Chapter 4 - Foundations of scalable systems

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Questions and Answers

What is the main purpose of using threads in a program?

To consolidate multiple methods into one.

To manage different variable scopes within a single execution.

To allow independent sequences of execution. (correct)

To create multiple instances of the same object.

Which interface must be implemented to define a thread in Java?

Executor

Runnable (correct)

Callable

ThreadHandler

What is created when executing a thread using the Runnable interface?

An abstract thread structure.

A separate class that inherits Thread.

A new instance of Runnable.

A Thread object constructed with a Runnable instance. (correct)

What method is called to initiate a thread's execution?

start() Signup and view all the answers

What does each thread have that allows it to manage its own execution?

Runtime stack Signup and view all the answers

How does a thread access the process' global data?

By sharing references with other threads. Signup and view all the answers

What is the role of the run() method in a thread?

To define the code that will be executed in the thread. Signup and view all the answers

Which of the following statements is true regarding a single-threaded process?

It executes all instructions through a single thread. Signup and view all the answers

What happens after the philosophers finish eating?

The chopsticks are returned to the table and the lock is released on each. Signup and view all the answers

In the Philosopher class, what is the purpose of the LogEvent method?

To simulate thinking and eating events with delays. Signup and view all the answers

What is the role of the synchronized blocks in the run method of the Philosopher class?

To ensure exclusive access to chopsticks while a philosopher is eating. Signup and view all the answers

How many philosophers are created in the for loop for threading?

Five philosophers. Signup and view all the answers

What is the expected behavior when a philosopher cannot pick up both chopsticks?

The philosopher waits until one chopstick is available. Signup and view all the answers

What is the primary focus when discussing multiple threads?

Synchronization Signup and view all the answers

How does the Java virtual machine (JVM) determine thread execution?

Through nondeterministic scheduling Signup and view all the answers

What is a term used to describe the ability of the JVM to interrupt a running thread?

Preemption Signup and view all the answers

In the provided example, which thread completed first during execution?

Thread-0 Signup and view all the answers

What can be inferred about the execution of threads in the given example?

Thread execution is dependent on the system's CPU scheduling. Signup and view all the answers

What happens when the JVM allocates an execution time slot to a thread?

The thread can be preempted by another thread. Signup and view all the answers

Which of the following best describes the term 'nondeterministic' in relation to threads?

The order of execution can vary with each program run. Signup and view all the answers

What scenario does the thread execution example illustrate?

Execution order can differ with each execution run. Signup and view all the answers

What implies that multiple threads can be in various states during their lifecycle?

Threads can be blocked, running, or terminated. Signup and view all the answers

How many threads were created in the provided example?

Three Signup and view all the answers

What is a fundamental challenge in concurrent programming?

Coordinating multiple threads to produce correct results Signup and view all the answers

What does nondeterministic execution of threads imply?

Thread execution can produce multiple outcomes Signup and view all the answers

What is one of the consequences of threads sharing global data?

It can lead to race conditions in a multithreaded environment Signup and view all the answers

What happens when multiple threads are executed on a single processor?

Their execution steps are interleaved Signup and view all the answers

Which of the following is a problem that needs to be avoided in all concurrent programs?

Race conditions Signup and view all the answers

Why might independent threads not be the typical scenario in multithreaded systems?

Threads usually need to share data and communicate Signup and view all the answers

What aspect of thread scheduling is determined by the scheduler?

When each thread runs based on an algorithm Signup and view all the answers

What must programmers ensure regarding their code in a multithreaded environment?

Code produces correct results regardless of execution order Signup and view all the answers

Which statement best describes race conditions?

They result from multiple threads accessing shared data unsafely Signup and view all the answers

What do threads typically need to do when processing requests in a web client scenario?

Communicate and coordinate work using shared data Signup and view all the answers

What is the main reason the request counter does not reach the expected value of 50,000?

The increment operation is not atomic at the machine level. Signup and view all the answers

What does the count variable in the RequestCounter class represent?

The total number of requests processed. Signup and view all the answers

Why is using a 5-second delay in the main method considered a bad idea?

It does not guarantee all threads will complete within that time frame. Signup and view all the answers

How is the counter value incremented in the RequestCounter class?

