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Concurrency Mutual Exclusion and Synchronization (PowerPoint) by SanjuDudeja

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Principles of Operating Systems

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									Concurrency: Mutual Exclusion
    and Synchronization
           Chapter 5
         Running Together!!
•   Look at the diagram; The issues are:
•   Communication among processes
•   Sharing resources
•   Synchronization of multiple processes
•   Allocation of processor time
Multiprogramming and
  Multiprocessing
             Concurrency
• Multiple applications
  – Multiprogramming to share the processor
    time
• Structured applications
  – Applications can be designed as a set of
    concurrent processes
• Operating-system structure
  – Operating system is also a set of processes
    or threads
Difficulties with Concurrency
• Sharing global resources including
  variables (read/write dependencies)
• Management of allocation of resources
  such as I/O devices
• Programming errors difficult to locate
  because of unpredictable behaviors
      A Simple Example
void echo()
{
char chin,chout;
  chin = getchar();
  chout = chin;
  putchar(chout);
}
This is a global procedure used
  by processes P1 and P2 which
  share chin as a common variable
      A Simple Example
 Process P1      Process P2
.                 .
chin = getchar(); .
. (Interrupt)     chin = getchar();
chout = chin;     chout = chin;
putchar(chout);   .
.                 putchar(chout);
.                 .
           The Solution
• Only one process should be allowed to
  use this procedure at any given time
• Process P1 interrupted while inside
  procedure echo
• Process P2 blocked from using echo so
  this problem cannot occur!!
• PROTECT GLOBAL VARIABLES BY
  CONTROLLING THE CODE
  Operating System Concerns
• Keep track of active processes (ch4)
• Allocate and deallocate resources
   –Processor time (scheduling)
   –Memory (Virtual Memory Mgmt.)
   –Files (File System)
   –I/O devices (I/O Mgmt.)
• Protect data and resources (Protection)
• Result of process must be independent of the
  speed of execution of other concurrent
  processes
        Process Interaction
• Processes unaware of each other (not
  designed to work together; competing)
• Processes indirectly aware of each other
  (Share access to an object; cooperating)
• Process directly aware of each other
  (designed to work together and use IPC
  or other ways to communicate)
            Competition
• Each process unaware and (its results
  are) unaffected by other processes
• However, the one who waits for a
  resource slows down; (starvation?)
• Non-sharable resources such as printers
  need to be allocated in mutual exclusion
  state (deadlock?) (critical section)
Cooperation Among Processes
         by Sharing
• Processes know that there are other
  processes out there
• Writing must be mutually exclusive
• Critical sections are used to provide data
  integrity
• Deadlock, starvation may occur
Cooperation Among Processes
    by Communication
• Messages are passed without sharing any
  resource
   – Mutual exclusion is not a control requirement
• Possible to have deadlock
   – Each process waiting for a message from the other
     process
• Possible to have starvation
   – Two processes sending message to each other
     while another process waits for a message
   Requirements for Mutual
          Exclusion
• Only one process at a time is allowed in
  the critical section for a resource
• A process that halts in its non-critical
  section must do so without interfering
  with other processes
• No deadlock or starvation
   Requirements for Mutual
          Exclusion
• A process must not be delayed access to
  a critical section when there is no other
  process using it
• No assumptions are made about relative
  process speeds or number of processes
• A process remains inside its critical
  section for a finite time only
 Providing Software Solution for Mutual
     Exclusion: Dekker’s Algorithm
• No support from OS except to serialize
  shared resource access
• Busy Waiting
  – Use a global variable called “turn”
  – Process is always checking to see if it can
    enter the critical section
  – Process can do nothing productive until it
    gets permission to enter its critical section
   Processes are Coroutines
• Designed to be able to pass execution
  control back and forth between
  themselves
• Inadequate to support concurrent
  processing
• Processes may suffer from slowing
  down as well as crashing all processes
            Second Attempt
• Each process can examine the other’s status
  with a boolean array flag[] but cannot alter it
• When a process wants to enter the critical
  section is checks the other processes first
• If no other process is in the critical section, it
  sets its status for the critical section
• This method does not guarantee mutual
  exclusion and it is not crash proof
• Each process can check the flags and then
  proceed to enter the critical section at the same
  time
Attempts
            Third Attempt
• Set flag to enter critical section before
  checking other processes
• If another process is in the critical section
  when the flag is set, the process is blocked
  until the other process releases the critical
  section. Mutual Exclusion is guaranteed!!
