Question

what is true about binary semaphore?

1)Binary semaphore has drawback called busy wait or spin lock.

2)Binary semaphore is applicable only for two processes

3)Binary semaphore has no drawback called busy wait or spin lock

4)none of the above

Comments

I think option 1 and 2 is correct

But option  2 is more accurate..

Please check..

Answer 1

For 1) and 3) from galvin and wikipedia, it says that

"Traditional semaphore has drawback of busy waiting aka spinlock but In a slightly modified implementation which is using the Queue overcome this problem" So 1) and 3) both are depends on implemenation.

For 2) although binary semaphor is applicable for only 2 processes but we can implement counting semaphor using Binary semaphor.

Comments

Binary semaphore can work for more than 2 processes. The 'binary' in binary semaphore comes from the fact that it allows only one process to enter a critical section or use a particular resource. Thus a resource can either be free (1) or not free (0). Whereas there can be multiple processes in busy-wait state waiting to acquire a lock on this process.

 

Consider an example with 3 processes.

Process 1 acquires a lock on the resource setting the semaphore value to 0.

Now process 2 tries to access the resource but fails since it can't acquire a lock so it busy-waits.

Same goes with process 3.

The above example can be scaled for n processes where n>=1

Answer 2

This link will solve your query...kindly check disavdantage.

http://gateoverflow.in/?qa=blob&qa_blobid=18345140180365630159

Comments

Which book?

Answer 3

  Answer should be (3)    struct Bsemaphore  {      enum value(0,1);      Queue type L;// list of all processes get                    // blocked while performing down operation usuccessfully         }    Bsemaphore s:  s.value=1;  Down(s);    Down(Bsemaphore s)  {      if(s.value==1)      {          s.value=0;      }      else      {          put the process in s.L and block it          sleep();      }  }      Up(Bsemaphore s)  {      if(s.L() is empty)      {          s.value=1;      }      else      {          select a process from s.L(only one process)          wakeup();      }  }

Answer 4

i think there is no such drawback instead it uses a diffrent mechanism which was made to overcome the problem of busy wait and spin lock . which is sleep procedure till someone is inside the critical section don't knock at the door just go to sleep when there will be a space the process will wake u up . the process of busy waiting is a charterstic  of lock variable not of semaphonre