Interprocess Communication

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Inter-process Communication

• Operating Systems - Lecture 4

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Inter Process Communication

• Process frequently have to communicate with each
  other
• Synchronisation
• Example Unix shell commands
• Sharing resources
• Example Writing to a file

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IPC issues

• Processes compete for exclusive access to a
  limited number of resources
• Reading and writing to a common shared database
• Waiting for an available resource

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Processes compete for exclusive access to a
limited number of resources
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Processes compete for exclusive access to a
limited number of resources
The issue of synchronisation was solved by
Dijkstra in 1965, by the introduction of
SEMAPHORES.
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Processes compete for exclusive access to a
limited number of resources
The issue of synchronisation was solved by
Dijkstra in 1965, by the introduction of
SEMAPHORES. The concept was illustrated by the
model called The Dining Philosophers Problem
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The Dining Philosophers Problem
A table
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The Dining Philosophers Problem
A table
Five diners
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The Dining Philosophers Problem
A table
Five diners
One fork each
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The Dining Philosophers Problem
Each diner needs 2 forks to eat
A table
Five diners
One fork each
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The Dining Philosophers Problem
Each diner needs 2 forks to eat
Each diner either wants to eat or think (sleep)
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The Dining Philosophers Problem
If all 5 diners pick up their left
fork simultaneously...
Each diner needs 2 forks to eat
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The Dining Philosophers Problem
If all 5 diners pick up their left
fork simultaneously...
Each diner needs 2 forks to eat
None will be able to pick up their right
fork. DEADLOCK
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The Dining Philosophers Problem
Attempt to solve the problem...
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The Dining Philosophers Problem
Pick up left fork, then check to see if right
fork is available
Attempt to solve the problem...
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The Dining Philosophers Problem
Pick up left fork, then check to see if right
fork is available
Attempt to solve the problem...
What if they ALL do that simultaneously?
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The Dining Philosophers Problem
A control array of 5 SEMAPHORES Each array
element is either set to EATING or HUNGRY
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The Dining Philosophers Problem
A control array of 5 SEMAPHORES Each array
element is either set to EATING or HUNGRY
EATING
HUNGRY
HUNGRY
EATING
HUNGRY
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The Dining Philosophers Problem
A control array of 5 SEMAPHORES Each array
element is either set to EATING or HUNGRY
EATING
HUNGRY
HUNGRY
A diner can only move into the EATING state if
both neighbours are HUNGRY
EATING
HUNGRY
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The Dining Philosophers Problem
A diner can only move into the EATING state if
both neighbours are HUNGRY HUNGRY diners must
BLOCK if the needed forks are busy.
EATING
HUNGRY
HUNGRY
EATING
HUNGRY
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Readers and Writers
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Readers and Writers
Database
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Readers and Writers
Database
Reader
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Readers and Writers
Database
Reader
Reader
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Readers and Writers
Database
Reader
Reader
Reader
Any number of Readers can access the
database simultaneously
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Readers and Writers
Database
Writer
Reader
Reader
Reader
What if a writer wants to access the database?
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Readers and Writers
Database
Writer
Reader
Reader
Reader
What if a writer wants to access the
database? This is only safe if there are no
readers...
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Readers and Writers
Database
Writer
What if a writer wants to access the
database? This is only safe if there are no
readers...
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Readers and Writers
Database
Access control semaphore
The reading/writing to the database is governed
by an Access control semaphore.
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Readers and Writers
Database
1
Reader
Access control semaphore
The reading/writing to the database is governed
by an Access control semaphore. So long as the
semaphore is not negative, the first reader sets
the semaphore to 1.
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Readers and Writers
Database
2
Reader
Access control semaphore
Reader
A second reader increments the value in the
semaphore
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Readers and Writers
Database
3
Reader
Access control semaphore
Reader
Reader
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Readers and Writers
Database
3
Reader
Access control semaphore
Reader
Writer
Reader
A writer cannot access the database while the
semaphore is positive.
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Readers and Writers
Database
2
Reader
Access control semaphore
Reader
Writer
A writer cannot access the database while the
semaphore is positive.
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Readers and Writers
Database
1
Reader
Access control semaphore
Writer
A writer cannot access the database while the
semaphore is positive.
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Readers and Writers
Database
0
Access control semaphore
Writer
A writer cannot access the database while the
semaphore is positive.
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Readers and Writers
Database
-1
Access control semaphore
The writer forces the semaphore negative
Writer
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Readers and Writers
Database
Reader
-1
Access control semaphore
No readers can access the database while
the semaphore is negative
Writer
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Readers and Writers
Database
0
Access control semaphore
When the writer leaves, the semaphore returns to
zero.
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Readers and Writers
Database
Reader
1
Access control semaphore
When the writer leaves, the semaphore returns to
zero. So Readers or one writer may gain access
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Concurrent Processes