• Deadlock is possible when two process set
  their flags to enter the critical section. Now
  each process must wait for the other process to
  release the critical section
           Fourth Attempt
• A process sets its flag to indicate its
  desire to enter its critical section but is
  prepared to reset the flag
• Other processes are checked. If they are
  in the critical region, the flag is reset and
  later set to indicate desire to enter the
  critical region. This is repeated until the
  process can enter the critical region.
           Fourth Attempt
• It is possible for each process to set its
  flag, check other processes, and reset its
  flag. This scenario is called
  “livelock”and it will not last very long
  so it is not deadlock. It is undesirable
  but with random waiting delay, this
  cycle can be broken
          Correct Solution
• Each process gets a turn at the critical
  section
• If a process wants the critical section, it
  sets its flag and may have to wait for its
  turn. If “turn” indicates this process’s
  turn, it can enter the critical section
• Both “turn” and “flag” provide a
  working solution!!
         Mutual Exclusion:
         Hardware Support
• Uniprocessor
  – A process runs until it invokes an operating-system
    service or until it is interrupted
  – Disabling interrupts guarantees mutual exclusion
  – Processor is limited in its ability to interleave
    programs because interrupts are being disabled
• Multiprocessing
     • disabling interrupts on one processor will not
       guarantee mutual exclusion
        Mutual Exclusion:
        Hardware Support
• Special Machine Instructions
  – Performed in a single instruction cycle
  – Not subject to interference from other
    instructions
  – Reading and writing
  – Reading and testing
       Mutual Exclusion:
       Hardware Support
• Test and Set Instruction (Atomic)
      boolean testset (int i) {
            if (i == 0) {
                   i = 1;
                   return true;
            }
            else {
                   return false;
            }
      }
        Mutual Exclusion:
        Hardware Support
• Exchange Instruction (Access to memory
  blocked for others)
  void exchange(int register,
                   int memory) {
      int temp;
      temp = memory;
      memory = register;
      register = temp;
  }
  Mutual Exclusion Machine
         Instructions
• Advantages
  – Applicable to any number of processes on
    either a single processor or multiple
    processors sharing main memory
  – It is simple and therefore easy to verify
  – It can be used to support multiple critical
    sections, each defined by its own variable
  Mutual Exclusion Machine
         Instructions
• Disadvantages
  – Busy-waiting consumes processor time
  – Starvation is possible when a process leaves
    a critical section and more than one process
    is waiting.
  – Deadlock
     • If a low priority process has the critical region
       and it is interrupted in favor of a higher priority
       process, the higher priority process will obtain
       the processor and may wait for the critical
       region
             Semaphores
• Special variable called a semaphore is
  used for signaling
• If a process is waiting for a signal, it is
  suspended until that signal is sent
• Wait and signal operations cannot be
  interrupted
• Queue is used to hold processes waiting
  on the semaphore
               Semaphores
• Semaphore is a variable that has an integer
  value
• It may be initialized to a nonnegative number
• Wait operation decrements the semaphore
  value and if the value becomes negative, the
  process is suspended
• Signal operation increments semaphore value
  and if it is still negative, a process waiting is
  unblocked
• Check out the binary semaphores
• What is a weak semaphore anyway?
 Producer/Consumer Problem
• One or more producers are generating
  data and placing these in a buffer
• A single consumer is taking items out of
  the buffer one at time
• Only one producer or consumer may
  access the buffer at any one time (No
  overlap allowed in buffer access)
          Producer
producer:
while (true) {
  /* produce item v */
  b[in] = v;
  in++;
}
         Consumer
consumer:
while (true) {
  while (in <= out)
    /*do nothing */;
  w = b[out];
  out++;
  /* consume item w */
}
Infinite Buffer
   Semaphores ensure mutual
         exclusion
• Producer performs wait on a semaphore before
  writing to the buffer
• If the buffer is being accessed by another
  producer or consumer, the producer will be
  blocked
• Consumer performs wait on a second
  semaphore that blocks it if the buffer is empty
• Thus the producer also has to perform signal
  on the second semaphore if it just put a value
  in an empty buffer. Why?
