Operating system subjective question for competition exam(4)

Q1.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.

Q2.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. 

Q3.Explain the popular multiprocessor thread-scheduling strategies.

1. Load Sharing: Processes are not assigned to a particular processor. A global
queue of threads is maintained. Each processor, when idle, selects a thread
from this queue. Note that load balancing refers to a scheme where work is
allocated to processors on a more permanent basis.
2. Gang Scheduling: A set of related threads is scheduled to run on a set of
processors at the same time, on a 1-to-1 basis. Closely related threads /
processes may be scheduled this way to reduce synchronization blocking,
and minimize process switching. Group scheduling predated this strategy.
3. Dedicated processor assignment: Provides implicit scheduling defined by
assignment of threads to processors. For the duration of program execution,
each program is allocated a set of processors equal in number to the number
of threads in the program. Processors are chosen from the available pool.
4. Dynamic scheduling: The number of thread in a program can be altered
during the course of execution. 

Q4.What is time-stamping?

It is a technique proposed by Lamport, used to order events in a distributed system
without the use of clocks. This scheme is intended to order events consisting of the
transmission of messages. Each system 'i' in the network maintains a counter Ci.
Every time a system transmits a message, it increments its counter by 1 and
attaches the time-stamp Ti to the message. When a message is received, the
receiving system 'j' sets its counter Cj to 1 more than the maximum of its current
value and the incoming time-stamp Ti. At each site, the ordering of messages is
determined by the following rules: For messages x from site i and y from site j, x
precedes y if one of the following conditions holds....(a) if Ti

Q5 What is a thread? Why is it that threads are faster to create than processes? What advantages do kernel threads provide over user threads?

: A thread is a single sequence stream within in a process. It is also
called lightweight processes. In a process, threads allow multiple executions of
streams. The CPU switches rapidly back and forth among the threads giving
illusion that the threads are running in parallel. A thread can be in any of several
states (Running, Blocked, Ready or Terminated).
An operating system that has thread facility, the basic unit of CPU utilization
is a thread. A thread has or consists of a program counter (PC), a register set, and a
stack space. Threads are not independent of one other like processes as a result
threads shares with other threads their code section, data section, OS resources
such as open files and signals.
Because a thread is smaller than a process, thread creation typically uses
fewer resources than process creation. Creating a process requires allocating a
process control block (PCB), a rather large data structure. The PCB includes a
memory map, list of open files, and environment variables. Allocating and
managing the memory map is typically the most time-consuming activity. Creating
either a user or kernel thread involves allocating a small data structure to hold a
register set, stack, and priority.
Advantage:
• Because kernel has full knowledge of all threads, Scheduler may decide to give
more time to a process having large number of threads than process having
small number of threads.
• Kernel-level threads are especially good for applications that frequently block.