By incrementing the count variable directly without safety. Signup and view all the answers

What can occur when one thread loads the counter value while another increments it?

The first thread updates the counter with the old value, losing the increment. Signup and view all the answers

What does atomic operation imply in the context of increments?

It cannot be interrupted and executes in a single step. Signup and view all the answers

What happens to the counter variable if Java manages multiple threads incorrectly?

The counter value may be lower than expected due to lost increments. Signup and view all the answers

What is the purpose of the RequestCounter class's main method?

To demonstrate how threads interact with a shared counter. Signup and view all the answers

What modification could improve the accuracy of the RequestCounter?

Implementing synchronized blocks or methods for counter access. Signup and view all the answers

What does creating 50,000 threads primarily test in this context?

The system's ability to manage an extensive number of threads efficiently. Signup and view all the answers

IBM launched the first multicore processor in 2001, featuring a chip with three CPUs.

False Signup and view all the answers

Modern programming languages tend to have diverse concurrency models for managing threads.

True Signup and view all the answers

Each software process typically has multiple threads of execution by default.

False Signup and view all the answers

Java's main() function defines the starting point of a single thread in a program.

True Signup and view all the answers

Concurrency models are a recent development in computer science, having been studied for less than 20 years.

False Signup and view all the answers

Threads run independently and asynchronously, which means their execution order is controlled by the programmer.

False Signup and view all the answers

Race conditions occur due to the independent execution of threads without shared resources.

False Signup and view all the answers

Multiple threads can share global data within a process, which can aid in coordinating their work.

True Signup and view all the answers

Deadlocks and race conditions are two fundamental problems that need to be avoided in concurrent programming.

True Signup and view all the answers

If all threads are completely independent, they do not require any coordination or shared data to function correctly.

True Signup and view all the answers

Match the following concepts with their explanations:

Concurrency = Executing multiple tasks simultaneously I/O event = An input or output operation that causes a program to wait CPU instruction = Basic operation executed by the processor Operating System Scheduling = Manages the execution of tasks on a CPU while others wait Signup and view all the answers

Match the following performance issues with their descriptions:

Wasted Processing Capacity = Time spent waiting for I/O rather than executing instructions Responsiveness in Applications = Ability of an application to handle tasks without delay Parallel Execution = Multiple activities processed at the same time Single Instruction Execution = Old limitation of CPUs where only one operation could occur at once Signup and view all the answers

Match the following programming challenges with their definitions:

Race Condition = An issue arising from multiple threads accessing shared data Concurrency Models = Frameworks and techniques for managing multiple threads Nondeterministic Execution = Unpredictable order of thread completion Thread Scheduling = Determining which thread runs at a given time Signup and view all the answers

Match the following terms with their corresponding actions:

Multicore Processor = Has multiple CPUs to improve performance Server Application = Structurally designed to handle multiple requests simultaneously Program I/O Wait = Occurs when a program is paused for a hardware response Linux Operating System = Efficiently manages the execution of multiple programs on a single CPU Signup and view all the answers

Match the following Java thread concepts with their descriptions:

Runnable interface = Defines a contract for classes to represent a thread's task start() method = Initiates the execution of a thread run() method = Contains the code that will be executed by the thread NamingThread class = An example implementation of the Runnable interface Signup and view all the answers

Match the following terms related to threads with their meanings:

Thread = An independent sequence of execution within a program Runtime stack = Manages local variables and method calls for each thread Global data = Data accessible by all threads in a process Execution context = The environment in which a thread runs its code Signup and view all the answers

Match the following actions with their outcomes in thread execution:

Creating a Thread object = Prepares a thread for execution but does not start it Calling run() directly = Executes the code in the main thread, not in a new thread Calling start() = Begins the execution of the thread's run() method Terminating a thread = Ends the independent execution of that thread Signup and view all the answers

Match the following thread states with their characteristics:

Runnable = The thread is ready to run and waiting for CPU time Blocked = The thread is waiting for a resource to become available Terminated = The thread has completed its execution Sleeping = The thread is paused temporarily and will resume after a specified time Signup and view all the answers

Match the following potential issues with threads in multithreading scenarios:

Race condition = Occurs when multiple threads access shared data without synchronization Thread starvation = When a thread is perpetually denied access to resources Deadlock = When two or more threads are blocked forever, waiting on each other Livelock = When threads keep changing states without making progress Signup and view all the answers

What happens when a producer attempts to add an item to a full buffer?

