[OS] Process 1

Process Part 1

Process

• An instance of program in execution
  • Program becomes process when executable file loaded into memory
• Program is passive entity stored on disk, process is active
• One program can become several processes (consider multiple users executing the same program)
• Each process is identified using its process ID (PID)

From program to process

• Each process has its own address space in memory
• The program code, also called text section
  • A register called Program Counter(PC) points to the instruction that is currently executing
• Data section containing global variables
• Stack containing temporary data
  • Function parameters, return addresses, local variables
  • Pointed by Stack Pointer (SP) register
• Heap containing memory dynamically allocated during run time

Process State

As a process executes, it changes state

• new : The process is being created ( by fork() )
• running : Instructions are being executed
• waiting : The process is waiting for some event to occur ( queue )
• ready : The process is waiting to be assigned to a processor ( queue )
• terminated : The process has finished execution

Process Control Block (PCB)

• Information associated with each process (also called task control block)
• Each PCB represents a process (loaded on main memory)
• Process state : running, waiting, etc …
• Program counter : location of the instruction to execute next
• CPU registers : contents of all process
• CPU scheduling information : priorities, scheduling queue pointers, etc …
• Memory managment information : memory allocated to the process
• Accounting information : CPU used, clock time, etc …
• I/O status information : I/O devices allocated to process, list of open files, etc …

Context Switch

• Context of a process represented in the PCB
• 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 using a context switch
• The system does no useful work while switching
• The more complex OS and the PCB, the longer the context switch (longer latency)
• Time dependent on hardware support

CPU switch

• Cpu do nothing while context switching

Threads (detail in CH4)

• Process has a single thread of execution
• All program have at least one thread
• Multiple threads
  • operate independently
  • program counter(PC) exist per thread
  • 1 PCB exist (1 process)

Process Scheduling

• Goal : Maximize CPU use, quickly switch processes onto CPU for time sharing
• Process scheduler selects among available processes for next execution on CPU
• Maintains scheduling queues of processes
  • 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 queue : set of processes waiting or an I/O device

Schedulers

• Short-term scheduler (CPU scheduler)
  • selects which process should be executed next and allocates CPU
  • sometimes the only scheduler in a system
  • invoked frequently, so must be fast
• Long-term scheduler (job scheduler)
  • selects which processes should be brought into the ready queue
  • invoked infrequently, so may be slow
• Processes can be described as either:
  • I/O bound process: spends more time doing I/O than computations
  • CPU bound process: spends more time doing computations
  • Long-term schedulers strives for good process mix of I/O and CPU bound jobs(make all busy)
• Medium-term scheduler (multiple programming)
  • Remove process from memory, store on disk, bring back in from disk to continue execution (swapping)