CS604 - Operating Systems - Lecture Handout 18 And 19

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Summary

  • Process Synchronization: the basic concept
  • The Critical Section Problem
  • Solutions for the Critical Section Problem
  • 2-Process Critical Section Problem solutions

Process Synchronization

Concurrent processes or threads often need access to shared data and shared resources. If there is no controlled access to shared data, it is often possible to obtain an inconsistent state of this data. Maintaining data consistency requires mechanisms to ensure the orderly execution of cooperating processes, and hence various process synchronization methods are used. In the producer-consumer problem that was discussed earlier, the version only allows one item less than the buffer size to be stored, to provide a solution for the buffer to use its entire capacity of N items is not simple. The producer and consumer share data structure ‘buffer’ and use other variables shown below:

Process Synchronization

The code for the producer process is:

code for the producer process

The code for the consumer process is:

code for the consumer process

Both producer and consumer routines may not execute properly if executed concurrently.
Suppose that the value of the counter is 5, and that both the producer and the consumer execute the statement counter++ and counter- - concurrently. Following the execution of these statements the value of the counter may be 4,5,or 6! The only correct result of these statements should be counter= =5, which is generated if the consumer and the producer execute separately. Suppose counter++ is implemented in machine code as the following instructions:

MOV R1, counter
INC R1
MOV counter, R1

whereas counter- - maybe implemented as:

MOV R2, counter
DEC R2
MOV counter, R2

If both the producer and consumer attempt to update the buffer concurrently, the machine language statements may get interleaved. Interleaving depends upon how the producer and consumer processes are scheduled. Assume counter is initially 5. One interleaving of statements is:

producer: MOV R1, counter (R1 = 5)
INC R1 (R1 = 6)
consumer: MOV R2, counter (R2 = 5)
DEC R2 (R2 = 4)
producer: MOV counter, R1 (counter = 6)
consumer: MOV counter, R2 (counter = 4)

The value of count will be 4, where the correct result should be 5. The value of count could also be 6 if producer executes MOV counter, R1 at the end. The reason for this state is that we allowed both processes to manipulate the variable counter concurrently.
A situation like this, where several processes access and manipulate the same data concurrently and the outcome of the manipulation depends on the particular order in which the access takes place, is called a race condition. To guard against such race conditions, we require synchronization of processes.
Concurrent transactions in a bank or in an airline reservation (or travel agent) office are a couple of other examples that illustrates the critical section problem. We show interleaving of two bank transactions, a deposit and a withdrawal. Here are the details of the transactions:

  • Current balance = Rs. 50,000
  • Check deposited = Rs. 10,000
  • ATM withdrawn = Rs. 5,000

The codes for deposit and withdrawal are shown in Figure 18.1.

Bank transactions—deposit and withdrawal

Figure 18.1 Bank transactions—deposit and withdrawal

Here is what may happen if the two transactions are allowed to execute concurrently, i.e., the transactions are allowed to interleave. Note that in this case the final balance will be Rs. 45,000, i.e., a loss of Rs. 5,000. If MOV Balance,A executes at the end, the result will be a gain of Rs. 5,000. In both cases, the final result is wrong.

Bank transactions—deposit and withdrawal 1

The Critical Section Problem

Critical Section: A piece of code in a cooperating process in which the process may updates shared data (variable, file, database, etc.).
Critical Section Problem: Serialize executions of critical sections in cooperating processes.
When a process executes code that manipulates shared data (or resource), we say that the process is in its critical section (for that shared data). The execution of critical sections must be mutually exclusive: at any time, only one process is allowed to execute in its critical section (even with multiple processors). So each process must first request permission to enter its critical section. The section of code implementing this request is called the entry section. The remaining code is the remainder section. The critical section problem is to design a protocol that the processes can use so that their action will not depend on the order in which their execution is interleaved (possibly on many processors).
There can be three kinds of solution to the critical section problem:

  • Software based solutions
  • Hardware based solutions
  • Operating system based solution

We discuss the software solutions first. Regardless of the type of solution, the structure of the solution should be as follows. The Entry and Exist sections comprise solution for the problem.

software solutions

Solution to the Critical Section Problem

A solution to the critical section problem must satisfy the following three requirements:

  1. Mutual Exclusion
    If process Pi is executing in its critical section, then no other process can be executing in their critical section.
  2. Progress
    If no process is executing in its critical section and some processes wish to enter their critical sections, then only those processes that are not executing in their remainder section can participate in the decision on which will enter its critical section next, and this selection cannot be postponed indefinitely.
  3. Bounded Waiting
    There exists a bound on the number of times that other processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is granted.

Assumptions

While formulating a solution, we must keep the following assumptions in mind:

  • Assume that each process executes at a nonzero speed
  • No assumption can be made regarding the relative speeds of the N processes.

2-Process Solutions to the Critical Section Problem

In this section algorithms that are applicable to two processes will be discussed. The processes are P0 and P1. When presenting Pi, we use Pj to denote the other process. An assumption is that the basic machine language instructions such as load and store are executed atomically, that is an operation that completes in its entirety without
interruption.

Algorithm 1

The first approach is to let the processes share a common integer variable turn initialized to 0 or 1. If turn = = i, then process Pi is allowed to execute in its critical section. The structure of the process Pi is as follows:

Algorithm 1

This solution ensures mutual exclusion, that is only one process at a time can be in its critical section. However it does not satisfy the progress requirement, since it requires strict alternation of processes in the execution of the critical section. For example, if turn= =0 and P1 is ready to enter its critical section, P1 cannot do so even though P0 may be in its remainder section. The bounded wait condition is satisfied though, because there is an alternation between the turns of the two processes.

Algorithm 2

In algorithm two, the variable turn is replaced with an array boolean flag[2]whose elements are initialized to false. If flag is true for a process that indicates that the process is ready to enter its critical section. The structure of process Pi is shown:

Algorithm 2

In this algorithm Pi sets flag[i]= true signaling that it is ready to enter its critical section. Then Pi checks to verify that process Pj is not also ready to enter its critical section. If Pj were ready, then Pi would wait until Pj had indicated that it no longer needed to be in the critical section (that is until flag[j]=false). At this point Pi would enter the critical section. On exiting the critical section, Pi would set flag[i]=false allowing the other process to enter its critical section. In this solution, the mutual exclusion requirement is satisfied. Unfortunately the progress condition is not met;
consider the following execution sequence:

T0: P0 sets flag[0]= true
T1: P1 sets flag[1]= true

Now both the processes are looping forever in their respective while statements.