CS计算机代考程序代写 gui javascript android Java IOS Module 4: Processes

Module 4: Processes

Chapter 3: Processes

Processes
Process Concept
Process Scheduling
Operations on Processes
Interprocess Communication
Examples of IPC Systems
Communication in Client-Server Systems

Objectives
To introduce the notion of a process — a program in execution, which forms the basis of all computation
To describe the various features of processes, including scheduling, creation and termination, and communication
To describe communication in client-server systems

Process Concept
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
Process – a program in execution; process execution must progress in sequential fashion

Process Concept
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
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

Process Concept (Cont.)

Process in Memory

Process 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

Diagram of Process State

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

CPU Switch From Process to Process

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

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

Threads

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 */
Process Representation in Linux

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

Process Scheduling

Ready Queue And Various I/O Device Queues

Representation of Process Scheduling

Schedulers
Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU
Sometimes the only scheduler in a system
Short-term scheduler is invoked frequently (milliseconds)  (must be fast)
Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue
Long-term scheduler is invoked infrequently (seconds, minutes)  (may be slow)
The long-term scheduler controls the degree of multiprogramming

Addition of Medium Term Scheduling

Some mobile systems (e.g., early version of iOS) 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

Multitasking in Mobile Systems

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

Multitasking in Mobile Systems

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  the longer the context switch

Time dependent on hardware support

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
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

