Processes:%20program%20 %20execution%20state

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• Processes program execution state
• Pseudoparallelism
• Multiprogramming
• Many processes active at once
• With switching
• process execution is not repeatable
• processes should make no assumptions about timing
• Process consists of
• Process core image program, data, run-time
  stack
• Program counter, registers, stack pointer
• OS bookkeeping information

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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.

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Diagram of Process State
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Diagram of Process State
X
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Diagram of Process State
X
X
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Diagram of Process State
X
X
X
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Process-Centered Operating System

• One way to view system a scheduler, with
  processes running on top
• Some of the processes are system processes

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Process Table
One entry per process.
Pr 0 Pr 1 Pr 2 Pr 4 ..Pr N
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PCB Process Management

• registers, program counter, program status word,
  stack pointer
• process state
• time process started, CPU time used, childrens
  CPU time used
• alarm clock setting
• pending signal bits
• pid
• message queue pointers, other flag bits

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PCB Memory-Related Information

• pointers to
• text
• Data
• stack segments
• Pointer to page table

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PCB File-Related Information

• root, working directory
• file descriptors
• User ID
• Group ID

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Process Control Block (PCB)
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Context Switch (or Process Switch)

• Currently executing process looses control of CPU
• Its context must be saved (if not terminated)
  into PCS.
• New process is chosen for execution.
• New process context is restored.
• New process is given control of the CPU.

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CPU Switch From Process to Process
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Process Scheduling 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.
• Process migration between the various queues.

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Ready Queue And Various I/O Device Queues
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Representation of Process Scheduling
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Low Level Interrupt Processing

• Each HW device has slot in interrupt vector (IV).
• On interrupt, HW pushes PC, PSW, one or more
  registers.
• Loads in new PC from IV.
• END OF HW
• Assembly code to save registers and info on stack
  (to PCB).
• Assembly sets up new stack for handling process.
• Calls C interrupt handling routing to complete
  interrupt processing.
• Scheduler is called and determines next process
  to run.
• Assembly language restores registers and other
  necessary items (e.g., memory map) of selected
  process and begins its execution.

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Schedulers

• Long-term scheduler (or job scheduler) selects
  which processes should be brought into the ready
  queue.
• Short-term scheduler (or CPU scheduler) selects
  which process should be executed next and
  allocates CPU.

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Schedulers (Cont.)

• Short-term scheduler is invoked very frequently
  (milliseconds) ? (must be fast).
• Long-term scheduler is invoked very infrequently
  (seconds, minutes) ? (may be slow).
• The long-term scheduler controls the degree of
  multiprogramming.

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Schedulers (Cont.)

• Processes can be described as either
• 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.

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Context Switch

• When CPU switches to another process, the system
  must save the state of the old process and load
  the saved state for the new process.
• Context-switch time is overhead the system does
  no useful work while switching.
• Time dependent on hardware support.

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Process Creation

• Parent process create children processes, which,
  in turn create other processes, forming a tree of
  processes.
• Resource sharing
• Parent and children share all resources.
• Children share subset of parents resources.
• Parent and child share no resources.
• Execution
• Parent and children execute concurrently.
• Parent waits until children terminate.

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Process Creation (Cont.)

• Address space
• Child duplicate of parent.
• Child has a program loaded into it.
• UNIX examples
• fork system call creates new process
• exec system call used after a fork to replace the
  process memory space with a new program.

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Unix Fork()
include ltstdio.hgt main(int argc, char argv)
int pid, j j 10 pid fork()
if (pid 0) /I am the child/ Do child
things else / I am the parent
/ wait(NULL) / Block execution until
child terminates /
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(No Transcript)
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fork()
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Processes Tree on a UNIX System
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exec Function calls

• Used to begin a processes execution.
• Accomplished by overwriting process imaged of
  caller with that of called.
• Several flavors, use the one most suited to
  needs.
• int execv( char path, char argvec)

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exec Function calls

• int execv( char path, char argvec)
• pathname Can be an executable program in your
  directory (application code) or a system program
  such as ls, cd, date, ..
• argvec Pointers to NULL terminated strings.
• First element should always be the name of the
  
• program, last element should always be NULL.
• Assume a.out b.out x.exe. By.by all in home
  directory.

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main (int argc, argv) int pid char
args2 pid fork() if (pid 0)
args0 ./a.out All executed by
args1 NULL child process
execv(./aout, args) printf(OOOpppssss.\n)
else printf(Not a problem!\n)
Executed by parent
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Process Termination

• Process executes last statement and asks the
  operating system to decide it (exit).
• Output data from child to parent (via wait).
• Process resources are deallocated by operating
  system.

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Process Termination

• Parent may terminate execution of children
  processes (abort).
• Child has exceeded allocated resources.
• Task assigned to child is no longer required.
• Parent is exiting.
• 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
• Computation speed-up
• Modularity
• Convenience

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Producer-Consumer Problem

• Paradigm for cooperating processes, producer
  process produces information that is consumed by
  a consumer process.
• unbounded-buffer places no practical limit on the
  size of the buffer.
• bounded-buffer assumes that there is a fixed
  buffer size.

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Constraints

• Do not over-write an item not yet consumed.
• Do not write to a full buffer
• Do not read from a previously read item.
• Do not read from an empty buffer.

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Bounded-Buffer Shared-Memory Solution

• Shared data
• define BUFFER_SIZE 10
• Typedef struct
• . . .
• item
• item bufferBUFFER_SIZE
• int in 0
• int out 0

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Bounded-Buffer Producer Process

• item nextProduced
• while (1)
• while (((in 1) BUFFER_SIZE) out)
• / do nothing /
• bufferin nextProduced
• in (in 1) BUFFER_SIZE

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Bounded-Buffer Consumer Process

• item nextConsumed
• while (1)
• while (in out)
• / do nothing /
• nextConsumed bufferout
• out (out 1) BUFFER_SIZE

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Producer
while (((in 1) BUFFER_SIZE) out)
Consumer Blocked
while (in out)
out
in
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Producer Blocked
while (((in 1) BUFFER_SIZE) out)
Consumer
while (in out)
O
O
O
O
O
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Producer
while (((in 1) BUFFER_SIZE) out)
Consumer Blocked
while (in out)
X X X X X
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Medium Term Scheduling