Chapter 4: Processes

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Chapter 4 Processes

• What is a process?
• Informally A program in execution
• Analogies A script Vs. a play. A recipe Vs.
  cooking.
• Formally Any CPU activity
• batch system jobs
• time-shared system tasks
• user programs
• batch programs
• interactive programs, etc.
• Assumption of Finite Progress
• Process execution must progress in a sequential
  fashion
• Execution CPU Burst, I/O Burst, CPU/Burst,
  CPU Burst with interrupts due to hardware (timer,
  etc.)
• Textbook uses the terms job and process almost
  interchangeably.

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Process Concept

• A process includes
• text
• The program code
• data section
• Explicitly declared memory/variables
• stack
• Implicitly declared memory
• activation record
• subroutine parameters
• local variables
• program counter
• Current instruction/point of execution

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Process State

• As a process executes, it changes state
• new The process is being created.
• running Instructions are being executed.
• waiting The process is waiting for some event
  to occur.
• ready The process is waiting to be assigned to
  a process.
• terminated The process has finished execution.
• Processes are generally I/O-bound or CPU-bound

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Process Control Block (PCB)

• Information associated with each process.
• Process state
• Program counter
• CPU registers
• CPU scheduling information
• priority
• pointers to scheduling queues
• Memory-management information
• base/limit registers
• Accounting information
• I/O status information
• pointers to open file table

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CPU Switch From Process to Process
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Process Scheduling Queues

• For a uniprocessor system, only one process can
  be active
• As processes appear or change state, they are
  placed in queues
• Job queue set of all processes in the system.
• Ready queue set of all processes residing in
  main memory, ready and waiting to execute.
• Device queues set of processes waiting for an
  I/O device.
• The OS controls process migration between the
  various queues
• Some migration events are require no decision
  making
• Some migration events require decision making
  (scheduling)

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Representation of Process Scheduling
Long Term Scheduler
Short Term Scheduler
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Ready Queue And Various I/O Device Queues

• Each process PCB is in exactly one queue
• Ready queue Ready to run
• I/O Queue Awaiting I/O completion (interrupt)

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Long-Term Schedulers

• Long-term scheduler (or job scheduler) selects
  which processes should be brought into the ready
  queue
• Long-term scheduler is invoked only on process
  creation/completion
• very infrequently called (seconds, minutes) ?
  (may be slow)
• controls the degree of multiprogramming
• Number of ready processes
• controls the process mix
• I/O-bound process spends more time doing I/O
  than computations, many short CPU bursts
• CPU-bound process spends more time doing
  computations few very long CPU bursts
• Long-term schedulers are not generally used in
  interactive time-sharing Operating Systems
• Commonly used in multiprogrammed batch systems

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Short-Term Schedulers

• Short-term scheduler (or CPU scheduler) selects
  which process should be executed next and
  allocates CPU
• Dispatches from ready queue based on priority
• Process executes until it creates an event or an
  interrupt occurs.
• Events I/O Request, fork a child, wait for an
  interrupt
• Interrupt Timeslice expired, I/O Completion,
  etc.
• If the process creates the event, it is removed
  from the ready queue and placed on an I/O or
  event wait queue
• Context Switch - a change of active process
• save/load CPU Registers to/from PCB (overhead)
• Short-term scheduler is invoked very frequently
• (milliseconds) ? (must be fast)
• Multi-programmed systems must provide short-term
  scheduling

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Medium-Term Schedulers

• Medium-Term schedulers - control the process mix
  and degree of multiprogramming for interactive
  time-shared systems
• All jobs are accepted (little or no long-term
  scheduling)
• Resource restrictions may not permit all jobs to
  be in memory concurrently
• Partially executed processes may be swapped out
  to disk and brought back into memory later.

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Process Creation

• A process may create new processes via system
  call
• fork()
• The creating process is called the parent
• The created process is called the child
• Parent process creates children processes, which,
  in turn create other processes, forming a tree of
  processes
• Parents may share resources with their children
• Option 1 Parent and children share all
  resources.
• Option 2 Children share subset of parents
  resources.
• Option 3 Parent and child share no resources.
• Execution
• Option 1 Parent and children execute
  concurrently
• Option 2 Parent waits until children terminate

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Process Creation (Cont.)

