Technical question Operating Systems 2016.
1. Explain tile 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 itselt 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
3. What is a binary semaphore? What is its use?
A binary semaphore is one, which takes oniy 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 Coffmans conditions that lead to a deadlock.
Mutual Exclusion: Only one process may use a critical resource at a time.
Hold & Wait: A process may be allocated some resources while waiting for others.
No Pre-emption: No resource can be forcible removed from a process holding it.
Circular Wait: A closed chain of processes exist such that each process holds at least
one resource needed by another process in the chain.
6. What are short—, tong— and medium—term scheduling?
Long term scheduler determines which programs arc admitted to the system for
processing. It controls the dcgrL’L’ o/muliiprogrammning, Once admitted, a job becomes a
Medium term scheduling is part of the swapping function. This relates to
processes that are in a blocked or suspended state. They are swapped out of real-memory
until they are ready to execute. The swapping-in decision is based on memory-
Short term scheduler, also know as a dispatcher executes most frequently, and
makes the tinest-grained decision of which process should execute next. This scheduler is
invoked whenever an event occurs. It may lead to interruption of one process by
7. What are turnaround time and response time?
Turnaround time is the interval between the submission of a job and its
completion. Response time is the interval between submission of a request, and the first
response to that request.