Units 3 & 4

Question 1

Shared resources

Answer 1

processes that are working together often need to access the same data or hardware resource

Question 2

Producer-consumer model

Answer 2

2 or more processes working with a shared resource

producer - writes data to the shared resource

consumer - reads data from the shared resource

Question 3

Condition synchronisation

Answer 3

mechanism to ensure that a process is blocked if some condition is not met

Question 4

Mutual exclusion

Answer 4

control access to shared data

only one process at a time had exclusive access to a shared resource

Question 5

What are race conditions?

Answer 5

when the final outcome of a process is dependent on the timing or sequence of other processes

Question 6

Atomic action

Answer 6

either happens completely or it does not happen at all

fine-grained or course-grained

Question 7

Critical region

Answer 7

a section of code in which a shared resource is being accessed

must be executed under mutual exclusion

Question 8

Entry protocol

Answer 8

code that must be executed before entering a critical region

prevents a process entering the critical region if another process is already in a critical region that accesses the same data resource

Question 9

Exit protocol

Answer 9

code that a process must execute immediately on completion of its critical region

signals the resource is now available to other processes

Question 10

Semaphore

Answer 10

data structure for solving synchronisation problems

like a permit - only one process at a time can hold the permit + a process must get hold of the permit before it can enter the critical region

Question 11

Binary semaphore

Answer 11

for mutual exclusive access to a single shared resource

several processes sharing a resource will share one semaphore

initial value of 1

Question 12

Counting semaphore

Answer 12

to protect access to multiple instances of shared data

initial value set to the number of resource instances available

Question 13

Blocking semaphore

Answer 13

to synchronise two processes

initialised to 0

Question 14

Monitors

Answer 14

data structures to encapsulate the shared data

shared data is accessible only through special monitor operations which are atomic

facilitate mutual exclusion

support condition synchronisation

Question 15

Monitor construct

Answer 15

well-defined interface

only operations defined in the interface can be used to access the monitor’s data

a monitor operation is equivalent to a critical region with entry + exit protocols

Question 16

In what ways can semaphores be used?

Answer 16

ensuring mutual exclusion from critical code

controlling access to shared resources

synchronising cooperating processes

Question 17

How can the incorrect use of semaphores lead to problems?

Answer 17

by forgetting to use semWait it is possible for a resource be become unprotected

by forgetting semSignal it is possible for a resource to become locked indefinitely

Question 18

Condition variables

Answer 18

special variables associated with the monitor that enable the monitor to handle condition synchronisation

have a queue - if a condition is not satisfied a process can be made to wait in a queue for that specific condition

Question 19

Thread safety

Answer 19

a class is said to be thread safe if it can be executed concurrently by several threads in a safe manner - without data getting into an inconsistent state

Question 20

Multiple readers, single writer policy

Answer 20

readers - threads that read the contents of a file

writers - threads that write to a file

need to keep track of how many threads are currently reading or writing - 2 variables:
      int activeReaders      (>= 0)
      int activieWriters        (1 or 0)

reader thread needs to check the value of activeWriters before it begins to read

a writer thread should check the values of both (one writer only)

Question 21

Writers-preferred policy

Answer 21

shared resource is given to a waiting writer (if there is one)

only if there is no writers waiting will it be given to readers

all waiting readers will be allowed access at the same time

Question 22

Readers-preferred policy

Answer 22

the shared resource is given to all waiting readers (if there are any) + is given to a writer only if no readers are waiting

Question 23

Alternating reader-writers policy

Answer 23

shared resource is given to a waiting writer when readers have finished + to readers when writers have finished

Question 24

Take a number policy

Answer 24

shared resource is given to threads in order of arrival

Question 25

Do you think the writers-preferred policy is fair?

Answer 25

possible that the reader processes never get a chance to run because writer processes always get priority

Question 26

What are the 4 conditions of deadlock?

Answer 26

1. mutual exclusion
2. hold while waiting
3. no pre-emption
4. cycle of processes

Question 27

How can deadlock be prevented?

Answer 27

by arranging for one of the 4 conditions to not be true

Question 28

Advantages of virtual machines

Answer 28

can be executed on various platforms

facilitate the developments of applications written in multiple languages

generated virtual code is smaller than the corresponding machine code

Question 29

Disadvantages of virtual machines

Answer 29

slower execution speed of the program

Question 30

Virtual machine (VM)

Answer 30

an abstract computing device that implements in software a hypothetical platform

compile programs for virtual platform + implement the virtual machine on top of various concrete platforms

Question 31

Interpreter

Answer 31

the implementation of a virtual machine consists of an interpreter

a program that executes another program

Question 32

Class loader

Answer 32

able to load a class file into the method area of an already running program

classes are loaded into the JVM only when they are needed - helps to minimise memory usage

Question 33

Bytecode verifier

Answer 33

devices receiving code should have some guarantee that it is safe to run that code

verifier scans the bytecode before it is executed

Question 34

Garbage collection

Answer 34

mechanism that reclaims the memory occupied by unused objects from the heap to keep memory usage as low as possible

Question 35

3 Java Platform editions

Answer 35

1. Enterprise edition (Java EE)
2. Standard edition (Java SE)
3. Micro edition (Java ME)
   * each edition is a set of tools, libraries, API specifications, etc for a certain kind of system

Question 36

Lock interface

Answer 36

locks have to be explicitly acquired and released by calling lock + unlock methods

if developer for gets to call unlock the application might deadlock - this cannot happen with the build-in object lock because they are automatically released by the JVM

Question 37

Condition interface

Answer 37

allow multiple explicit condition variables for each lock

Question 38

Countdown latch

Answer 38

CountDownLatch class

implements a thread execution barrier with a counter

if counter positive, barrier down + threads arriving at latch have to wait

if counter 0, barrier up + all waiting threads resume execution

has two operations: wait + notify

Question 39

Factory methods

Answer 39

a method that creates objects of a given type C

typically are static methods and C is an interface

used when:

 - the creation of a C object is complex
 - the exact implementation of C should be kept hidden

Question 40

What is one of the central concerns for correctness of multithreaded applications?

Answer 40

changes made to shared variables by one thread must be visible to all other threads reading those variables

Question 41

Through which method calls can a Java thread release control?

Answer 41

sleep
yield
wait
join