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Process Description and Control Chapter 3 Process Requirements of an Operating System Interleave execution of several processes to maximize processor utilization while providing reasonable response time Allocate resources to processes Support interprocess communication and user creation of processes Process Also called a task or job It is a program in execution it is an active entity Two-State Process Model Dispatch Enter Not Running Running Exit Pause State transition diagram Queue Enter Dispatch Exit Processor Pause Queuing diagram Reasons for Process Creation Submission of a batch job User logs on Created by OS to provide a service such as printing Spawned by an existing process Reasons for Process Termination Normal completion e.g., user logs off, requests termination Error and fault conditions e.g., time limit exceeded, memory unavailable OS intervention e.g., when deadlock occurs Parent request Processes Ready prepared to execute Blocked waiting for I/O Dispatcher cannot just select the process that has been in the queue the longest because it may be blocked A Five-State Model Running process currently being executed Ready prepared to execute Blocked waiting for some event to occur New created but not admitted to job pool Exit released from job pool Five-State Process Model Dispatch New Admit Ready Release Running Time-out Event Occurs Blocked Event Wait Exit Single Blocked Queue Ready Queue Release Dispatch Admit Processor Time-out Event Wait Event Occurs Blocked Queue Multiple Blocked Queues Ready Queue Release Dispatch Admit Processor Time-out Event 1 Wait Event 1 Occurs Event 1 Queue ... Event n Wait Event n Occurs Event n Queue Suspended Processes Processor is faster than I/O so all processes could be waiting for I/O Swap these processes to disk to free up more memory Blocked state becomes suspend state when swapped to disk Two new states Blocked, suspend Ready, suspend Process State Transition Diagram with Two Suspend States Ne w Admit Admit Suspend Dispatch Activate Ready, suspend Ready Suspend Time out Event Occurs Event Occurs Activate Blocked, suspend Running Blocked Event Wait Exit Operating System Control Structures Tables are constructed for each entity the operating system manages memory tables I/O tables file tables process tables Memory Tables Allocation of main memory to processes Allocation of secondary memory to processes Protection attributes for main or virtual memory Information needed to manage virtual memory I/O Tables I/O device is available or assigned Status of I/O operation Location in main memory being used as the source or destination of the I/O transfer File Tables Existence of files Location on secondary memory Current Status (e.g., open) Attributes (e.g., permissions) Sometimes this information is maintained by a file-management system Process Table Process image is a collection: user program program or set of programs to be executed user data locations for local and global variables, defined constants run-time stack LIFO stack to keep track of procedure calls, parameters passed process control block (PCB) attributes used by the OS for process control Contents of the Process Control Block Process identification user id, process id, id of parent Process state information user visible registers control and status registers stack pointers Contents of PCB (cont’d) Process control information scheduling and state information data structuring interprocess communication process privileges memory management resource ownership and utilization Typical Functions of an Operating System Kernel Process management Memory management I/O management Support functions Process Management Process creation and termination Process scheduling and dispatching Process switching Process synchronization and support for inter-process communication Management of process control blocks Memory Management Allocation of address space to processes Swapping Page and segment management I/O Management and Support Functions Buffer management Allocation of I/O channels and devices to processes Interrupt handling Accounting Monitoring Steps in Process Creation Assign a unique process identifier Allocate space for the process Initialize process control block Set up appropriate linkages e.g., add new process to linked list used for scheduling queue Other maintain an accounting file When to Context Switch Interrupts react to an asychronous external event e.g., clock, I/O, memory fault Trap associated with execution of current instruction e.g, handle error or exceptional condition Supervisor call explicit request, e.g., open file Steps in a Context Switch Save context of “old” process Update “old” PCB with the new state and any accounting information Move “old” PCB to appropriate queue Select a “new’’ process for execution Update PBC of “new” process Update memory management tables Restore context of “new” process Execution of the OS Nonprocess kernel execute kernel outside of any process OS code is executed as a separate entity that operates in privileged mode Execution within user processes OS software within context of a user process process executes in privileged mode when executing OS code Process-based operating system major kernel functions are separate processes UNIX Process State Transition Diagram fork Created Preempted return to user not enough memory (swapping system only) enough memory User Running preempt return system call, interrupt interrupt, interrupt return swap out reschedule process Ready to Run in Memory Ready to Run Swapped swap in Kernel Running sleep wakeup wakeup exit Zombie Asleep in Memory swap out Sleep, Swapped
