Lecture 17, Tue 06/04

Threads

Threads

• A decent tutorial covering a lot of details on C++ threads: https://thispointer.com/c-11-multithreading-part-1-three-different-ways-to-create-threads/

Recall our discussion on processes

• A computer program in execution.
• A process contains the memory and state of the program being run on a computer.
• Threads are different than processes such that:
  • A program unit that is executed independently of other parts of your program.
  • A process may create many threads
    • An OS manages threads similar to a process, BUT
      • threads shares the memory space of the main process.
  • A C++ application starts with a single main thread.

Threads and hardware

• For multi-core processors, you can design your program such that several parts of it work concurrently.
• Each processor (core) runs instructions in parallel.
  • For single-core architectures, it provides the illusion that something is working concurrently, but the processor is shared among all programs.
  • A a single thread can only be executed on a single core at any given time.

Concurrency

• Concurrency is important in computer science.
  • For example, working on “parts of a problem” concurrently can improve performance if the parts can be done independently.
  • Many applications use concurrency for performance reasons, for example:
    • Web servers process requests concurrently
    • We browsers fetch / render images concurrently
  • Embarrassingly Parallel problems are usually a good fit for concurrent computing
    • Monte-carlo simulations are embarrassingly parallel – the result is the average (expected) scenario based on independent executions.

Example of creating a thread

#include <iostream>
#include <thread>
#include <unistd.h>
#include <string>

using namespace std;

void callback1(string val, int num) {
	cout << "----" << endl;
	cout << "Thread with str val: " << val << endl;
	cout << "num: " << num << endl;
	cout << "Thread id: " << this_thread::get_id() << endl;
	sleep(5);
	cout << "----" << endl;
}

int main() {
	std::thread t1(callback1, "one", 1); // add args to function here
	cout << "t1 id: " << t1.get_id() << endl;
	cout << "Done." << endl;

	return 0;
}

• The above program terminates abnormally because when a thread is destructed, it must either be joined back to the parent thread, or the parent thread needs to detach it.
• By default, the main thread does not wait for any thread to finish execution.
• A thread may wait for another thread to terminate execution before continuing its own execution by calling .join() on the thread.
• A thread may also call .detach on a thread to allow it to run independently.
  • In the example above, main terminates before t1 finishes
    • Since main terminates and t1 is destructed from the stack, the process terminates (std::terminate) because t1 was not joined back to the main thread, or t1 was not detached from the main thread.
• The code below shows how to either join or detach the thread in order to avoid abnormal termination:

int main() {
	std::thread t1(callback1, "one", 1); // add args to function here
	cout << "t1 id: " << t1.get_id() << endl;

	// main thread waits for t1 to finish execution before resuming
	// its own execution
	//t1.join();
	
	// or main thread detaches t1 and does not wait for t1 to finish.
	// Since t1 is not detached, program will not terminate abnormally.
	//t1.detach();

	cout << "Done." << endl;

	return 0;
}

Example of a thead creating a thread

• See if you can trace the output and understand the order of execution:

void callback2(bool create) {
	cout << "***" << endl;
	cout << "Starting thread: " << this_thread::get_id() << endl;
	sleep(5);

	if (create) {
		thread subt1(callback2, false);
		cout << "Creating thread: " << subt1.get_id() << endl;
		subt1.join();
	}
	cout << "Finishing thread: " << this_thread::get_id() << endl;
	cout << "***" << endl;
}

int main() {
	thread t2(callback2, true);
	cout << "main::t2 id: " << t2.get_id() << endl;
	t2.join();
	cout << "Done." << endl;
	return 0;
}

Race Conditions

• Careful programming has to be taken into consideration when dealing with multiple threads
• If threads share memory with each other (for example, an object), read / write order conflicts can happen.
• A high-level scenario:
  • A multi-threaded bank software program depends on atomic transactions
    • Atomic transaction means either everything is executed or nothing is.
  • Imagine a joint bank account contains a $100 balance
    • Person x deposits $10 and Person y withdraws $10 at the same time.
    • Person y obtains the current balance of $100 before withdrawing $10.
    • The OS switches to Person x after Person y obtains the current balance of $100.
    • Person x reads the current balance of $100 and deposits $10 (current balance = $110)
    • The OS switches back to Person y and finishes withdrawing $10 using Person y’s current balance ($100), and updates the current balance to $90.
    • Customer loses money … :(

Example of a Race Condition

# Makefile
bank:
	g++ -o main -std=c++11 main.cpp bank.cpp

// Bank.h

class Bank {
public:
	Bank();
	void deposit(double amount);
	double getBalance();
private:
	double totalBalance;
};

// Bank.cpp

#include "Bank.h"

Bank::Bank() {
	totalBalance = 0;
}

void Bank::deposit(double amount) {
	totalBalance += amount;
}

double Bank::getBalance() {
	return totalBalance;
}

#include <iostream>
#include <thread>
#include "Bank.h"

using namespace std;

void run(Bank* b) {
	for (int i = 0; i < 10000; i++) { b->deposit(10); }
}

int main() {
	Bank* bank = new Bank();
	thread t1(run, bank);
	thread t2(run, bank);
	t1.join();
	t2.join();
	cout << "---" << endl;
	cout << bank->getBalance() << endl;
	return 0;
}

• Expected balance is $200000 by the end of execution, but generally is not (unless we get lucky…).

Mutex (mutual exclusion)

• a multi-threaded locking mechanism that only allows a single thread to execute code.
  • Only one thread may acquire the lock at any given time.
  • If a thread obtains a lock on a mutex, no other thread can access the code until the lock is released (unlocked).
    • One way to provide atomic transactions and avoid race conditions between threads.
    • Another common way is to use Semaphores, but we won’t cover this construct in this class.

Updated code using a mutex object

// Bank.h
#include <mutex>

class Bank {
public:
	Bank();
	void deposit(double amount);
	double getBalance();
private:
	double totalBalance;
	std::mutex mutex;
};

// Bank.cpp

#include "Bank.h"

Bank::Bank() {
	totalBalance = 0;
}

void Bank::deposit(double amount) {
	mutex.lock();
	totalBalance += amount;
	mutex.unlock();
}

double Bank::getBalance() {
	return totalBalance;
}

#include <iostream>
#include <thread>
#include "Bank.h"

using namespace std;

void run(Bank* b) {
	for (int i = 0; i < 10000; i++) { b->deposit(10); }
}

int main() {
	Bank* bank = new Bank();
	thread t1(run, bank);
	thread t2(run, bank);
	t1.join();
	t2.join();
	cout << "---" << endl;
	cout << bank->getBalance() << endl;
	return 0;
}

• Using the mutex locking object, we now consistently have an ending balance of $200000.

Deadlock

• A situation where two (or more) threads are waiting on each other to release a resource, where each of the threads have a lock on the dependent resources.
• Execution is halted and cannot move forward in this situation.