process sync

Question

const int n = 5;
int count = 0;
void test(){
    for i = 1 to n
    count += 2;
}
main()   {
    Par begin 
        test();
        test();
        test();
    Par end   
}

What can be the maximum and minimum value of count after completion of the program?
 

Comments

i think minimum 10 and maximum 30

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after completion count will always be 30.. right ??

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means we have to take the function calling in the sequence only.??

Answer 1

Minimum should be 4. Maximum should be 30.

As explained, it's read, increment count and store(3 steps) during execution of :

count+=2;

3 test() calls with 5 iterations for each. For simplicity let's consider 3 processes P1, P2 and P3 will execute one test each.

P1: Test() call (first iteration): loads '0', increments it by 2, [gets preempted before storing it back]

P2: Allow all 5 iterations for test() to run. The count value will now be 10 

P3: 4 iterations of third test() runs normally and gets preempted. So, now the value of count is 18.

Now,  P1 is starts again from where it left off. It has count value of 2 incremented locally. It stores back 2 and gets preempted.

Now, P3 resumes execution of it's final iteration. It loads the value of count as 2, increments it to 4 and gets preemted

Now, P1 executes remaining 4 iterations making the value of count as 10.

Finally, P3 executes the 'store' for it's last iteration. It stores back 4 (since it has count value as 4 locally incremented) 

So, minimum turns out to be 4

The maximum will be 30 (execution happens sequentially)

Comments

Nice and easy explanation thank you!!!!

Answer 2

Count += 2  treated as three instruction,

1. Read count

2. count + 2            //increment locally

3. Store                  // Globally

All 3 test() inside par begin par end means these function executes concurrently.

Program execution for max value.of count : run serially all Test(). Count becomes 30..

Now for minimum value :

Execute each test() for i=1 utill count +2 ..

Each test() increment count value to count +2 LOCALLY & still waiting for all test() to increment der local count by 2.

After incrementing local count let 1st test write count to 2 GLOBALLY.

Then 2nd test write count to 2 (because count +2 done locally so count value is still 2 for this test() ) . It overwrites count value by 2.

Then 3rd test() executes in same way as 2nd test() .

For n=1 all 3 test() executed and count value is 2..

 

Do same same for n=2,3,4,5

Final value of count is 10.

Comments

i have a doubt .

I think the max and min both should be 30 . 

Because when the 1st test()  is called then it should executed completely after that main will come to 2nd test()  

and after 2nd test() then 3rd test() will be executed . 

1st test();

2nd test();

3rd test(); 

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All 3 test() are inside "PAR BEGIN, PAR END" so they execute concurrently..

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oooo par means they can execute concurrently .  thanks :) 

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^^ PAR : PARALLEL

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Thanks a lot

Answer 3

Min should Be 4 and Max should be 30

Answer 4

MIn is 10 when each thread reads the same count value each time and updated

Max is 30 when threads run one after the other.