Concurrency

Question 1

What is interference?

Answer 1

Interference may occur when multiple threads try to modify shared
variables at approximately the same time (thus they need to share the same data
memory to do this)

Question 2

What is an OS process

Answer 2

The student should probably draw a diagram how a computer has a number of OS
processes, the JVM runs in one of these processes, and this JVM can then support a
number of individual threads [1 mark]. The OS process is allocated independent
resources such as it own memory space for machine code, stack and data, thus
each process runs a different executable [1 mark]

Question 3

What is synchronization?

Answer 3

This can be eliminated by synchronizing critical
sections of the code that modify the shared variables [1 mark] – this only permits
one thread access to the shared variables at a time within a critical section [1
mark]. The synchronization mechanism uses locking where only one thread at a
time can have the lock

Question 4

Intrinsic Lock

Answer 4

The thread will get the lock at the start of the
critical section of code and give it back at the end, thus only allowing a single
thread to run in the critical section [1 mark].

Question 5

What is the concurrency abstraction?

Answer 5

Based on having a number of processes, the abstraction models the overall system by interleaving the execution of atomic atoms of processes.

Question 6

What is a abstract process?

Answer 6

Totally ordered sequence of atomic atoms.

Question 7

What is a sequential program in the concurrency abstraction?

Answer 7

Consists of single process. assuming a fixed input, program is deterministic

Question 8

What is a concurrent program in regards to the concurrency abstraction?

Answer 8

Consists of two or more processes carrying out atomic actions

Question 9

What is an atomic action?

Answer 9

No divisible in terms of processing

Question 10

What is interleaving?

Answer 10

describe a MODEL in which atomic actions from different processes are carried out.

Question 11

Do atomic actions get carried out simultaneously?

Answer 11

No, in the concurrency abstraction they are interleaved.

Question 12

Is a++ an atomic action?

Answer 12

No, consists of a LW a SW and a addi

Question 13

What is a context switch?

Answer 13

When the OS moves from processing one process to the a different process

Question 14

What is an OS process?

Answer 14

Used to run separate tasks which do not interfere with each other, each process has its own virtual memory address space for executable machine code, data, and stack

Question 15

How are threads and OS processes different?

Answer 15

Threads share the same address space, thus share the same code

Question 16

Draw the OS processes, JVM and Java Threads

Answer 16

DRAW IT

Question 17

What does the JVM do?

Answer 17

Runs a single process on top of the OS, allows spawning of threads of execution

Question 18

What does a thread have its own copy of?

Answer 18

Program counter register for its location in the running program
Other registers fro local variables being worked upon
Stack space for procedure call parameters and other local variables

Question 19

When does context switching occur?

Answer 19

Happens while thread code is being executed.

Question 20

What are the 6 different states of a thread?

Answer 20

NEW
A thread that has not yet started is in this state.
RUNNABLE
A thread executing in the Java virtual machine is in this state.
BLOCKED
A thread that is blocked waiting for a monitor lock is in this state.
WAITING
A thread that is waiting indefinitely for another thread to perform a
particular action is in this state.
TIMED_WAITING
A thread that is waiting for another thread to perform an action for up
to a specified waiting time is in this state.
TERMINATED
A thread that has exited is in this state.

Question 21

Draw the thread lifecycle

Answer 21

DRAW IT

Question 22

What does race condition refer to?

Answer 22

Indicate bad effects resulting from unfortunate thread timings

Question 23

Define interference

Answer 23

Destructive update caused by arbitrary interleaving of read write actions.

Question 24

What can make a method mutually exclusive?

Answer 24

Syncronized

Question 25

Describe a synchronized methoed

Answer 25

Method locks the object whose method has been called, other threads cannot enter other synchronised methods on the same object

Question 26

What is a reentrant lock?

Answer 26

Java runtime system allows a thread to reacquire the lock that it already holds, eliminates possibility of a single thread deadlocking itself.

Question 27

How is the lock implemented?

Answer 27

Spinlocks. 0 if unlocked 1 if locked. To lock a thread while(spinlock_mem != 0){;}
spinlock_mem = 1;

Question 28

How to we prevent interference?

Answer 28

Critical sections of code need to synchronized.

Question 29

What does safely published mean?

Answer 29

Both the reference to the object and the objects latest state will be made visible to other threads at the same time.

Question 30

What are the 4 ways to safely publish an object?

Answer 30

Initialise the object reference as a static initialiser
Storing reference to it in a volatile field
Storing a reference to it into a final field.
Storing a reference to it is properly guarded by a lock