Processes

Chapter 3

Objectives

• Identify the separate components of a process and illustrate how they are represented and scheduled in an operating system.
• Describe how processes are created and terminated in an operating system, including developing programs using the appropriate system calls that perform these operations.
• Describe and contrast interprocess communication using shared memory and message passing.
• Design programs that uses pipes and POSIX shared memory to perform interprocess communication.
• Write program using the fork() system call
• Describe client-server communication using sockets and remote procedure calls.
• Design kernel modules that interact with the Linux operating system.

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Process Concept

• An operating system executes a variety of programs that run as a process.
• Process – a program in execution; process execution must progress in sequential fashion. No parallel execution of instructions of a single process
• A process consists of multiple parts such as:
• 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

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Process Concept (Cont.)

• Program is passive entity stored on disk (executable file); process is active
• Program becomes process when an executable file is 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

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Process in Memory

Memory Layout of a C Program

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

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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

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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
• Explore in detail in Chapter 4 (Our next chapter)

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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 */

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Process Scheduling

• Process scheduler selects among available processes for next execution on CPU core
• Goal -- Maximize CPU use, quickly switch processes onto CPU core
• Maintains scheduling queues of processes
• Ready queue – set of all processes residing in main memory, ready and waiting to execute
• Wait queues – set of processes waiting for an event (i.e., I/O)
• Processes migrate among the various queues

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Ready and Wait Queues

Representation of Process Scheduling

CPU Switch From Process to Process

• A context switch occurs when the CPU switches from one process to another.

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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 pure 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
• Some hardware provides multiple sets of registers per CPU 🡺 multiple contexts loaded at once

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Multitasking in Mobile Systems

• 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
• 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

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Operations on Processes

• System must provide mechanisms for:
• Process creation
• Process termination

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Process Creation

• Parent process creates 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

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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
• Parent process calls wait()waiting for the child to terminate

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A Tree of Processes in Linux

C Program Forking Separate Process

Creating a Separate Process via Windows API

Process Termination

• Process executes last statement and then asks the operating system to delete it using the exit() system call.
• Returns status data from child to parent (via wait())
• Process’ resources are deallocated by operating system
• Parent may terminate the execution of children processes using the abort() system call. Some reasons for doing so:
• Child has exceeded allocated resources
• Task assigned to child is no longer required
• The parent is exiting, and the operating systems does not allow a child to continue if its parent terminates

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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

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Android Process Importance Hierarchy

• Mobile operating systems often have to terminate processes to reclaim system resources such as memory. From most to least important:
• Foreground process
• Visible process
• Service process
• Background process
• Empty process
• Android will begin terminating processes that are least important.

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Multiprocess Architecture – Chrome Browser

• 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

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

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Communications Models

• a) Shared memory. (b) Message passing.

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Producer-Consumer Problem

• Paradigm for cooperating processes:
• producer process produces information that is consumed by a consumer process
• Two variations:
• unbounded-buffer places no practical limit on the size of the buffer:
• Producer never waits
• Consumer waits if there is no buffer to consume
• bounded-buffer assumes that there is a fixed buffer size
• Producer must wait if all buffers are full
• Consumer waits if there is no buffer to consume

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IPC – 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 Chapters 6 & 7.

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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

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Bounded-Buffer – Shared-Memory Solution

• Shared data

#define BUFFER_SIZE 10

typedef struct {

. . .

} item;

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item buffer[BUFFER_SIZE];

int in = 0;

int out = 0;

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• Solution is correct, but can only use BUFFER_SIZE-1 elements

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Producer Process – Shared Memory

item next_produced; �

while (true) {

/* produce an item in next produced */

while (((in + 1) % BUFFER_SIZE) == out)

; /* do nothing */

buffer[in] = next_produced;

in = (in + 1) % BUFFER_SIZE;

}

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Consumer Process – Shared Memory

item next_consumed; �

while (true) {� while (in == out)

; /* do nothing */� next_consumed = buffer[out];

out = (out + 1) % BUFFER_SIZE;�

/* consume the item in next consumed */

}

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What about Filling all the Buffers

• Suppose that we wanted to provide a solution to the consumer-producer problem that fills all the buffers.
• We can do so by having an integer counter that keeps track of the number of full buffers.
• Initially, counter is set to 0.
• The integer counter is incremented by the producer after it produces a new buffer.
• The integer counter is and is decremented by the consumer after it consumes a buffer.

