Scheduling

Question 1

How many threads can each CPU run at a time?

Answer 1

1

Question 2

Is thread scheduling consistently applied or platform dependent?

Answer 2

Platform-dependent.

Java does not require implementations to schedule threads in a particular way.

Question 3

When must programmers be careful when coding with threads?

Answer 3

When the order of thread execution is vital to the overall program execution.

Question 4

How does the JVM scheduler deal with threads of the same priority?

Answer 4

First-in-first-out

Question 5

How many states of threads are there?

Answer 5

4

Question 6

What are the characteristics of the ‘new’ state?

Answer 6

Thread object created, but thread as such does not yet exist because ‘start’ has not yet been called

Question 7

What are the characteristics of the ‘runnable’ state?

Answer 7

After start, thread becomes ‘runnable.’
Default state for threads.
Many threads can be in ‘runnable’ state but only one per CPU will be executing

Question 8

What are the characteristics of the ‘blocked’ state?

Answer 8

If thread calls sleep, wait or join, or if blocked when trying to read data or waiting for access to a lock on a synchronised object, it is moved to the blocked state.

Question 9

What are the characteristics of the ‘terminated’ state?

Answer 9

When a thread’s run method is finished, it moves to the terminated state.

Question 10

How does the scheduler determine which thread to run?

Answer 10

From the number of threads in the runnable state, it will be the one with the highest priority.

Question 11

What will the scheduler do when a higher priority thread enters the runnable state?

Answer 11

The scheduler will pre-empt the currently running executing thread and execute the new thread

Question 12

How does the schedulers deal with threads of the same priority?

Answer 12

Either run until blocked or terminate or possibly time-sliced.

Question 13

How does starvation occur?

Answer 13

When a thread never manages to run.

Question 14

How are shared resources dealt with when threads use them to ensure consistent and correct access to the shared data?

Answer 14

Locks

Question 15

If a thread with a lower priority started to executed and obtained a lock on a shared resource but was then interrupted by a higher priority thread which also needed access to the locked resource, what would need to occur?

Answer 15

Priority Inheritance

Question 16

What is meant by task precedence?

Answer 16

Some threads need to execute before others to ensure for the correct execution of the program as a whole

Question 17

What are timing constraints?

Answer 17

In a real-time system, each thread may have its own deadline, some will take longer than others to complete, some may operate on a periodic basis

Question 18

What is priority inversion?

Answer 18

Where lower priority threads can execute and terminate before higher priority threads due to locks placed on resources making some threads unable to execute

Question 19

What is priority inheritance?

Answer 19

When a lower priority thread has a lock on a sync. resource which is required by a higher priority thread, the lower priority thread’s priority is temporarily and silently raised to allow it to finish and release the lock

Question 20

If threads of the same priority pre-empt each other, what is this known as?

Answer 20

Round-Robin scheduling

Question 21

Is round-robin scheduling required in JVM?

Answer 21

No

Question 22

Are Java threads fair?

Answer 22

No, higher priority threads will ensure lower priority ones will not if they are in the same pool

Question 23

In Java, with no round-robin scheduling, are threads of the same priority guaranteed to pre-emptively time-sliced?

Answer 23

No

Question 24

What happens if Round-Robin scheduling is used to deal with multiple threads of the same priority?

Answer 24

For all threads to complete it will take the same overall time.

Question 25

What happens if Round-Robin scheduling is not used to deal with multiple threads of the same priority?

Answer 25

One thread could be finished very early, and another would take a very long time.