3 process management


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Process Management

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3 process management

  1. 1. Processes1. Process Concept2. Process Scheduling3. Operations on Processes4. Interprocess Communication
  2. 2. 1. Process Concept• An operating system executes a variety of programs: – Batch system – jobs – Time-shared systems – user programs or tasks• The terms job and process are used almost interchangeably• 1.1 The Process – a program in execution; process execution must progress in sequential fashion – A process includes: • program counter - current activity • Stack - temporary data(function, parameters, return addresses, and local variables) • data section - global variables • heap - dynamically allocated memory during process run time Loganathan R, CSE, HKBKCE 2
  3. 3. 1. Process Concept Contd… Process in Memory • A program by itself is not a process • A program is a passive entity, a file containing a list of instructions stored on disk (often called an executable file) • A process is an active entity, with a program counter specifying the next instruction to execute and a set of associated resources Loganathan R, CSE, HKBKCE 3
  4. 4. 1. Process Concept Contd…1.2 Process State• As a process executes, it changes state, state of a process is defined in part by the current activity of that process – 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 Loganathan R, CSE, HKBKCE 4
  5. 5. 1. Process Concept Contd…1.3 Process Control Block (PCB) also called task control block contains information associated with each process• Process state - new, ready, running, waiting, halted• Program counter - the address of the next instruction to be executed• CPU registers - accumulators, index registers, stack pointers, and general- purpose registers• CPU scheduling information - process priority, pointers to scheduling queues and scheduling parameters• Memory-management information - value of the base and limit registers, the page tables, or the segment tables• Accounting information - the amount of CPU and real time used, time limits, account numbers, job or process numbers• I/O status information - list of I/O devices allocated to the process, a list of open files1.4 Threads• A process is a program that performs a single thread of execution• Single thread of control allows the process to perform only one task at one time• Multiple threads of execution perform more than one task at a time Loganathan R, CSE, HKBKCE 5
  6. 6. 1. Process Concept Contd…Process Control CPU Switch From Process to ProcessBlock (PCB) Loganathan R, CSE, HKBKCE 6
  7. 7. 2. Process Scheduling• Multiprogramming and Time Sharing switch the CPU among processes so frequently that users can interact with each program• Process Scheduler selects an available process (possibly from a set of several available processes) for program execution on the CPU2.1 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. Each device has its own device queue• Processes migrate among the various queues Loganathan R, CSE, HKBKCE 7
  8. 8. 2. Process Scheduling Contd…Ready Queue And Various I/O Device Queues Loganathan R, CSE, HKBKCE 8
  9. 9. 2. Process Scheduling Contd…• Representation of Process Scheduling is Queuing Diagram• Rectangle represents a queue, Circle represent the resource and arrow indicate the flow of processes• New process is initially put in the ready queue and waits there until it is selected for execution, or is dispatched• Once the process is allocated the CPU and is executing, the events may occur are: The process could issue an I/O request and then be placed in an I/O queue The process could create a new sub process and wait for the sub processs Termination The process could be removed forcibly from the CPU, as a result of an interrupt, and be put back in the ready queue Loganathan R, CSE, HKBKCE 9
  10. 10. 2. Process Scheduling Contd…2.2 Schedulers• The selection processes from those queue is carried out by the appropriate scheduler• Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue  Executes much less frequently, minutes may separate the creation of one new process and the next  controls the degree of multiprogramming (the number of processes in memory)  take more time to decide which process should be selected for execution  select a good process mix of I/O-bound(spends more of its time doing I/O) and CPU-bound (spends more of its time doing computations) processes)  Time-sharing systems (UNIX &Windows) have no long-term scheduler but simply put every new process in memory for the short-term scheduler• Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU  select a new process for the CPU frequently i.e. executes at least once every 100 milliseconds  Must be fast. i.e. If it takes 10 milliseconds to decide to execute a process for 100 milliseconds, then 10/(100 + 10) = 9 percent of the CPU is being wasted simply for scheduling the work Loganathan R, CSE, HKBKCE 10
