Chapter 3: ProcessConcept

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Chapter 3 Process-Concept
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Chapter 3 Process-Concept

• Process Concept
• Process Scheduling
• Operations on Processes
• Cooperating Processes
• Interprocess Communication
• Communication in Client-Server Systems

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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
• A process includes
• program counter
• stack
• data section

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Process in Memory
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Multiple Processes in Memory
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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 process
• terminated The process has finished execution

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Diagram of Process State
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Process 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 information

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Process Control Block (PCB)
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Multiple Processes in Memory
PCB of D
PCB of C
free memory
PCB of B
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CPU Switch From Process to Process
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Process 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
• Processes migrate among the various queues

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Ready Queue And Various I/O Device Queues
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Representation of Process Scheduling
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Schedulers

• Long-term scheduler (or job scheduler)
• On batch systems
• batch job queue
• Selects which processes should be brought into
  the ready queue
• Short-term scheduler (or CPU scheduler)
• For multitasking (time-sharing)
• selects which process should be executed next and
  allocates CPU

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Addition of Medium Term Scheduling

• Medium-term scheduler
• on time-sharing systems
• Temporarily remove process from memory (swap out)
• Free up memory and reduce degree of
  multiprogramming to improve performance (see ch.
  8)

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Schedulers (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

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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
• Context-switch time is overhead the system does
  no useful work while switching
• Time dependent on hardware support

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Process 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 parents resources
• Parent and child share no resources
• Execution
• 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

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Process Creation
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A tree of processes on a typical Solaris
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C Program Forking Separate Process

• int main()
• Pid_t pid
• / fork another process /
• pid fork()
• if (pid lt 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")
• exit(0)

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• main()
•     int i     int n     pid_t childpid
      n 3     for (i 1 i lt n  i)
         childpid fork()        if (childpid gt
  0) break    
• printf(d, i)
• wait(NULL)

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• main()
•     int i     int n     pid_t childpid
      n 3     for (i 1 i lt n  i)
         childpid fork()        if (childpid
  0) break    
• printf(d, i)

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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

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Cooperating Processes and IPC

• 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
• Cooperating processes require an interprocess
  communication (IPC) mechanism to exchange data
  and synchronize operations

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Communications Models
a) Message passing b) Shared memory
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IPC (Interprocess Communication)

• Shared memory
• Shared memory segment
• Semaphore
• Message Passing
• Pipe
• Message queue (mailbox)
• Sockets
• Remote procedure call (RPC)

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Shared-Memory Systems

• Processes communicate by reading and writing data
  in shared memory area.
• A process creates a shared-memory segment in its
  address space.
• Other processes attach that segment to their
  address space

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Shared Memory

• Common chunk of read/write memory
• among processes

Proc. 2
Proc. 1
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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

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Bounded-Buffer Shared-Memory Solution

• Shared data
• define BUFFER_SIZE 10
• Typedef struct
• . . .
• item
• item bufferBUFFER_SIZE
• int in 0
• int out 0
• Solution is correct, but can only use
  BUFFER_SIZE-1 elements

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Bounded-Buffer Insert() Method

• while (true) / Produce an item /
• while (((in (in 1) BUFFER_SIZE)
  out)
• / do nothing -- no free buffers /
• bufferin item
• in (in 1) BUFFER SIZE

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Bounded Buffer Remove() Method

• while (true)
• while (in out)
• // do nothing -- nothing to
  consume
• // remove an item from the buffer
• item bufferout
• out (out 1) BUFFER_SIZE

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UNIX Shared Memory System Calls

• include ltsys/ipc.hgt
• include ltsys/shm.hgt
• / allocates a shared memory segment /
• int shmget(key_t key, int size, int shmflg)
• / attach and detach shared memory segment /
• char shmat ( int shmid, char shmaddr, int
  shmflg )
• char shmdt ( int shmid, char shmaddr, int
  shmflg )
• / shared memory control /
• int shmctl(int shmid, int cmd, struct shmid_ds
  buf)

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Shared Memory Segment (1)

