Operating Systems

1. Explain the concept of Reentrancy?
   It is a useful, memory-saving technique for multiprogrammed timesharing systems. A Reentrant Procedure is one in which multiple users can share a single copy of a program during the same period. Reentrancy has 2 key aspects: The program code cannot modify itself, and the local data for each user process must be stored separately. Thus, the permanent part is the code, and the temporary part is the pointer back to the calling program and local variables used by that program. Each execution instance is called activation. It executes the code in the permanent part, but has its own copy of local variables/parameters. The temporary part associated with each activation is the activation record. Generally, the activation record is kept on the stack.

   Note: A reentrant procedure can be interrupted and called by an interrupting program, and still execute correctly on returning to the procedure.
2. Explain Belady's Anomaly?
   Also called FIFO anomaly. Usually, on increasing the number of frames allocated to a process virtual memory, the process execution is faster, because fewer page faults occur. Sometimes, the reverse happens, i.e., the execution time increases even when more frames are allocated to the process. This is Belady's Anomaly. This is true for certain page reference patterns.
3. What is a binary semaphore? What is its use?
   A binary semaphore is one, which takes only 0 and 1 as values. They are used to implement mutual exclusion and synchronize concurrent processes.
4. What is thrashing?
   It is a phenomenon in virtual memory schemes when the processor spends most of its time swapping pages, rather than executing instructions. This is due to an inordinate number of page faults.
5. List the Coffman's conditions that lead to a deadlock.
   1. Mutual Exclusion: Only one process may use a critical resource at a time.
   2. Hold & Wait: A process may be allocated some resources while waiting for others.
   3. No Pre-emption: No resource can be forcible removed from a process holding it.
   4. Circular Wait: A closed chain of processes exist such that each process holds at least one resource needed by another process in the chain.