  11. 11. 2. Process Scheduling Contd…Medium Term Scheduler• Intermediate level of scheduling, removes processes from memory (and from active contention for the CPU) to reduce the degree of multiprogramming• The process can be reintroduced later into memory, and its execution can be continued where it left off(Swapping) Loganathan R, CSE, HKBKCE 11
  12. 12. 2. Process Scheduling Contd…2.3 Context Switch• When CPU switches to another process, the system must save the state(PCB) of the old process and load the saved state for the new process is known as context switch• Context-switch time is overhead; the system does no useful work while switching• Time dependent on hardware support – Sun UltraSPARC provide multiple sets of registers, which requires changing the pointer to the current register set Loganathan R, CSE, HKBKCE 12
  13. 13. 3. Operations on Processes3.1 Process Creation• Parent process create children processes via create-process systemcall, which, in turn create other processes, forming a tree of processes• Processes are identified according to a unique process identifier(or pid), which is typically an integer number A tree of process on a Solaris system Loganathan R, CSE, HKBKCE 13
  14. 14. 3. Operations on Processes Contd…• When a Process creates new process,• Resource sharing • Parent and children share all resources • Children share subset of parent’s resources • Parent and child share no resources• Execution possibilities • Parent and children execute concurrently • Parent waits until children terminate • Address space possibilities • Child duplicate of parent • Child has a program loaded into it Loganathan R, CSE, HKBKCE 14
  15. 15. 3. Operations on Processes Contd…Process Creation UNIX examples • fork system call creates new process • exec system call used after a fork to replace the process’s memory space with a new program int main() { pid_t pid; pid = fork(); if (pid < 0) { /* error occurred */ fprintf(stderr, "Fork Failed"); exit(-1); } else if (pid == 0) { /* child process */ execlp("/bin/ls", "ls", NULL); } else { /* parent process */ /* parent wait for the child to complete */ wait (NULL); printf ("Child Complete"); Process Creation exit(0); } } C Program Forking Separate Process Loganathan R, CSE, HKBKCE 15
  16. 16. 3. Operations on Processes Contd…3.2 Process Termination• Process executes last statement and asks the operating system to delete it (using exit()) – Output data from child to parent (via wait()) – Process’s 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 – If parent is exiting • Some operating system do not allow child to continue if its parent terminates – All children terminated - cascading termination Loganathan R, CSE, HKBKCE 16
  17. 17. 4. Interprocess Communication• Processes executing concurrently in the operating system may be either independent processes or cooperating 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 – break into subtasks, each of which will be executing in parallel • Modularity - dividing the system functions into separate processes or threads • Convenience - individual user may work on many tasks at the same time• Cooperating processes require an interprocess communication (IPC) mechanism to exchange data and information Loganathan R, CSE, HKBKCE 17
  18. 18. 4. Interprocess Communication Contd…• Two fundamental models of Interprocess communication• Shared memory • a region of memory that is shared by cooperating processes is established then exchange information takes place by reading and writing data to the shared region• Message passing • communication takes place by means of messages exchanged between the cooperating processes Message passing Shared memory Loganathan R, CSE, HKBKCE 18
  19. 19. 4. Interprocess Communication Contd…4.1 Shared memory Systems• A shared-memory region resides in the address space of the process creating the shared-memory• Other processes that wish to communicate using this shared-memory segment must attach it to their address space• The form of the data and the location are determined by these processes and are not under the operating systems control• Example : Producer-Consumer Problem - Paradigm for cooperating processes, producer process produces information that is consumed by a consumer process • Example : compiler may produce assembly code, which is consumed by an assembler, in turn, may produce object modules, which are consumed by the loader• To allow producer and consumer processes to run concurrently, we must have available a buffer of items that can be filled by the producer and emptied by the consumer • unbounded-buffer places no practical limit on the size of the buffer (customer may wait) • bounded-buffer assumes that there is a fixed buffer size(both waits) Loganathan R, CSE, HKBKCE 19
