Philadelphia University
Lecturer: Dr. Nameer N. EL-Emam
Internal Examiner: Dr. Raad Alwan
Module Name: Adv.Operating System
Module Number: 750434
First Exam
12/4/2015
Faculty of Information Technology
Department of CS
Marking Scheme
Semester two 2014-2015
one hour
Basic Part
Objective: The aim of this part is to check student abilities to understand the basic of DOS.
Q1/( 10 marks):
Answer the following questions for DOS concepts:
1-What is the meaning of the coherent system? Why it is important in DOS while it is
not a substantial concept in another system?
Coherent system: A system that can cover all requirements on the global
behavior of the target system, such as:
 Stability,
 fault-tolerance and failure recovery,
 concurrency control,
 commitment,
 and consistency of replicated data.
Every thing in DOS is automatic controlled to cover all processes' demands by
transparency (without user, interfering) but in another system, some kind of
decisions are taken by users.
2- The main functions in DOS are:
a) Governing resources. b) Control.,
c) Resource/process allocated.
Distinguish between them and show why checking the state of resources
(idle/busy) is not type of control function?



Governing resources : Governing Regulations and Administrative
Regulations to manage the system.
Resource allocator: Allocate resources (software and hardware) of the
computer system and manage them efficiently.
Control : Controls execution of user programs and operation of I/O
devices.
Checking the state of resources (idle/busy) is not a type of control, it implements
the resource allocation while control is just make control on execution with out
optimization which process is better to execute at the specific time.
3- If transparency has been satisfied in DOS, then explain why the system should be
homogeneous.
Step1: Let us define the features of transparency = { Access, Location,
Migration, Replication, Concurrency, Failure, Persistence};
Step2:If transparency has been satisfied, then
the above features should be implemented on homogeneous or
heterogeneous:
Step3:If features can not be implemented on heterogeneous then
It must be implemented on homogeneous
Now we should check if the above features can be implemented through a
heterogeneous network system or not.







Access: Hide how a resource is accessed and where accessing.
(Not existed in heterogeneous).
Location: Hide where a resource is located (Not existed in
heterogeneous).
Migrate: It cannot be migrated if heterogeneous systems are used.
Replicate: The users cannot tell how many copies exist. (Not existed
in heterogeneous).
Concurrency: The users cannot tell how many copies exist. (Not
existed in heterogeneous).
Failure: Hide the recovery the blocked resources by another
processor(fault tolerance) (Not implemented in
heterogeneous).
Persistence: Hide whether a resource (software) is in the memory or
on the disk is impossible to implement in
heterogeneous
4- Show the difference between micro kernel and monolithic kernel and why uniprocessor system using micro kernel, show what is the main components to make a
micro kernel equivalent with monolithic kernel?


Monolithic Kernel : All parts of a kernel like the Scheduler, File System, Memory
Management, Networking Stacks, Device Drivers, ….etc, are maintained in one unit
within the kernel is called Monolithic Kernel
Micro Kernal: Includes all system processes that hold very important parts like IPC (Inter
process Communication), basic scheduler, basic memory handling, basic I/O primitives
etc., are put into the kernel. Communication happens via message passing. Others are
maintained as server processes in User Space.
To make a micro kernel equivalent with monolithic kernel, we should expand the memory of
uni- processor that holds all managements with all services as a master on many slaves.
5- How to extend resource management machine in centralized OS. Why it is not
necessary in DOS?
In centralized : using virtualization by making computer appears to be more than its real
configuration (this is extended resources) // appears to be many processors and
appear has a large memory.
In DOS: it is not necessary to extend management with new services due to have migration to
work stations that have the specific services.
Familiar Part
Objective: The aim of this part is to check student abilities to understand type of threads in DOS.
Q2/( 10
marks):
Answer the following questions for the threading concepts:
1- When the library decided to do each of the following function (creating
threads, destroying threads, and scheduling threads).
o
o
o
Create threads : When we need to define an independent stream of instructions
that can be scheduled to run on different processors in parallel.
Destroying threads : If thread is completed or problem in thread execution.
Scheduling threads: When threads are ready for execution, specific policy
should be used to dispatch threads in order to maximize or minimize
performance measures.
2- How library create a thread? (Write proposed pseudo code).
Step1: Define thread ID;
Step2: Specify fixed area to save thread execution context that includes private storage
(thread-local storage TLS);
Step3: Define signal mask of thread, this mask has two values (blocked, unblocked);
Step4: Define the priority of thread;
Step5: Define the address of thread’s stack;
3- If library is defined in the user space and one thread has been destroyed, then the
size of process that holds this thread has been reduced. So, how the kernel can
detect the new size of process? If the rest threads depending on the destroyed
thread, how the kernel overcome this problem?
Let S is the size of process P, where S is known by kernel.
Let P ={T1,T2,…Tm}, where Ti is the ith thread.
Then kernel knows address space of process P that is defined in PCB, the space
(P) and the size (S) includes m threads.
P
m

Ti
,S 
m
S(T )
i
i 1
i 1
If Tm is destroying, then the new size of process NS defined in PCB is equal to:
NS 
m 1
S(T )
i
i 1
Then the kernel can detect the new size of process NS, where this new size is define in
PCB and the information of PCB is updated by the kernel.
If the rest threads depending on the destroyed thread, how the kernel overcome this problem?
 Due to the dependency, the system kills the process with all threads.
4- LWP does not depend on system resources. Why?
The main function of LWP is to support user threads by services/ resources
and through this concept we can reduce system overhead instead of calling
those services by threads from kernel directly.
LWP does not need resource to run its code, but it needs resources/services
to transfer them to user because the kernel create LWP for this issue.
5- Why unbound threads model is better than bound threads model with respect to
system overhead? Show your answer by one clear example.
Bound thread is said to have system contention scope
- It contends with all threads in system so if we have n threads then we
need to make system call n times.
Unbound thread has process contention scope
It contends with threads in same process so if we have n threads
belong to m processes , where n>m then we need to make m times
system call (less than n times).