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Producer

while (true) {� /* produce an item in next produced */

while (counter == BUFFER_SIZE)

; /* do nothing */

buffer[in] = next_produced;

in = (in + 1) % BUFFER_SIZE;

counter++;

}

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Consumer

while (true) {

while (counter == 0)

; /* do nothing */

next_consumed = buffer[out];

out = (out + 1) % BUFFER_SIZE;

counter--;

/* consume the item in next consumed */

}

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Race Condition

• counter++ could be implemented as�� register1 = counter� register1 = register1 + 1� counter = register1
• counter-- could be implemented as�� register2 = counter� register2 = register2 - 1� counter = register2

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• Consider this execution interleaving with “count = 5” initially:

S0: producer execute register1 = counter {register1 = 5}�S1: producer execute register1 = register1 + 1 {register1 = 6} �S2: consumer execute register2 = counter {register2 = 5} �S3: consumer execute register2 = register2 – 1 {register2 = 4} �S4: producer execute counter = register1 {counter = 6 } �S5: consumer execute counter = register2 {counter = 4}

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Race Condition (Cont.)

• Question – why was there no race condition in the first solution (where at most N – 1) buffers can be filled?
• More in Chapter 6.

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IPC – Message Passing

• Processes communicate with each other without resorting to shared variables

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• IPC facility provides two operations:
• send(message)
• receive(message)

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• The message size is either fixed or variable

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Message Passing (Cont.)

• If processes P and Q wish to communicate, they need to:
• Establish a communication link between them
• Exchange messages via send/receive
• Implementation issues:
• 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?

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Implementation of Communication Link

• Physical:
• Shared memory
• Hardware bus
• Network
• Logical:
• Direct or indirect
• Synchronous or asynchronous
• Automatic or explicit buffering

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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

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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

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Indirect Communication (Cont.)

• Operations
• Create a new mailbox (port)
• Send and receive messages through mailbox
• Delete a mailbox
• Primitives are defined as:
• send(A, message) – send a message to mailbox A
• receive(A, message) – receive a message from mailbox A

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Indirect Communication (Cont.)

• 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.

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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

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Producer-Consumer: Message Passing

• Producer

message next_produced;� while (true) {� /* produce an item in next_produced */�

send(next_produced);

}

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• Consumer

message next_consumed;� while (true) {� receive(next_consumed)�� /* consume the item in next_consumed */� }

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Buffering

• Queue of messages attached to the link.
• Implemented in one of three ways

1. Zero capacity – no messages are queued on a link.�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

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Examples of IPC Systems - POSIX

• POSIX Shared Memory
• Process first creates shared memory segment�shm_fd = shm_open(name, O CREAT | O RDWR, 0666);
• Also used to open an existing segment
• Set the size of the object

ftruncate(shm_fd, 4096);

• Use mmap() to memory-map a file pointer to the shared memory object
• Reading and writing to shared memory is done by using the pointer returned by mmap().

IPC POSIX Producer

IPC POSIX Consumer

Examples of IPC Systems – Windows

• Message-passing centric via advanced local procedure call (LPC) facility
• Only works between processes on the same system
• Uses ports (like mailboxes) 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

Pipes

• 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.

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Ordinary 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

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• Windows calls these anonymous pipes

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Named 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

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Communications in Client-Server Systems

• Sockets
• Remote Procedure Calls

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Sockets

• A socket is defined as an endpoint for communication
• Concatenation of IP address and port – a number included at start of message packet to differentiate network services on a host
• The socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8
• Communication consists between a pair of sockets
• All ports below 1024 are well known, used for standard services
• Special IP address 127.0.0.1 (loopback) to refer to system on which process is running

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Socket Communication

Sockets in Java

• Three types of sockets
• Connection-oriented (TCP)
• Connectionless (UDP)
• MulticastSocket class– data can be sent to multiple recipients
• Consider this “Date” server in Java:

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Sockets in Java – The equivalent Date client�

Remote Procedure Calls

• Remote procedure call (RPC) abstracts procedure calls between processes on networked systems
• Again uses ports for service differentiation
• 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 performs the procedure on the server
• On Windows, stub code compile from specification written in Microsoft Interface Definition Language (MIDL)

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Remote Procedure Calls (Cont.)

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• Data representation handled via External Data Representation (XDL) format to account for different architectures
• Big-endian and little-endian
• Remote communication has more failure scenarios than local
• Messages can be delivered exactly once rather than at most once
• OS typically provides a rendezvous (or matchmaker) service to connect client and server

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Execution of RPC

The End!