• TO WAKE UP THE CONSUMER
• See the program and discussion in the textbook p223
Producer with Circular Buffer
producer:
while (true) {
  /* produce item   v */
  while ((in + 1)   % n == out)
    /* do nothing   */;
  b[in] = v;
  in = (in + 1) %   n
}
   Consumer with Circular
         Buffer
consumer:
while (true) {
  while (in == out)
    /* do nothing */;
  w = b[out];
  out = (out + 1) % n;
  /* consume item w */
}
          Need a Haircut?
• But the barber shop has only three chairs
  for customers so only three barbers are
  available
• The waiting area has the capacity of four
  customers
• Only 20 customers can be
  accommodated within the shop at any
  time
Barbershop Problem
      Barbershop Problem
• When a barber is free, the longest
  waiting customer on the sofa moves to
  the chair
• The longest standing customer then sits
  on the sofa
• Payment is accepted one at a time
• Barbers cut hair, accept payment and
  sleep if no customers are around
        Barbershop Problem
•   /* program barbershop1 */
•   semaphore max_capacity = 20;
•   semaphore sofa = 4;
•   semaphore barber_chair = 3;
•   semaphore coord = 3;
•   semaphore cust_ready = 0, finished = 0,
    leave_b_chair = 0, payment= 0, receipt =
    0;
                             Customer
•   void customer ()
•   {
•   wait(max_capacity);
•   enter_shop();
•   wait(sofa);
•   sit_on_sofa();
•   wait(barber_chair);
•   get_up_from_sofa();
•   signal(sofa);
•   sit_in_barber_chair;
•   signal(cust_ready);
•   wait(finished);
•   leave_barber_chair();
•   signal(leave_b_chair);
•   pay();
•   signal(payment);
•   wait(receipt);
•   exit_shop();
•   signal(max_capacity)
•   }
                       Barber
•   void barber()
•   {
•   while (true)
•   {
•   wait(cust_ready);
•   wait(coord);
•   cut_hair();
•   signal(coord);
•   signal(finished);
•   wait(leave_b_chair);
•   signal(barber_chair);
•   }
•   }
                       Cashier
•   void cashier()
•   {
•   while (true)
•   {
•   wait(payment);
•   wait(coord);
•   accept_pay();
•   signal(coord);
•   signal(receipt);
•   }
•   }
       What is ensured here?
•   Shop and Sofa Capacity
•   Barber Chair capacity
•   Putting exactly one customer in a chair
•   Holding customer in chair
•   Forcing customer to pay before leaving
•   Switching barber to become a cashier
•   Speed not taken into account; see
    discussion starting on page 232
               Monitors
• Monitor is a software module
• Chief characteristics
  – Local data variables are accessible only by
    the monitor
  – Process enters monitor by invoking one of
    its procedures
  – Only one process may be executing in the
    monitor at a time
  – Wait and signal different from semaphores;
    a signal may be lost if no one waiting
        Message Passing
• Enforce mutual exclusion
• Exchange information

 send (destination, message)
 receive (source, message)
          Synchronization
• Sender and receiver may or may not be
  blocking (waiting for message)
• Blocking send, blocking receive
  – Both sender and receiver are blocked until
    message is delivered
  – Called a rendezvous
  – Tight synchronization
          Synchronization
• Non-blocking send, blocking receive
  – Sender continues processing such as
    sending messages as quickly as possible
  – Receiver is blocked until the requested
    message arrives (such as a server process)
• Non-blocking send, non-blocking
  receive
  – Neither party is required to wait
• Non-blocking send is dangerous!!
              Addressing
• Direct addressing
  – send primitive includes a specific identifier
    of the destination process
  – receive primitive could know ahead of time
    which process a message is expected
  – receive primitive could use source
    parameter to return a value when the
    receive operation has been performed
              Addressing
• Indirect addressing
  – messages are sent to a shared data structure
    consisting of queues
  – queues are called mailboxes
  – one process sends a message to the mailbox
    and the other process picks up the message
    from the mailbox
  – Sender and receiver are decoupled!!
  – 1-N, N-1, 1-1 and N-N messages possible
Message Format
    Readers/Writers Problem
• Any number of readers may
  simultaneously read the file
• Only one writer at a time may write to
  the file
• If a writer is writing to the file, no reader
  may read it
• Read Section 5.7

								
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