It blocks until space becomes available in the buffer. Signup and view all the answers

What is one drawback of using polling in the producer-consumer problem?

It increases resource usage, especially CPU cycles. Signup and view all the answers

What Java methods are used to implement signaling between threads in the producer-consumer problem?

wait() and notify() Signup and view all the answers

How do consumers in the producer-consumer problem know when new items are available?

They receive a signal from producers when an item is added. Signup and view all the answers

What is the purpose of using a synchronized block when calling wait()?

To ensure that conditions are met before waiting. Signup and view all the answers

What happens when a consumer retrieves an item from a buffer?

It reduces the buffer size and sends a signal to producers. Signup and view all the answers

What is a key difference between blocking operations and busy waiting?

Blocking operations do not consume resources, while busy waiting does. Signup and view all the answers

In a real-time application, why is minimizing resource usage important?

It can lead to faster processing times and better performance. Signup and view all the answers

What scenario describes a situation where consumers have to wait for items to become available?

When consumers process items slower than producers can add them. Signup and view all the answers

Study Notes

Thread Basics

A single thread has access to a program's environment and resources, such as open file handles and network connections.
Threads are independent sequences of execution in a process with their own runtime stacks for managing method calls and local variables.
Each thread has access to the process's global data and environment.
In Java, threads can be defined by creating a class that implements the Runnable interface and defines a run() method.

Creating and Executing Threads

To execute a thread in Java, create a Thread object using an instance of the Runnable interface and call the start() method to invoke the execution context.

Nondeterminism in Thread Execution

The JVM scheduler controls the order of thread execution.
The order of thread execution is nondeterministic, meaning programmers cannot control the order in which threads are executed.
The JVM scheduler can preempt threads, interrupting their execution to give other threads a chance to run.
Preemption is a crucial component of thread scheduling, ensuring that all threads have a chance to make progress.

Race Conditions

Race conditions occur when multiple threads access shared data, leading to potential inconsistencies and incorrect results.
Threads access shared data structures to coordinate their work and communicate status.
In Java, this shared data includes both global and static data.
Race conditions arise because the execution of multiple threads can be interleaved, making it difficult to predict the order in which statements are executed.
To illustrate race conditions, the text presents an example of 50,000 threads updating a shared counter. The program often fails to produce the correct result due to non-atomic increments, where the three operations involved in incrementing a counter can be interrupted by other threads.

Deadlocks

Deadlocks occur when threads are unable to proceed because they are waiting for resources held by other threads that are also blocked.
The text uses the classic Dining Philosophers problem as an analogy to exemplify deadlocks.
The Dining Philosophers problem involves five philosophers who need to pick up two chopsticks to eat. They are seated around a circular table with a single chopstick placed between each philosopher.
The deadlock occurs when all philosophers pick up their left chopsticks simultaneously, preventing any further action because they are now waiting for the right chopstick, which is held by a neighboring philosopher.
Deadlocks can be resolved using strategies such as resource ordering, timeouts, and deadlock detection.

Multi-core Processors

IBM introduced the first multi-core processor in 2001, featuring two CPUs
Multi-core chips allow software with parallel activities to execute concurrently on each core, enhancing processing capacity
Modern laptops can have up to 16 CPUs (cores)

Threads

Threads are the primary mechanism for structuring software systems as concurrent activities
Every programming language has its own threading mechanism, but the underlying semantics are broadly similar
Threads are asynchronous and run independently until they are complete, and the scheduler decides which thread runs next

Concurrency Models

Various concurrency models exist to structure and coordinate concurrent activities, including:
- Thread-based concurrency: uses independently executing threads with locks to access shared resources
- Actor Model: uses "actors" that communicate by sending asynchronous messages
- CSP (Communicating Sequential Processes): uses channels for synchronous message passing

Problems with Threads

Race Conditions: Occur when multiple threads modify shared data structures simultaneously, leading to unpredictable results.
- Interleaving: Execution of threads is interleaved by the CPU, meaning their statements are not necessarily executed in a predictable order, increasing the risk of race conditions.
- Critical Sections: Critical sections are parts of code that update shared data structures and must be executed atomically to prevent race conditions. The synchronized keyword in Java defines a critical section to ensure only one thread can access it at a time.
Deadlocks: Occur when multiple threads are blocked indefinitely because they require resources that are hold by each other, leading to a standstill.

