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Operating System Concepts and Techniques
Lecture 12
Interprocess communication-1
M. Naghibzadeh
Reference
M. Naghibzadeh, Operating System Concepts and Techniques, First ed., iUniverse Inc., 2011.
To order: www.iUniverse.com, www.barnesandnoble.com, or www.amazon.com
The need for communication
Communication is essential among all societies
Processes (or threads) form a society in the world
inside computers
They can communicate by using a variety of
techniques, such as shared memory and message
passing
It is natural to assume dependent processes
synchronize their activities
Even independent processes synchronize using shared
resources in order not to interfere with one another
We will discuss methodologies and tools for safe and
efficient communication between processes
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Race condition
It is difficult to believe correct programs may
sometimes produce incorrect results, i.e., race
condition occurs
Consider two transactions, one wants to withdraw
$1000 and the other $2000 from a checking account
Transaction 1:
1-1 Read account 123’s record
1-2 Subtract $1000 from balance
1-3 Write account 123’s record
Transaction 2:
2-1 Read account 123’s record
2-2 Subtract $2000 from balance
2-3 Write account 123’s record
 With round robin scheduler the execution of
processes could interleave in numerous ways
 The system only guarantees that when a machine
instruction starts it will continue to the end
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Lost update
What if their execution is interleaved in the following
way
1-1
1-2
2-1
2-2
2-3
1-3
Read account 123’s record //Balance is $10000
Subtract $1000 from balance //Balance is $9000 but not saved
Read account 123’s record //Balance is $10,000
Subtract $2000 from balance //Balance is $8,000 but not saved
Write account 123’s record
//Balance is $8,000
Write account 123’s record
//Balance is $9,000
The result is that the transaction two’s update is lost
This couldn’t have happened if transactions are
executed one after the other
The same situation may happen within the OS, for
example, a message may be lost instead of an update
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Critical region
Most computer resources are shared in
a long time but must be used by a
single user exclusively in short times


A classroom chair may be used by many students during one
day, however at any given time only one person uses it
The case is similar for printer, scanner, a record of a
database, a queue of waiting processes etc.
Such a resource is called a critical resource
Critical region or critical section of a program is that
part of the program which uses a critical resource
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Mutual Exclusion (ME)
In order to safely use a critical resource we must
follow the following steps



Get permission to enter critical region
Enter critical region and use the resource
Leave the critical region and free the resource
By proper usage of critical resources race condition
will not happen, hence correct programs produce
correct results
By guaranteeing a long-term shared resource to be
used solely for short terms such that no more than
one process is using the resource in any given time
we say, mutual exclusion problem is solved
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Acceptable ME solusions
Not all solutions to ME problem are acceptable; an
acceptable solution must have the following four
properties




The number of processes simultaneously using a resource must be,
at the most, equal to the maximum number of allowable processes
Mutual exclusion must be upheld disrespectful of relative execution
speed of processes and the number of processors.
A process may only block other processes from using a resource
when inside its critical
A process does not have to wait forever to get its chance to use a
needed critical resource
We can now talk about techniques and tools
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Disabling interrupt Method
when a process is running, if it disables the whole
interrupt system, the process can keep running
The steps to insure exclusive usage of a critical
resource in a single-processor are:



Disable interrupt
Use the critical resource
Enable interrupt
Difficulties:


Only the interrupt system of one processor is disabled,
doesn’t work in multiprocessor environment
The whole system is degraded to single programming
Disable interrupt is a privileged instruction
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Strict alternation Method
Use a shared variable such as turn


It can have only one value at any given time, zero or one
Process zero executes the following statement before using
the resource, a similar statement is for Process one
GetPermission: If (turn != 0) goto GetPermission;
Or similarly
GetPermission: while (turn !=0);
Change turn when finished with using the resource
In the following two processes alternate using a
resource infinite number of times, initial value of turn
determines which process will use it first

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Turn =0;
// Process 0
while (true) {
GetPermission: if (turn != 0) goto GetPermission;
/* Critical section where the resource is used by this process */
turn = 1; //Set turn to process 1 to get permission and use the resource
}
// Process 1
while (true) {
GetPermission: if (turn != 1) goto GetPermission;
/* Critical section where the resource is used by this process */
turn = 0; // Set turn to process 0 to get permission and use the resource
}
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Difficulties of Strict alternation Method
It suffers from busy-waiting

If one process is using the resource and the processor
switches to the other process, it keeps checking to see when
its turn comes up; making processor busy while waiting for
an event
It is only good for two processes
Processes must alternate to use the resource
If one accepts its difficulties he is welcome to use it
Peterson’s method removes the strict alternation
nature of this method
Advantage: It is applicable to multiprocessor systems
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Peterson’s method
Peterson used the shared variable turn and a twoelement array desire that are defined as
int turn;
int desire [2];
Every process which needs to use the resource will
call the GetPermission procedure with its process id
which is eithr zero or one
void GetPermission (int process)
{
int other;
// Variable Other defines other process’s number
other = 1-process;
desire [process] = true; //This process wants to use the resource
turn = process;
while (turn== process && desire [other] == true) ; //Busy-wait
}
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Peterson’s method…
To leave its critical region and free resource the
process will call
void FreePermission (int process)
{
desire [process] = false;
}
Advantages:


removes strict alternation
Good for multiprocessor systems
Disadvantages:


It suffers from busy-waiting
It is only good for two processes
If one accepts its difficulties he is welcome to use it
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Summary
Uncontrolled attempts by concurrently running processes
(or threads) to use a shared resource may cause collision
which may cause race processes may produce incorrect
results
We introduced many methods to guarantee mutual
exclusive (hence race-free) resource usage
Disabling interrupt method works well for single processor
systems; this can only run in a protected mode to run a
very short procedure
A strict alternation method is for synchronizing two
processes to alternate when using a shared resource
Peterson and Dekker methods remove the restriction of
alternate shared resource usage by processes
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Find out
The properties of an acceptable ME problem solution
An example system running concurrent processes
which has race
How strict alternation could be extended to three
processes
An example system for which we can use strict
alternation
The difference between race and critical region
How atomicity could be implemented by software
What makes you feel Peterson’s method works well
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Any questions?
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