Chapter 13 - Concurrency

Question 1

what are the 4 different levels of concurrency

Answer 1

machine instruction level

statement level

unit level

program level

Question 2

what is machine instruction level concurrency

Answer 2

executing two more machine instructions concurrently

Question 3

what is statement level concurrency?

Answer 3

executing two or more high-level language statements simultaneously

Question 4

what is unit level concurrency?

Answer 4

executing two or more subprogram units simultaneously

Question 5

6 motivations for concurrency

Answer 5

• speed up execution
• increase program flexibility
• resource management in OS
• task management in OS
• solving large-scale problems
• simulates actual physical systems that consist of multiple concurrent subsystems

Question 6

what is a scalable concurrent algorithm?

Answer 6

an algorithm with which the speed of execution increases when more processors are available

Question 7

what is a thread of control?

Answer 7

a sequence of program points reached as control flows through the program

Question 8

what is physical concurrency?

Answer 8

multiple independent processors (multiple threads of control)

Question 9

what is logical concurrency?

Answer 9

the appearance of physical concurrency is presented by time-sharing one processor

Question 10

what is hidden concurrency and what are 3 kinds of it?

Answer 10

• concurrency that is built into processor architectures
• pipelining of instruction (run instruction A, while preparing instruction B, while fetching instruction C)
• separate lines (buses) for instructions and data
• prefetching of instruction and data

Question 11

3 differences between a task (aka a process) and a subprogram

Answer 11

Tasks are:

• implicitly started
• invoking unit need not wait for the task to finish to resume itself
• upon completion of the task, control may or may not return to the invoker

Question 12

what is a lightweight vs a heavyweight task?

Answer 12

lightweight tasks all run in a single address space and heavyweight tasks each run in their own address space

Question 13

what is a program “guard”

Answer 13

a linguistic device that allows the guarded code to be executed only when a specified condition is true

Question 14

what is a task descriptor?

Answer 14

-a data structure that stores all of the relevant information about the execution state of a task

Question 15

what is cooperation synchronization

Answer 15

when two processes must perform their work in some mandated order

Question 16

what competition synchronization

Answer 16

when two processes need to access some resource but must not do it at the same time

Question 17

what is the Ada “accept” statement (3 things)

Answer 17

-its represents the entry point for a task using “Rendevous” for synchronization

each accept statement has an associated process queue

-the “server” waits when the accept statement is reached until a process meets the “when” statement requirements and is in one of its process queues

Question 18

rendezvous actor task vs server task

Answer 18

server task - a task with accept clauses

actor task - a task without accept clauses

Question 19

what is the Rendevous “pragma” keyword?

Answer 19

-assigns priority to tasks in the specification

Question 20

what is the Java “synchronized” keyword

Answer 20

when applied to methods within an object it disallows those methods from being run concurrently on different threads

when to applied to set of statements then those statements cannot be run concurrently by different threads

Question 21

4 java thread class methods

Answer 21

start- call the run method implemented in whatever runnable object

yield- a hint to the scheduler that the current thread is willing to yield its current use of the processor if needed. Moves thread to ready state

sleep- moves the thread into the blocked state

join- forces a method to delay its execution until the run method of another thread has completed its execution

Question 22

What is the equivalent of P and V for Java semaphores?

Answer 22

s = new Semaphore(0) --> constructor with initial value of zero
f = new Semaphore(MAX) 

s.acquire() //same as P
[blah, blah, blah]
f.release() //same as V

Question 23

2 ways to define a Java class with run() for threads

Answer 23

• create a class that extends the Thread class and override run (only if the class has no parents)
• create a that implements Runnable interface and define the functionality of run()

Question 24

what is the Java “notifyAll” method?

Answer 24

-it tells all other threads that some event that a thread may have been waiting for has possibly occurred

Question 25

what are atomic Integer, Boolean, and Long types?

Answer 25

-they are just like Integer, Boolean, and Long classes but their operations are non-interruptible