Session 5 - Process Synchronization

8/8/2019 Session 5 - Process Synchronization

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OPERATING SYSTEMSDesign and Implementation

Chapter 2Processes

Instructor:Hadi Salimi

Computer Engineering Department

IRAN University of Science and [email protected]

IPC

Processes frequently need to

communicate with other processes

For example, in a shell pipeline, the output

of the first rocess must be assed to the

Operating Systems - By: Hadi Salimi - IUST-CE 25/19/2009

second process. This is called Inter-Process

Communication orIPC.


2/14

Race Conditions


Two processes want to access shared memory at same time. What

happens if they try to access it simultaneously?

Race Conditions

Situations like this are called race

conditions.

What will happen if two processes execute

the followin code?


X=0;

Read(x);

X++;

Write(x);


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Critical Sections

We should prohibit more than one process

from reading and writing the shared data

at the same time.

In other words what we need is mutual


exclusion.

The part of the program where the shared

memory is accessed is called the critical

section orcritical region.

Solution Criteria

Conditions to hold to have a good solution:

No two processes may be simultaneously

inside their critical regions.

No assumptions may be made about speeds


or the number of CPUs.No process running outside its critical section

may block other processes.

No process should have to wait forever to

enter its critical region.


4/14

Mutual Exclusion

Busy Waiting

Disabling interrupts

Strict alternation

Petersons solution


TSL instruction

Sleep and Wakeup

Semaphores

Monitors

Message passing

Disabling Interrupts

Let each process to disable all interrupts

just after entering its critical section and

re-enable them just before leaving.

Is it wise to let the rocesses do such a ob?


What will happen if one process never turnthe interrupts on?

What if the program is running on a

multiprocessor?


5/14

Lock Variables

Does the following code solve the

problem?

If (lock ==0) then


lock = 1;

Begin Critical Section

lock = 0;

End Critical Section

Strict Alternation


Which condition is violated?


6/14

Petersons Algorithm


TSL Instruction

It is another proposal that requires a little

help from hardware.

Many computers have a TEST AND LOCK

instruction that works as follows:


Reads the memory into a register and storesa non-zero value into it.


7/14

TSL (cont.)


Sleep and Wakeup

The described solutions requiring busy

waiting.



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Sleep and Wakeup

Semaphores

Monitors Message Passing


Sleep and Wakeup

Despite busy waiting methods which

waste CPU cycles, in this method

processes may sleep orwakeup using

system calls.


Lets clarify this approach using anexample, namely, producers and

consumers.


9/14

Sleep and Wakeup

Consider two

processes

which produce

and consume

Consumer

Process

Producer

Process


items from/to

a buffer with

size N.

Producer/Consumer



10/14

Problems

This solution is also wrong.

Consider the situations in which both theproducer and the consumer access the

shared variable count simultaneousl .


This may cause both of the processes go

sleep.

Semaphores

In many problems there is a need to count

an event, like producing an item or

consuming it.

Accessin to this counter should be


protected against concurrent processes. Such a protected counter is called a

semaphore which has more features.


11/14

Semaphores (cont.)

Two operators are defined on a

semaphore: Down and Up (generalizations

ofsleep and wakeup)


If (x > 0)

x--;

else

Sleep() };

If (there is any waiting process)Pick a process from queue and make it

ready;

else

x++ };

Semaphores (cont.)

How to protect a critical section usingsemaphores?

int s = 1;


Critical Section

Up(s);


12/14

Semaphores (cont.)

Consider a resource which can be shared by 3

processes. How accessing this device can be

protected using semaphores?

int x = 3;


Down(x);

Accessing the shared resource.

Up(x);

Producer/Consumer

What happens if the

down operator first


applied on mutex?


13/14

Monitors

The unfortunate situation which causes no

process to proceed is called deadlock.

We study it in detail in the next chapter.


careful one must be when using

semaphores.

Monitors (cont.)

To make it easier to write correct

programs, a higher level primitive called

monitoris introduced.

It is a collection of rocedures variables


and data structures that are all grouped ina package.

An important property:

Only one process can be active in a monitor

at any time.


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Monitors (cont.)

monitorexample

int x;procedure producer(x)


end;

procedure consumer(x)

end;

Monitors (cont.)

Monitors are a programming language

construct, so the compiler should handle

calls to procedures.

When a rocess calls a monitor


procedure, the first few instructions of theprocedure will check to see if any other

process is currently active or not.

Session 5 - Process Synchronization

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