chapter 3:Process concept Flashcards
(4)Process Concept, how execute
An operating system executes a variety of programs: Batch system – jobs Time-shared systems – user programs or tasks Textbook uses the terms job and process almost interchangeably The program code, also called text section Current activity including program counter, processor registers Stack containing temporary data Function parameters, return addresses, local variables Data section containing global variables Heap containing memory dynamically allocated during run time
(4)Process Concept, what is process and its parts ?
Process – a program in execution; process execution must progress in sequential fashion Multiple parts: The program code, also called text section Current activity including program counter, processor registers Stack containing temporary data Function parameters, return addresses, local variables Data section containing global variables Heap containing memory dynamically allocated during run time Program is passive entity stored on disk (executable file), process is active
(4)Process Concept, what is a program ?
Program is passive entity stored on disk (executable file), process is active Program becomes process when executable file loaded into memory Execution of program started via GUI mouse clicks, command line entry of its name, etc One program can be several processes Consider multiple users executing the same program
(5)Process in Memory
(6)Process State, what are the states ?
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 processor
- terminated: The process has finished execution
(7)Diagram of Process State
(8)Process Control Block (PCB)
Information associated with each process
(also called task control block)
Process state – running, waiting, etc
Program counter – location of instruction to next execute
CPU registers – contents of all process-centric registers
CPU scheduling information- priorities, scheduling queue pointers
Memory-management information – memory allocated to the process
Accounting information – CPU used, clock time elapsed since start, time limits
I/O status information – I/O devices allocated to process, list of open files
(9)CPU Switch From Process to Process
(10)Threads
So far, process has a single thread of execution
Consider having multiple program counters per process
Multiple locations can execute at once
- Multiple threads of control -> threads
Must then have storage for thread details, multiple program counters in PCB
See next chapter
(11)Process Representation in Linux
Represented by the C structure task_struct
pid t pid; /* process identifier */
long state; /* state of the process */
unsigned int time slice /* scheduling information */
struct task struct *parent; /* this process’s parent */
struct list head children; /* this process’s children */
struct files struct *files; /* list of open files */
struct mm struct *mm; /* address space of this process */
(12)Process Scheduling
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 queues – set of processes waiting for an I/O device
- Processes migrate among the various queues
(13)Ready Queue And Various
I/O Device Queues
(14)Representation of Process Scheduling-Queuing diagram
Queuing diagram represents queues, resources, flows
(15)Schedulers TYPES ?
- *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
- Sometimes the only scheduler in a system
(15)Schedulers- Short-term scheduler is invoked…?
Short-term scheduler is invoked very frequently (milliseconds) ⇒ (must be fast)
(15)Schedulers- Long-term scheduler is invoked…….?
Long-term scheduler is invoked very infrequently (seconds, minutes) ⇒ (may be slow)
The long-term scheduler controls the degree of multiprogramming
(15) Schedualers-process
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
Long-term scheduler strives for good process mix
(16)Addition of Medium Term Scheduling
Medium-term scheduler can be added if degree of multiple programming needs to decrease
- Remove process from memory, store on disk, bring back in from disk to continue execution: swapping
(17)Multitasking in Mobile Systems
Some systems / early systems allow only one process to run, others suspended
Due to screen real estate, user interface limits iOS provides for a
- Single foreground process- controlled via user interface
- Multiple background processes– in memory, running, but not on the display, and with limits
- Limits include single, short task, receiving notification of events, specific long-running tasks like audio playback
(17)Multitasking in Mobile Systems-Android ?
Android runs foreground and background, with fewer limits
- Background process uses a service to perform tasks
- Service can keep running even if background process is suspended
- Service has no user interface, small memory use
(18)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 via a context switch
Context of a process represented in the PCB
Context-switch time is overhead; the system does no useful work while switching
- The more complex the OS and the PCB -> longer the context switch
Time dependent on hardware support
- Some hardware provides multiple sets of registers per CPU -> multiple contexts loaded at once
(19)Operations on Processes
System must provide mechanisms for process creation, termination, and so on as detailed next
(20)Process Creation
Parent process create children processes, which, in turn create other processes, forming a tree of processes
Generally, process identified and managed via a process identifier (pid)
Resource sharing options
- Parent and children share all resources
- Children share subset of parent’s resources
- Parent and child share no resources
Execution options
- Parent and children execute concurrently
- Parent waits until children terminate
(21)A Tree of Processes in Linux