Process 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

C Program Forking Separate Process
int main() {
pid_t pid;
/* fork another process */
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 will wait for the child to complete */ wait (NULL); printf ("Child Complete"); } return 0; } Creating a Separate Process via Windows API Process Termination Process executes last statement and asks the operating system to delete 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 If parent is exiting Some operating system do not allow child to continue if its parent terminates All children terminated - cascading termination Process Termination Some operating systems do not allow child to exists if its parent has terminated. If a process terminates, then all its children must also be terminated. cascading termination. All children, grandchildren, etc. are terminated. The termination is initiated by the operating system. The parent process may wait for termination of a child process by using the wait()system call. The call returns status information and the pid of the terminated process pid = wait(&status); If no parent waiting (did not invoke wait()) process is a zombie If parent terminated without invoking wait , process is an orphan Many web browsers ran as single process (some still do) If one web site causes trouble, entire browser can hang or crash Google Chrome Browser is multiprocess with 3 different types of processes: Browser process manages user interface, disk and network I/O Renderer process renders web pages, deals with HTML, Javascript. A new renderer created for each website opened Runs in sandbox restricting disk and network I/O, minimizing effect of security exploits Plug-in process for each type of plug-in Multiprocess Architecture – Chrome Browser Interprocess Communication Processes within a system may be independent or cooperating Cooperating process can affect or be affected by other processes, including sharing data Reasons for cooperating processes: Information sharing Computation speedup Modularity Convenience Cooperating processes need interprocess communication (IPC) Two models of IPC Shared memory Message passing Communications Models (a) Message passing. (b) shared memory. 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 Modularity Convenience Producer-Consumer Problem Paradigm for cooperating processes, producer process produces information that is consumed by a consumer process unbounded-buffer places no practical limit on the size of the buffer bounded-buffer assumes that there is a fixed buffer size Bounded-Buffer – Shared-Memory Solution Shared data #define BUFFER_SIZE 10 typedef struct { . . . } item; item buffer[BUFFER_SIZE]; int in = 0; int out = 0; Bounded-Buffer – Producer while (true) { /* Produce an item */ while ((((in + 1) % BUFFER SIZE)== out) /*NOP*/ ; /* i.e., no free buffers */ buffer[in] = item; in = (in + 1) % BUFFER SIZE; } Bounded Buffer – Consumer while (true) { while (in == out) /*NOP*/; // nothing to consume // remove an item from the buffer item = buffer[out]; out = (out + 1) % BUFFER SIZE; return item; } Solution is correct, but can only use how many slots when full? Ans: BUFFER_SIZE-1 Interprocess Communication – Shared Memory An area of memory shared among the processes that wish to communicate The communication is under the control of the users processes not the operating system. Major issues is to provide mechanism that will allow the user processes to synchronize their actions when they access shared memory. Synchronization is discussed in great details in Chapter 5. Interprocess Communication – Message Passing 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/receive Implementation of communication link physical (e.g., shared memory, hardware bus) logical (e.g., logical properties) Implementation 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? Direct Communication Processes must name each other explicitly: 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 Indirect Communication Messages are directed and received from mailboxes (also referred to as ports) Each mailbox has a unique id Processes can communicate only if they share a mailbox Properties of communication link Link established only if processes share a common mailbox A link may be associated with many processes Each pair of processes may share several communication links Link may be unidirectional or bi-directional Indirect Communication Operations create a new mailbox send and receive messages through mailbox destroy a mailbox Primitives are defined as: send(A, message) – send a message to mailbox A receive(A, message) – receive a message from mailbox A Indirect Communication Mailbox sharing P1, P2, and P3 share mailbox A P1, sends; P2 and P3 receive Who gets the message? Solutions 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. Synchronization Message passing may be either blocking or non-blocking Blocking is considered synchronous Blocking send has the sender block until the message is received Blocking receive has the receiver block until a message is available Non-blocking is considered asynchronous Non-blocking send has the sender send the message and continue Non-blocking receive has the receiver receive a valid message or null Synchronization Message passing may be either blocking or non-blocking Blocking is considered synchronous Blocking send -- the sender is blocked until the message is received Blocking receive -- the receiver is blocked until a message is available Non-blocking is considered asynchronous Non-blocking send -- the sender sends the message and continue Non-blocking receive -- the receiver receives: A valid message, or Null message Different combinations possible If both send and receive are blocking, we have a rendezvous Producer-consumer becomes trivial message next_produced; while (true) { /* produce an item in next produced */ send(next_produced); } message next_consumed; while (true) { receive(next_consumed); /* consume the item in next consumed */ } Synchronization (Cont.) Buffering Queue of messages attached to the link; implemented in one of three ways 1. Zero capacity – 0 messages Sender must wait for receiver (rendezvous) 2. Bounded capacity – finite length of n messages Sender must wait if link full 3. Unbounded capacity – infinite length Sender never waits Examples of IPC Systems - POSIX POSIX Shared Memory Process first creates shared memory segment segment id = shmget(IPC PRIVATE, size, S IRUSR | S IWUSR); Process wanting access to that shared memory must attach to it shared memory = (char *) shmat(id, NULL, 0); Now the process could write to the shared memory sprintf(shared memory, "Writing to shared memory"); When done a process can detach the shared memory from its address space shmdt(shared memory); Examples of IPC Systems - Mach Mach communication is message-based Even system calls are messages Each task gets two mailboxes at creation- Kernel and Notify Only three system calls needed for message transfer msg_send(), msg_receive(), msg_rpc() Mailboxes needed for commuication, created via port_allocate() Examples of IPC Systems – Windows XP Message-passing centric via local procedure call (LPC) facility Only works between processes on the same system Uses ports (like mboxes) to establish and maintain communication channels Communication works as follows: The client opens a handle to the subsystem’s connection port object The client sends a connection request The server creates two private communication ports and returns the handle to one of them to the client The client and server use the corresponding port handle to send messages or callbacks and to listen for replies Local Procedure Calls in Windows XP Communications in Client-Server Systems Sockets Remote Procedure Calls Remote Method Invocation (Java) Sockets A socket is defined as an endpoint for communication Concatenation of IP address and port The socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8 Communication consists between a pair of sockets Socket Communication Remote Procedure Calls Remote procedure call (RPC) abstracts procedure calls between processes on networked systems Stubs – client-side proxy for the actual procedure on the server The client-side stub locates the server and marshalls the parameters The server-side stub receives this message, unpacks the marshalled parameters, and peforms the procedure on the server Execution of RPC Remote Method Invocation Remote Method Invocation (RMI) is a Java mechanism similar to RPCs RMI allows a Java program on one machine to invoke a method on a remote object Marshalling Parameters Acts as a conduit allowing two processes to communicate Issues: Is communication unidirectional or bidirectional? In the case of two-way communication, is it half or full-duplex? Must there exist a relationship (i.e., parent-child) between the communicating processes? Can the pipes be used over a network? Ordinary pipes – cannot be accessed from outside the process that created it. Typically, a parent process creates a pipe and uses it to communicate with a child process that it created. Named pipes – can be accessed without a parent-child relationship. Pipes Pipes Ordinary Pipes allow communication in standard producer-consumer style Producer writes to one end (the write-end of the pipe) Consumer reads from the other end (the read-end of the pipe) Ordinary pipes are therefore unidirectional Require parent-child relationship between communicating processes Windows calls these anonymous pipes See Unix and Windows code samples in textbook Pipes Named Pipes are more powerful than ordinary pipes Communication is bidirectional No parent-child relationship is necessary between the communicating processes Several processes can use the named pipe for communication Provided on both UNIX and Windows systems Print process A message queue kernel (a) (b) process A shared memory kernel process B m0 m1 m2 ...m3 mn process B