  20. 20. 4. Interprocess Communication Contd…Bounded-Buffer – Shared-Memory Solution• Shared buffer is implemented as a circular array with two logical pointers: in and out• The buffer is empty when in == out; the buffer is full when ((in + 1) % BUFFER_SIZE) == out. #define BUFFER_SIZE 10 typedef struct { ... } item; item buffer[BUFFER_SIZE]; int in = 0; int out = 0; Loganathan R, CSE, HKBKCE 20
  21. 21. 4. Interprocess Communication Contd…Code for the producer and consumer processes• Producer process item nextProduced; while (true) { /* produce an item in nextProduced */ while ( ( ( in + 1) % BUFFER-SIZE) == out) ; /* do nothing */ buffer[in] = nextProduced; in = (in + 1) % BUFFER_SIZE; }• Consumer process item nextConsumed; while (true) { while (in == out) ; //do nothing nextConsumed = buffer[out]; out = (out + 1) % BUFFEFLSIZE; /* consume the item in nextConsumed */ } Loganathan R, CSE, HKBKCE 21
  22. 22. 4. Interprocess Communication Contd…4.2 Message-Passing Systems• 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 ProcessP and Q wish to communicate, they need to: – establish a communication link(Not Phsical) between them – exchange messages via send/receive• Implementation of communication link – physical (e.g., shared memory, hardware bus or network) – logical (e.g., logical properties) Loganathan R, CSE, HKBKCE 22
  23. 23. 4. Interprocess Communication Contd…• Methods for logically implementing a link and send() / receive () operations • Direct or indirect communication • Synchronous or asynchronous communication • Automatic or explicit buffering4.2.1 Naming - Processes must name each other explicitly in Direct Communication • send (P, message) – send a message to process P • receive(Q, message) – receive a message from process Q• Properties 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• In Symmetry addressing - both the sender process and the receiver process must name the other i.e. send(P, message) and receive (Q, message)• In asymmetry addressing - only the sender names the recipient • send(P, message)—Send a message to process P • receive(id, message)—-Receive a message from any process; id will be the name of the sender process (disadvantage – Changing id of a process necessitates all references to the old id) Loganathan R, CSE, HKBKCE 23
  24. 24. 4. Interprocess Communication Contd…• Messages are sent to and received from mailboxes or ports • Each mailbox has a unique id • Processes can communicate only if they share a mailbox • Primitives : send(A, message and receive(A, message) where A is a mailbox• Properties of communication link • Link established only if processes share a common mailbox • A link may be associated with more than two processes • Each pair of processes may share several communication links • Link may be unidirectional or bi-directional• Mailbox sharing • P1, P2, and P3 share mailbox A and P1, sends to A; Who gets the message? • Allow a link to be associated with at most two processes • Allow only one process at a time to execute a receive operation • Allow the system to select arbitrarily the receiver. Sender is notified who the receiver was. The system may define an algorithm(RR) for selection • A mailbox may be owned either by a process or the OS • If the mailbox is owned by a process, it is distinguished as owner (only receive) & user (only send) • A mailbox owned by OS has an existence of its own and allows a process to – create a mailbox , send and receive messages through mailbox and destroy a mailbox • The process that creates a new mailbox is the owner and it can only receive messages, ownership and receiving privilege may be passed to other processes through appropriate system calls Loganathan R, CSE, HKBKCE 24
  25. 25. 4. Interprocess Communication Contd…4.2.2 Synchronization• Different design options for implementing send() and receive () primitives• Message passing may be either blocking or non-blocking also known as synchronous or asynchronous.• Blocking is considered synchronous – Blocking send - sender block until the message is received – Blocking receive - receiver block until a message is available – rendezvous between the sender and the receiver• Non-blocking is considered asynchronous – Non-blocking send - sender send the message and continue – Non-blocking receive - receiver receive a valid message or null Loganathan R, CSE, HKBKCE 25
  26. 26. 4. Interprocess Communication Contd…4.2.3 Buffering• Messages exchanged by communicating processes reside in a temporary queue• Queue of messages attached to the link is implemented in one of three ways – Zero capacity – The queue length is zero, no messages can wait. Sender must wait for receiver (rendezvous) – Bounded capacity – finite length of n messages. Sender must wait if link full – Unbounded capacity – infinite length Sender never waits Loganathan R, CSE, HKBKCE 26