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Overview

• Examples of concurrent programming
• Resource management
• Process synchronisation
• Competing processes
• Semaphore mechanism
• Deadlocks
• Inter-process communications

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Examples of concurrent programming
1. Processes work completely independently
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Examples of concurrent programming
Shared database
2. Processes share common resource
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Examples of concurrent programming
3. Processes work together to form one application
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Resource management

• What resources can processes share?
• CPU, main memory,
• I/O devices
• data items in memory
• messages from one process to another

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Mutual Exclusion
Sharing serially reusable resources. The
problem is ensuring only one process has the
resource at any one time...
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Mutual Exclusion
Sharing serially reusable resources. The
problem is ensuring only one process has the
resource at any one time...
Cant have the resource
Has the resource
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Mutual Exclusion
Sharing serially reusable resources. The
problem is ensuring only one process has the
resource at any one time...
Cant have the resource
Has the resource
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What can go wrong here?

• Imagine two resources X and Y
• and that processes require both X and Y at the
  same time
• If one process already has X and the other
  process already has Y
• We get a DEADLOCK

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Deadlock
Resource X
This process has been allocated resource X and is
waiting for resource Y
Resource Y
This process has resource Y and is waiting for
resource X
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Inter process communication
Waits
Runs
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Inter process communication
Waits
Runs
Sends a signal to say completion of event Waits
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Inter process communication
Waits
Runs
Sends a signal to say completion of event Waits
Runs
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Competing processes

Seat on Flight Booking Database
Reads availability
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Competing processes

Seat on Flight Booking Database
Reads availability
Reads availability
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Competing processes

User A
Seat on Flight Booking Database
Reads availability Updates booking system
Reads availability
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Competing processes

User B
Seat on Flight Booking Database
Reads availability Updates booking system
Reads availability Updates booking system
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Competing processes

User B
Seat on Flight Booking Database
Reads availability Updates booking system
Reads availability Updates booking system
Need to synchronise processes so the critical
regions dont run at the same time
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Critical Regions
Process A Process B
Neither process in Critical Region Both
processes can safely run
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Critical Regions
Process A Process B
Process A enters its Critical Region Process B
cannot now enter - and should Wait
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Critical Regions
Process A Process B
Process A exits its Critical Region Process B can
now enter
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How can we control concurrent processes?
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The Open Gate concept
A process asks to see if the Gate is open before
proceeding
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The Open Gate concept
While gate_open FALSE do nothing enddo set
gate_open to FALSE ltcritical region codegt set
gate_open to TRUE
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The Open Gate concept

• Time problems
• There might be a gap between checking the Gate
  and setting gate_open to FALSE
• There is also a very costly loop while waiting.

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Semaphore mechanism

• A fail-safe system
• With no costly loops.
• Uses Operating System scheduler to make process
  WAIT.

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Semaphore mechanism
Semaphore
Shared Resource
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Semaphore mechanism
Semaphore
Shared Resource
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Semaphore mechanism - WAIT
Semaphore
If s gt 0 then s s - 1 else BLOCK process
Shared Resource
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Semaphore mechanism - Signal
Semaphore
If any processes are waiting on s start one of
these processes else ss1
Shared Resource
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Inter-process communications
Synchronisation Message/data exchange