Example of Race Conditions

The text illustrates this using a RequestCounter example where multiple threads update a shared counter.
The increment operation isn't atomic at the machine level, leading to lost updates and incorrect results due to thread interleaving.
Even infrequent occurrences of race conditions can lead to undesirable outcomes.

Dining Philosophers Problem

The "Dining Philosophers Problem" illustrates deadlocks:
- Five philosophers are sitting around a table, each needing two chopsticks to eat
- Each chopstick is held by two philosophers, creating the potential that each philosopher will grab one chopstick and wait indefinitely for the neighboring philosopher to release the second chopstick needed.
This problem demonstrates how threads can become deadlocked when accessing shared resources.

Concurrency in Web Applications

Concurrency in web applications is like managing the flow of customers in a busy coffee shop, where a single barista might struggle to handle complex orders, causing delays and frustration.
Implementing concurrency in software allows the processing of multiple requests simultaneously, mimicking the benefits of having multiple baristas in the coffee shop, leading to increased efficiency and quicker service.
Modern CPUs can execute millions of instructions per second. However, when a program needs to access a hardware device - like a disk drive or network card - it must wait for the data, causing a significant slowdown as the CPU remains idle.
Operating systems exploit this idle time by scheduling other programs to execute while one program waits for input/output (I/O), greatly enhancing efficiency and CPU utilization.

Thread Management and Execution

Concurrency in Java is achieved using threads, which are independent units of execution within a program.
Each thread has its own call stack to manage its own local data and method calls, but also shares access to the process's global data and environment.
To create a new thread in Java, a class that implements the Runnable interface and defines the run() method is used.
The start() method is used to initiate the thread, creating a new execution context for it to run independently. Invoking run() directly will execute the code within the thread's method, but will not create a new thread, effectively keeping the program single-threaded.
The join() method allows one thread to wait for another thread to complete its execution before proceeding.

Deadlock: A Concurrency Problem

Deadlock occurs when multiple threads are blocked indefinitely, waiting for resources held by other threads, creating a circular dependency.
Deadlock is a common problem in concurrent programs, and a simple example is the classic "Dining Philosophers Problem" where five philosophers at a round table, each needing two chopsticks to eat, can end up in a situation where all are blocked and no progress is possible.

Addressing Deadlock

Deadlock can sometimes be mitigated using timeouts on blocking operations. If a thread waits too long for a resource, it may release the resource it holds and try again later, potentially breaking the deadlock.
However, timeouts are not always effective and can lead to performance issues, making designing deadlock-free solutions a preferable approach.
A deadlock-free solution involves designing resource allocation protocols to eliminate circular dependencies, ensuring that one or more threads can always make progress.
In the Dining Philosophers problem, a deadlock-free solution involves forcing a specific philosopher always to pick up their right chopstick first, breaking the circular dependency and ensuring that at least one philosopher can always get both chopsticks.

Producer-Consumer Problem

Illustrates a common scenario where producers generate data and consumers consume it.
A shared buffer acts as a intermediary between the producers and the consumers.
Producers add items to the buffer, consumers retrieve items from it.
Buffer capacity is limited, leading to potential waiting for producers or consumers.
Producers must wait if the buffer is full, and consumers must wait if it is empty.

Polling vs. Blocking

Polling involves repeatedly checking if a resource is available.
Blocking suspends a thread until a resource is available.

Blocking Operations

Blocking is more efficient than polling because it avoids unnecessary resource consumption.
Threads in a blocking state consume no resources until the condition is met.

Java Primitives for Signaling

wait(): Called by a thread when a required condition is not true.
notify(): Called when the condition becomes true, signaling the wait()ing thread.
Used to implement guarded blocks, which ensure a condition holds before a thread resumes execution.
Example: Using a condition "empty" to block a consumer thread if the buffer is empty.

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Test your knowledge on the basics of threads in Java, including how to create and execute them. Understand thread management, nondeterminism, and the role of the JVM scheduler in handling multiple threads. Challenge yourself with questions on implementation and functionality.