• include ltsys/ipc.hgt
• include ltsys/shm.hgt
• include ltsys/stat.hgt
• include ltsys/types.hgt
• include ltsys/wait.hgt
• define BUFFER_SIZE 10
• typedef struct
• int pc_bufferBUFFER_SIZE
• int in
• int out
• my_buffer

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Shared Memory Segment (2)

• int main()
• / the identifier for the shared memory segment
  /
• int segment_id
• / a pointer to the shared memory segment /
• my_buffer shared_memory1, shared_memory2
• / the size (in bytes) of the shared memory
  segment /
• int segment_size, i
• pid_t pid
• segment_size sizeof(my_buffer)

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Shared Memory Segment (3)

• / allocate a shared memory segment /
• segment_id shmget(IPC_PRIVATE, segment_size,
  S_IRUSR S_IWUSR)
• / IPC_PRIVATE create new IPC
• S_IRUSR, S_IWUSR permission user readable,
  writeable
• /
• / attach the shared memory segment /
• shared_memory1 (my_buffer )
  shmat(segment_id, NULL, 0)
• printf("shared memory segment d attached at
  address p\n", segment_id, shared_memory1)
• / Initialize in and out values /
• shared_memory1 -gt in 0
• shared_memory1 -gt out 0
• pid fork()

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Shared Memory Segment (4)

• if (pid 0) / child process is producer
  /
• / attach the shared memory segment /
• shared_memory2 (my_buffer )
  shmat(segment_id, NULL, 0)
• for (i 0 i lt 20 i)
• while ((shared_memory2-gtin 1)
  BUFFER_SIZE
• shared_memory2-gtout) / Do
  Nothing /
• / Add item to buffer /
• shared_memory2-gtpc_buffershared_memory2
  -gtin i
• / Update in pointer. /
• shared_memory2-gtin (shared_memory2-gtin
  1)BUFFER_SIZE
• / End for produces 20 items /
• / detach the shared memory segment /
• shmdt((void)shared_memory2)
• exit(0)

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Shared Memory Segment (5)

• else / parent process is consumer /
• for (i 0 i lt 20 i)
• while (shared_memory1-gtin
  shared_memory1-gtout)
• / Busy polling. Do nothing useful.
  /
• / Update out pointer. /
• shared_memory1-gtout
  (shared_memory1-gtout1)BUFFER_SIZE
• / End for consumes 20 items /
• / detach the shared memory segment /
• shmdt((void)shared_memory1)
• wait (NULL)
• / remove the shared memory segment /
• shmctl(segment_id, IPC_RMID, NULL)
• exit (0)
• return 0

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Message-Passing Systems

• 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)

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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?

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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

• 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

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Indirect Communication

• Mailbox sharing
• P1, P2, and P3 share mailbox A
• P1, sends P2 and P3 receive
• Who gets the message?
• Possible 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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Pipe

• Create complicate transformation by feeding
  standard output of one filter into standard input
  of another filter
• ls -l gt my.file sort -n 4 lt myfile
• ls -l sort -n 4
• Two filters (ls and sort) do not share file
  descriptor table
• Standard output of ls is connected to standard
  input of sort through a pipe

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Pipe

• include ltunistd.hgt
• int pipe(int fildes2)
• pipe(fildes) creates a communication buffer
• The pipe can be accessed through file
  descriptors fildes0 and fildes1
• Data is read from fildes0 and written to
  fildes1
• Data passes through a pipe in one direction on a
  first-in-first-out basis

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Pipe

• define MAXLINE 80
• main()
• int n, fd2
• char lineMAXLINE
• pipe(fd)
• if (fork()gt0)
• close(fd0)
• write(fd1, "hello\n", 6)
• else
• close(fd1)
• n read(fd0, line, MAXLINE)
• write(1, line, n)

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Message Queue (1)

