4.1Operating System ConceptsCSMC 412Operating SystemsProf. Ashok K Agrawala© 2004 Ashok AgrawalaSet 44.2Operating System ConceptsProcesses Process Concept Process Scheduling Operations on Processes Cooperating Processes Interprocess Communication Communication in Client-Server Systems4.3Operating System ConceptsProcess Concept An operating system executes a variety of programs:) Batch system – jobs) Time-shared systems – user programs or tasksTextbook uses the terms job and process almost interchangeably. Process – a program in execution; process execution must progress in sequential fashion. A process includes:) program counter ) stack) data section) CodeProcess is an active entity4.4Operating System ConceptsProcess 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 processor) terminated: The process has finished execution.4.5Operating System ConceptsDiagram of Process State4.6Operating System ConceptsProcess Control Block (PCB)Information associated with each process. Process state Program counter CPU registers CPU scheduling information Memory-management information Accounting information I/O status information4.7Operating System ConceptsProcess Control Block (PCB)4.8Operating System ConceptsCPU Switch From Process to Process4.9Operating System ConceptsProcess 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.4.10Operating System ConceptsReady Queue And Various I/O Device Queues4.11Operating System ConceptsRepresentation of Process Scheduling4.12Operating System ConceptsSchedulers 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.4.13Operating System ConceptsAddition of Medium Term Scheduling4.14Operating System ConceptsSchedulers (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. 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.4.15Operating System ConceptsContext 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.4.16Operating System ConceptsProcess 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 parent’s resources.) Parent and child share no resources.Execution) Parent and children execute concurrently.) Parent waits until children terminate.4.17Operating System ConceptsProcess 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.4.18Operating System ConceptsProcesses Tree on a UNIX System4.19Operating System ConceptsProcess Termination Process executes last statement and asks the operating system to terminate it (exit).) Output data from child to parent (via wait).) Process’ resources are deallocated by operating system.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.4.20Operating System ConceptsCooperating 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) Convenience4.21Operating System ConceptsProducer-Consumer Problem Paradigm for cooperating processes, producer process produces information that is consumed by a consumerprocess.) unbounded-buffer places no practical limit on the size of the buffer.) bounded-buffer assumes that there is a fixed buffer size.4.22Operating System ConceptsInterprocess Communication (IPC) Mechanism for processes to communicate and to synchronize their actions. Message system – processes communicate with each other without resorting to shared variables. IPC facility provides two operations:) send(message) – message size fixed or variable ) receive(message) If P and Q wish to communicate, they need to:) establish a communication link between them) exchange messages via send/receiveImplementation of communication link) physical (e.g., shared memory, hardware bus)) logical (e.g., logical properties)4.23Operating System ConceptsImplementation Questions How are links established? Can a link be associated with more than two processes? How many links can there be between every pair of communicating processes? What is the capacity of a link? Is the size of a message that the link can accommodate fixed or variable? Is a link unidirectional or bi-directional?4.24Operating System ConceptsDirect Communication Processes must name each other explicitly:) send (P, message) – send a message to process P) receive(Q, message) – receive a message from process QProperties of communication link) Links are established automatically.) A link is associated with exactly one pair of communicating processes.) Between each pair there exists exactly one link.) The link may be unidirectional, but is usually bi-directional.4.25Operating System ConceptsIndirect Communication Messages are directed and
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