Interprocess Communication

1
Interprocess Communication

• 1. Ways of passing information
• 2. Guarded critical activities (e.g. updating
  shared data)
• 3. Proper sequencing in case of dependencies
• 2 and 3 apply to threads as well.

2
Race Conditions

• Two processes want to access shared memory at the
  same time.
• The final result depends on who runs precisely
  when.

3
Critical Regions (1)

• Part of the program where shared memory is
  accessed.
• Four conditions to provide correct and efficient
  communication
• 1. Mutual exclusion No two processes
  simultaneously in critical region
• 2. No assumptions made about speeds or numbers of
  CPUs
• 3. Progress No process running outside its
  critical region may block another process
• 4. Fairness No process must wait forever to
  enter its critical region (assuming fair
  scheduling!)

4
Critical Regions (2)

• Mutual exclusion using critical regions

5
Attempts for Mutual Exclusion Using Busy Waiting

• 1. Disabling all interrupts (only by kernel!)
• 2. Lock variables gt fatal race condition
• 3. Strict alternation using spin locks - violates
  condition 3
• 4. Petersons solution
• 5. Test-and-set locks (TSL)
• Priority inversion problem (H loops while L is in
  critical section)

6
Strict Alternation

• Proposed solution to critical region problem
• (a) Process 0. (b) Process 1.

7
Petersons Solution

• Interested(process)False gt process is not in
  and does not want to enter critical section
• If both are interested, a process can enter only
  if it is the others turn.

8
Test-and-set lock

• Entering and leaving a critical region using the
• TSL instruction
• Atomic instruction, implemented in hardware

9
Sleep and Wakeup

• Producer-consumer problem with fatal race
  condition

10
Semaphores

• Integer variable with two atomic operations
• down if 0, then go to sleep
• if gt0, then decrement value
• up increment value and let a sleeping process
  to perform a down
• Implementation by disabling all interrupts by the
  kernel.

11
Semaphores

• The producer-consumer problem using semaphores

12
Mutexes

• Implementation of mutex_lock and mutex_unlock
• for synchronization of threads in user space

13
Monitors (1)

• Example of a monitor - only one process inside
  the monitor at any time

14
Monitors (2)

• Outline of producer-consumer problem with
  monitors
• only one monitor procedure active at one time
• buffer has N slots
• Condition variables with wait and signal

15
Message Passing

• The producer-consumer problem with N messages

16
Barriers

• Use of a barrier
• processes approaching a barrier
• all processes but one blocked at barrier
• last process arrives, all are let through

17
Dining Philosophers

• Philosophers eat/think
• Eating needs 2 forks
• Pick one fork at a time
• How to prevent deadlock

18
A nonsolution to the dining philosophers problem
19
Deadlock-free code for Dining Philosophers (1)
20
Deadlock-free code for Dining Philosophers (2)
21
The Readers and Writers Problem
22
The Sleeping Barber Problem
23
Solution to the Sleeping Barber Problem