Exam 2 Review - Chapters 4 to 6, Tutorials for Mutex Lock and Semaphore

Question 1

What is a thread?

Answer 1

A basic unit of computation, AKA a lightweight process

Question 2

What do multi-threaded applications consist of?

Answer 2

They have multiple threads within a single process, each having their own program counter, stack, and set of registers; all share common code and data

Question 3

What are the benefits of using multi-threaded processes?

Answer 3

Improves responsiveness, resource sharing, economy, and scalability

Question 4

How do multithreading models work?

Answer 4

User threads are supported above the kernel, and kernel threads are supported by the OS

Question 5

What are the different kinds of multithreading models?

Answer 5

Many-to-one, one-to-one, and many-to-many

Question 6

Why is many-to-many the most popular multithreading model?

Answer 6

A blocking system call wouldn’t block the entire process, as well as allowing the OS to create a sufficient number of kernel threads

Question 7

What is assigned to a thread when it’s created?

Answer 7

A thread ID, stack, and starting address for execution

Question 8

What parameters does pthread_create() have?

Answer 8

In order, ChildID, Thread_attributes, function_name, arguments

Question 9

What is the purpose of pthread_join()?

Answer 9

Wait for the target thread to terminate before further processing in main thread

Question 10

How do you compile and create and executable file with pthread?

Answer 10

cc -o programName programName.c -pthread

Question 11

Why do we care for process synchronization?

Answer 11

So processes can execute concurrently with minimal problems (OS must provide mechanisms to ensure the orderly execution of cooperating processes)

Question 12

What problem arises with concurrent data access?

Answer 12

May result in data inconsistency

Question 13

What is the Producer-Consumer (Bounded Buffer) problem?

Answer 13

A data buffer shared by two processes; each can update counter, risks a race condition

Question 14

What is a race condition?

Answer 14

Several processes access and manipulate the same data concurrently and the outcome depends on the particular order of execution

Question 15

What is the critical section?

Answer 15

It’s a segment of code in each process where the process may be changing common variables, updating table, etc.

Question 16

What is Peterson’s solution for fixing the race condition?

Answer 16

Uses the idea of locking to protect critical regions via locks; meets the three requirements: mutual exclusion, progress, bounded waiting. Uses flags as locks

Question 17

What are mutex locks?

Answer 17

Based on the idea of locking, protects a critical section by first acquire() a lock and then release() the lock (boolean variable indicating lock availability, and methods must be atomic); considered the simplest synchronization tools

Question 18

What are semaphores?

Answer 18

Similar to mutex locks, binary but uses an integer value to indicate lock availability

Question 19

How can you use semaphores with no busy waiting?

Answer 19

Instead of keeping a process busy waiting, have it block itself (suspend itself), then wake it up when a semaphore is ready

Question 20

What is a deadlock?

Answer 20

Multiple processes are blocked, each waiting for a resource that can only be freed by one of the other blocked processes

Question 21

What methods can you use to resolve deadlocks?

Answer 21

FIFO (every process will eventually get its turn to be removed) or LIFO (early processes may never be removed, starved)

Question 22

What solutions can be used for the Dining-Philosophers Problem?

Answer 22

Allow at most 4 philosophers to sit at the table; pick up chopsticks only if both are available (picking up done in critical section); use asymmetric solution, odd-numbered picks up first the left then right chopstick, even-numbered picks up first right then left chopstick

Question 23

What is a CPU-I/O burst cycle?

Answer 23

A process execution consisting of a cycle of CPU execution (performing calculations) and I/O wait (waiting for data transfer in or out the system)

Question 24

What does the CPU (short-term) scheduler do?

Answer 24

Handle removal of running process from CPU and the selection of another process, as well as selecting from among the processes in ready
queue, and allocates the CPU to one of them

Question 25

What is preemptive scheduling?

Answer 25

A process given the CPU may have it taken away

Question 26

What is nonpreemptive scheduling?

Answer 26

A process given the CPU may not have it taken away

Question 27

What does a dispatcher do?

Answer 27

Switches context, switches to user mode, and jumps to the proper location in the newly loaded program

Question 28

What is the criteria for selecting a scheduling algorithm?

Answer 28

Maximize CPU utilization and throughput, minimize turnaround time, waiting time, and response time

Question 29

What’s the average wait time for first-come first-serve?

Answer 29

(P1 wait time + P2 wait time + … + Pn wait time)/n; Average time can be long

Question 30

What is the convoy effect?

Answer 30

A short process comes behind a long process

Question 31

What is the average wait time for shortest job first?

Answer 31

Same as FCFS but organize processes by shortest to largest; one of the fastest algorithms

Question 32

How do you calculate shortest remaining time first?

Answer 32

AWT = Delta((Completion Time - Arrival Time) - Burst Time)/n

Question 33

What is priority scheduling?

Answer 33

A priority number is associated with each process, and the CPU is allocated to the CPU with the highest priority

Question 34

What is round robin?

Answer 34

Each process gets a small amount of CPU time (time quantum q). After this has elapsed, the process is preempted and added to the end of the ready queue (no process waits more than (n-1)q time units

Question 35

How does multilevel queue work?

Answer 35

Processes in the ready queue is partitioned into multiple sub-lists, based on some criteria (RR for foreground queue and FCFS for background queue), each one independent

Question 36

How does multilevel feedback queue work?

Answer 36

Like multilevel queue but processes flow from one to another, can be organized with different scheduling algorithms like RR leading to FCFS

Question 37

How do symmetric and asymmetric processing differ?

Answer 37

For asymmetric, only one processor controls all activities and for symmetric, each processor is self-scheduling

Question 38

What is load balancing?

Answer 38

Attempts to keep workload evenly distributed