Q1
1. An operating system is the most important software that runs on a computer and provides
services to applications. It manages the computer's memory, processes, and all of its
software and hardware. It also allows a user to communicate with the computer without
knowing how to speak the computer's language and its details.
? The computer can be a networked system with many servers
? It hides the “messy” details using a level of abstraction
? It presents the user with a simplified machine, much easier to use
? It executes user’s programs on a specific platform
? It uses the computer hardware and resources efficiently
2. Client obtains service by sending messages to server processes through microkernel.
Benefits
? Uniform interface on request made by a process
all services are provided by means of message passing (through the microkernel)
? Extensibility
allows the addition of new services
? Portability/Maintainability
changes needed to port the system to a new processor are made within the microkernel -
not in the other services
Weakness
In a micro kernel architecture applications communicate with service via the kernels interprocess communication channels. As Operating System services execute in user space the
kernel must perform multiple context switches in order to service the request. In the
monolithic kernel architecture, the core services are implemented in the kernel, no context
switch is required (only an escalation of privilege). Due to this micro kernels are often slower
compared to monolithic kernels, i.e. less efficient
3.
? External fragmentation occurs when holes appear in
otherwise contiguous memory blocks
? Internal fragmentation occurs when the block of memory
allocated to a process is larger than the memory that process
requires
4.
(1)First fit:
A:9-12
B:1
(2)Best fit:
A:15-18
B:1
(3)Worst fit:
A:9-12
B:15
5.
Advantages:
1.Cheaper in large disk subsystems, RAID1 need as twice as many disks are needed to store
data.
2. Good general performance
Disadvantages:
1. RAID1 has better security as it provide a redundancy of all data.
2. Need additional CPU performance to calculate parity
6.
Limited power in processor
Limited Space
Q2
a.(1)
fork() creates a new process
? Requires no arguments, returns identifier of child process
? Creates an exact copy of the parents address space for new process.
? Child process continues executing from where the fork() call was made in the parent
process
? Returns zero to the newly created child process
(2)
pipe() creates a pipe, a unidirectional data channel that can be used for interprocess
communication. The array pipefd is used to return two file descriptors referring to the ends
of the pipe. pipefd[0] refers to the read end of the pipe. pipefd[1] refers to the write end of
the pipe. Data written to the write end of the pipe is buffered by the kernel until it is read
from the read end of the pipe.
(3)
0
After the fork function, the child process and the parent process use separate addresses in
memory, and changes to the parameters in the child process do not affect the parent process.
(4)
0 1 2 3 4
b.
What are threads?
? Multiple concurrent paths of execution within process
? Lightweight process
? Unit of dispatching (execution)
? Threads in a process share address space
? Separate concurrency from protection
Why use multithreading?
Primary Benefits
? Responsiveness : applications can continue executing while parts are blocked
? Resource sharing : Threads share code and data
? Economy : Cheaper to create threads than processes
? Multiprocessor utilisation : threads can utilise parallel execution
Separates resource ownership and scheduling
? Multithreading is rarely used without requiring some access to shared resources
Application threads must map to a kernel execution thread
? User level : Many to one mapping
? Kernel level : One to One mapping
User level threads
Threads exist only in user space, kernel knows nothing about them
? Require a runtime package to implement the threads and keep track of them
? Can be used on systems that do not support threading
Benefits
? Cheap to create
? Cheap context switches
? Thread yields don’t result in process context switch
? Application specific scheduling
Drawbacks
? Blocking system calls can block entire process
? Errors propagate to whole process
? No clock interrupts, scheduler runs at same level
as thread
? No benefits from multi-processor systems
Kernel level threads
Kernel has full knowledge of all threads and processes
? A kernel thread is created for each user thread
? Requires specific kernel support
Benefits
? Kernel has full control of thread scheduling.
May give more time to processes with more
threads.
? Errors stop threads not processes
? Kernel handles blocking system calls
Drawbacks
? Operations much slower than user level
threads
? Significant overhead for kernel. It must
manage and schedule processes and threads.
c.
HOW
the output is dependent on the sequence or timing of other uncontrollable events. It becomes
a bug when events do not happen in the order the programmer intended.
Results:
Execution may fail sporadically and with strange, unreproducible output
Solving requires mutual exclusion
? Ensure only one process or thread can do the same thing to a shared variable or file
? Access to shared resources must be synchronised to avoid inconsistencies
Critical Section
Part of a cooperating processes (or threads) where a shared
resource is accessed and must be executed atomically
Problem solution requires:
?Mutual exclusion: No two processes or threads can be in their related
critical sections at the same time
?Progress: Progress cannot be delayed by another process or thread not
in a critical section
?Bounded Waiting: No process or threads should wait forever to enter its
critical section
MapReduce
Long Tail phenomena
Possible mitigation strategy:
Monitoring
Scheduling: Load Balancing
Fault-tolerance