• UNIX example
• fork system call creates new process
• Created child process is a duplicate of parent
• Same memory image, nearly identical PCB
• The OS must provide some mechanism to allow
  modification
• exec family of system call used after a fork to
  overload the process memory space with new
  text/program
• setuid() allows a change of owner
• Consider the login process

pid_t pid pid fork() if (pidlt0)
fprintf(stderr,Fork Error\n exit(1) if (pid
0) / child block - execve(), etc. / else
/ parent block / - wait(), etc. / /
Common Block /
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Process Termination

• Process executes last statement and asks the
  operating system to delete it (exit)
• Output data from child to parent (via wait)
• Process resources are deallocated by operating
  system
• Memory, open files, I/O buffers, etc.
• Process can be terminated by interrupts or other
  events
• Bus error, Seg Fault, etc.
• Processes can be terminated by signals
• Parent may terminate child with signals
  (abort(),kill())
• Child has exceeded allocated resources
• Task assigned to child is no longer required
• Parent sends a termination signal to all childer
  on exit
• Operating system does not allow child to continue
  if its parent terminates
• Cascading termination

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Cooperating Processes

• Independent process cannot affect or be affected
  by the execution of another process.
• Cooperating process can affect or be affected by
  the execution of another process
• Advantages of process cooperation
• Information sharing
• files, memory, partial results, etc.
• Computation speed-up
• Parallel implementations
• Modularity
• Os system programs, daemons, etc.
• Convenience
• User-level multi-tasking CNTL-Z, bg, , fg, jobs

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Producer-Consumer Problem

• Paradigm for cooperating processes, producer
  process produces information that is consumed by
  a consumer process.
• General model for processes that run concurrently
  and must share a synchronized buffer.
• unbounded-buffer places no practical limit on the
  size of the buffer.
• bounded-buffer assumes that there is a fixed
  buffer size.
• The buffer is provided by the OS through IPC or
  shared memory

Producer REPEAT wait until buffer not full()
Produce item (takes time) UNTIL FALSE
Consumer REPEAT wait until buffer not
empty() Consume item (takes time) UNTIL FALSE
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Threads

• A process is an exact duplicate of its parent
  with its own (replicated) memory space, stack
  space, and PCB.
• It is useful to allow the creation of
  tightly-coupled children that dont require
  unique copies of the code/data memory or OS
  resources
• A thread (or lightweight process) is a basic unit
  of CPU utilization it consists of
• program counter
• register set
• stack space
• A thread shares with its peer threads its
• code section
• data section
• operating-system resources
• collectively know as a task.
• A traditional or heavyweight process is equal to
  a task with one thread

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Why use Threads?

• In a multiple threaded task, while one server
  thread is blocked and waiting, a second thread in
  the same task can run.
• Threads provide a mechanism that allow sequential
  processes to make blocking system calls while
  also achieving parallelism.
• Cooperation of multiple threads in same job
  confers higher throughput and improved
  performance.
• Applications that require sharing a common buffer
  (i.e., producer-consumer) benefit from thread
  utilization.
• Possible immediate advantage to running code on
  multi-processor system
• Thread context switch is fast
• Only PC and GPRs must be restored
• No memory management necessary (no cache purge
  required)
• BEWARE There is no security between threads
• Debugging can be difficult

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Threads Support

• Kernel-supported threads (Mach and OS/2) the OS
  is aware of threads, each thread has a PCB and
  is scheduled by the OS
• Advantage
• Multiple System Calls in operation concurently
• Disadvantage
• Context switch is slower requires OS
  intervention
• Fairness issues for CPU time
• User-level threads supported above the kernel,
  via a set of library calls at the user level
  (Project Andrew from CMU).
• Advantage
• Fast context switch - no OS overhead
• Disadvantage
• Kernel sees only one process, one system call
  halts all threads.
• Some operating systems offer a hybrid approach
  (Solaris 2).

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Threads Support in Solaris 2

• Hybrid approach implements both user-level and
  kernel-supported threads (Solaris 2). Solaris 2
  is a version of UNIX with support for threads at
  the kernel and user levels, symmetric
  multiprocessing, and real-time scheduling.
• Light Weight Process (LWP) intermediate level
  between user-level threads and kernel-level
  threads.
• Resource needs of thread types
• LWP PCB with register data, accounting and
  memory information, switching between LWPs is
  relatively slow.
• User-level thread only need stack and program
  counter no kernel involvement means fast
  switching. Kernel only sees the LWPs that
  support user-level threads.
• Kernel thread small data structure and a stack
  thread switching does not require changing memory
  access information relatively fast.

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Threads in Solaris 2
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Interprocess Communication (IPC)

• IPC is covered in CEG434/634
• Skip section 4.6