[Operating System] Process Management (1)

Process Management (1)

Process Concept

• Job (Application, Execution/Image file)
  • A bundle of program and data to be executed
  • An entity before submission for execution
• Process
  • An entity that is registered to kernel for execution
  • Kernel manages the processes to improve overall system performance

A process includes

• Program code → text
• Global data → data
• Temporary data → stack
  • Local variables, function parameters, return addresses
• Heap
  • Memory area that is dynamically allocated during execution
• Values of the processor registers including program counter and general registers
• Etc

• A program in execution
• An entity that is registered and being managed by kernel
• An entity that is admitted to request and use system resources
• An entity that is allocated the PCB
• An active entity
  • Request/allocate/release system resources during execution

Resource Concept

• A passive entity that is allocated to and released by the processes under the control of the kernel
• Classification of the resources

System States

• Modeling of system states
  • Interaction between the processes and resources
  • What is in the system?
    • Processes
    • Resources
    • Kernel

PCB (Process Control Block)

• Keeps several information about each process that is registered to the kernel
• Maintained in kernel space

Information in PCB

• PID (Process Identification Number)
• Process state
• Scheduling information
  • Process priority, scheduling parameters, etc.
• Memory management information
  • Base/limit registers, page tables, segment tables, etc.
• I/O status information
  • List of I/O devices allocated, list of open files, etc.
• Accounting information
• Context save area
  • Space for saving register context of the process

• In Unix
  • Process table slot
  • U-area
• In Linux
  • Process descriptor (task_struct)

struct task_struct {
  unsigned long state;
  int prio;
  unsigned long policy;
  struct task_struct *parent;
  struct list_head tasks;
  pid_t pid;
  ...
}

• Notes
  • PCB information is different for each OS
  • PCB access speed is important for overall system performance

Process States

• Determined by the process-resource interaction states

Process State Transition Diagram

Created state

• User’s job/command request is registered to the kernel
• PCB allocated
• New process is being created
• Transient state

• Kernel
  • Check available memory space
  • Transit the process to ready or suspended ready state

• Process creation in Unix system
  • By fork() system call

• Process creation
  • By system call
  • Parent process, child process
  • Tree-structured process hierarchy

Ready state

• Allocated all resources necessary except processor
• Waiting processor allocation
• Can execute immediately when the processor is allocated
• Dispatch, schedule
  • Transition from ready state to running state

Running state

• Owns processor now (executing its program on the processor)
• Owns all resources necessary

• Preemption
  • Transition from running state to ready state
  • Due to the expiration of time quantum (time slice) or appearance of higher priority process
• Block/sleep
  • Transition from running state to asleep state
    • Waiting for resource allocation, I/O completion, or special event, during execution (running)

Blocked/asleep state

• Requested some resources other than processor or memory and waiting for them
• Resource request through system call
  • System call $\equiv$ SVC (SuperVisor Call)
    • Kernel provides application processes with system call I/F so that they can request resources
• Wakeup
  • Transition from asleep state to ready state
  • Due to allocation of resources or occurrence of events

  Immediate transition from asleep state to running state is not allowed. Why?

Suspended state

• Suspended ready state
  • Swapped out of memory from ready state
  • Memory image is transferred to swap device, which is a special file system
• Suspended asleep state
  • Swapped out of memory from asleep state

• swap-out(suspend)
  • Transition due to losing memory
• swap-in(resume)
  • Transition due to getting memory

Terminated/zombie state

• Completing process execution
• Releasing all resources allocated
• Only having some information in PCB (in kernel space, memory)
• Process termination in Unix system
  • By exit() system call

Process States in Unix

Process States in Linux