• include ltsys/wait.hgt
• include ltsys/types.hgt
• include ltsys/msg.hgt
• include ltsys/ipc.hgt
• define TRUE 1
• define FALSE 0
• define BLOCKING 0 // blocking send/receive,
  msgflg value
• define MYPORT 12345 // unique value to create
  key
• define FIRST_MSG 0 // receive first message
  in queue
• define DATA_T 1 // type code for a data
  message
• define PERMS 0644 // access permission
• define MSG_SIZE 100 // max. size of message
• int ret_code

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Message Queue (2)

• // the message buffer record
• typedef struct msgbuffer
• long mtype // message type (must be gt 0)
• char the_messageMSG_SIZE // the message
• Message, Messageptr
• // message queue parameters
• key_t myKey // unique key
• int queueId // message queue identifier
• int flags
• size_t msgsize sizeof(Message)
• int main(int argc, char argv)
• Messageptr message1
• char msgMSG_SIZE
• char msgptr

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Message Queue (3)

• // create an IPC key for our message queue
• myKey ftok(".", MYPORT)
• // create the message queue
• // IPC_CREAT - create a queue with the given
  key
• // if it does not already exist.
• // IPC_EXCL - return error if queue with the
  given
• // key already exists.
• // PERMS - specify permissions for queue.
• flags IPC_CREAT IPC_EXCL PERMS
• queueId msgget(myKey, flags)
• // get a message from the user
• msgptr fgets(msg, MSG_SIZE, stdin)

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Message Queue (4)

• // fork a new process
• if(fork())
• // Parent send a message to the queue
• message1 (Messageptr)malloc(msgsize)
• message1-gtmtype DATA_T
• strncpy(message1-gtthe_message, msgptr,
  MSG_SIZE-1)
• ret_code msgsnd(queueId, message1,
  msgsize, BLOCKING)
• printf("Message sent\n")
• sleep(2) // sleep before parent cleans up
  the message queue

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Message Queue (5)

• else // Child read message from queue
• sleep(1)
• printf("Inside child process\n")
• message1 (Messageptr)malloc(msgsize)
• msgrcv(queueId, message1, msgsize,
• FIRST_MSG, BLOCKING)
• printf("Child process read message type d
  s",
• message1-gtmtype, message1-gtthe_messag
  e)
• exit(0)
• // close the message queue
• msgctl(queueId, IPC_RMID, NULL)
• return 0
• / end main /

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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

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Buffering

• Queue of messages attached to the link
  implemented in one of three ways
• 1. Zero capacity 0 messagesSender must wait
  for receiver (rendezvous)
• 2. Bounded capacity finite length of n
  messagesSender must wait if link full
• 3. Unbounded capacity infinite length Sender
  never waits

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Client-Server Communication

• Sockets
• Remote Procedure Calls
• Remote Method Invocation (Java)

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Sockets

• A socket is defined as an endpoint for
  communication
• Concatenation of IP address and port
• The socket 161.25.19.81625 refers to port 1625
  on host 161.25.19.8
• Communication consists between a pair of sockets

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Socket Communication
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Socket Communication

• Server
• Create it with socket()
• Name it with bind()
• Queue client connection requests with listen()
• Accept client connection requests with accept()
• Client
• Create it with socket()
• Request a connection to a listening server with
  connect()

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import java.net. import java.io. public
class DateServer public static void
main(String args) try
ServerSocket sock new ServerSocket(6013)
// now listen for connections while
(true) Socket client
sock.accept() PrintWriter
pout new PrintWriter(client.getOutputStream(),
true) // write
the Date to the socket
pout.println(new java.util.Date().toString())
// close the socket and resume
listening for more connections
client.close()
catch (IOException ioe)
System.err.println(ioe)
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import java.net. import java.io. public
class DateClient public static void
main(String args) try
// this could be changed to an IP name or address
other than the localhost Socket sock
new Socket("127.0.0.1",6013)
InputStream in sock.getInputStream()
BufferedReader bin new BufferedReader(new
InputStreamReader(in)) String
line while( (line bin.readLine())
! null) System.out.println(line)
sock.close()
catch (IOException ioe)
System.err.println(ioe)
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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.

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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.

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Marshalling Parameters
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End of Chapter 3