - one CPU supports multiple
  processes
• True parallelism - processes run on multiple CPUs

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Concurrent Process

• Two kinds of communication paradigms
• - Shared-variable model
• - Message-passing model
• Most systems incorporate a mixture of the two.

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Interprocess Communication (IPC)
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How are links established?

• Direct communication
• send(process_id, message) receive(process_id,
  message)

Example producer and consumer problem
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Direct communication
Process producer repeat ... Produce an
item in nextp ... Send(consumer,nextp) ... unt
il false
Process consumer repeat receive(producer,next
c) ... Consume the item in nextc ... until
false
This is a symmetry in addressing
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How are links established?

• Indirect Communication
• send(mailbox, message) receive(mailbox,
  message)

- Associate with more than two processes.
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Whats the capacity of the links?

• Buffering three ways to implement queue
• - Zero capacity
• - Bounded capacity
• - Unbounded capacity
• A zero-capacity buffer means processes must
  handshake
• in order to communicate.