CS110: Principles of Computer Systems

Autumn 2021
Jerry Cain
PDF

Introduction to Threads

Multiple threads are often spawned to subdivide and collectively solve a larger problem.
Consider the scenario where 10 ticket agents answer telephones—as they might have before the internet came along—at United Airlines to jointly sell 250 airline tickets.
Each ticket agent answers the telephone, and each telephone call always leads to the sale of precisely one ticket.
Rather than requiring each ticket agent sell 10% of the tickets, we'll account for the possibility that some ticket sales are more time consuming than others, some ticket agents need more time in between calls, etc. Instead, we'll require that all ticket agents keep answering calls and selling tickets until all have been sold.
Here's our first stab at a main function. Full program is right here.

int main(int argc, const char *argv[]) {
  thread agents[10];
  size_t remainingTickets = 250;
  for (size_t i = 0; i < 10; i++)
    agents[i] = thread(ticketAgent, 101 + i, ref(remainingTickets));
  for (thread& agent: agents) agent.join();
  cout << "End of Business Day!" << endl;
  return 0;
}

Introduction to Threads

As with most multithreaded programs, the main thread elects to spawn child threads to subdivide and collaboratively solve the full problem at hand.
In this case, the main function declares the primary copy of the remaining ticket count—aptly named remainingTickets—and initializes it to 250.
The main thread then spawns ten child threads to run some ticketAgent thread routine, yet to be fully defined. Each agent is assigned a unique id number between 101 and 110, inclusive, and a reference to remainingTickets is shared with each thread.
As is typical, the main thread blocks until all child threads have finished before exiting. Otherwise, the entire process might be torn down even though some child threads haven't finished.

int main(int argc, const char *argv[]) {
  thread agents[10];
  size_t remainingTickets = 250;
  for (size_t i = 0; i < 10; i++)
    agents[i] = thread(ticketAgent, 101 + i, ref(remainingTickets));
  for (thread& agent: agents) agent.join();
  cout << "End of Business Day!" << endl;
  return 0;
}

Introduction to Threads

The ticketAgent thread routine accepts an id number (used for logging purposes) and a reference to the remainingTickets.
It continually polls remainingTickets to see if any tickets remain, and if so, proceeds to answer the phone, sell a ticket, and publish a little note about the ticket sale to cout.
handleCall, shouldTakeBreak, and takeBreak are all in place to introduce short, random delays and guarantee that each test run is different than prior ones. Full program is (still) right here.

static void ticketAgent(size_t id, size_t& remainingTickets) {
  while (remainingTickets > 0) {
    handleCall(); // sleep for a small amount of time to emulate conversation time.
    remainingTickets--;
    cout << oslock << "Agent #" << id << " sold a ticket! (" << remainingTickets 
         << " more to be sold)." << endl << osunlock;
    if (shouldTakeBreak()) // flip a biased coin
      takeBreak();         // if comes up heads, sleep for a random time to take a break
  }
  cout << oslock << "Agent #" << id << " notices all tickets are sold, and goes home!" 
       << endl << osunlock;
}

Introduction to Threads

Presented below right is the abbreviated output of a confused-ticket-agents run.
In its current state, the program suffers from a serious race condition.
Why? Because remainingTickets > 0 test and remainingTickets-- aren't guaranteed to execute within the same time slice.
If a thread evaluates remainingTickets > 0 to be true and commits to selling a ticket, the ticket might not be there by the time it executes the decrement. That's because the thread may be swapped off the
CPU after the decision to sell
but before the sale, and during
the dead time, other threads—
perhaps the nine others—all
might get the CPU and do
precisely the same thing.
The solution? Ensure the
decision to sell and the sale
itself are executed without
competition.

poohbear@myth61:$ ./confused-ticket-agents 
Agent #110 sold a ticket! (249 more to be sold).
Agent #104 sold a ticket! (248 more to be sold).
Agent #106 sold a ticket! (247 more to be sold).
// some 245 lines omitted for brevity 
Agent #107 sold a ticket! (1 more to be sold).
Agent #103 sold a ticket! (0 more to be sold).
Agent #105 notices all tickets are sold, and goes home!
Agent #104 notices all tickets are sold, and goes home!
Agent #108 sold a ticket! (4294967295 more to be sold).
Agent #106 sold a ticket! (4294967294 more to be sold).
Agent #102 sold a ticket! (4294967293 more to be sold).
Agent #101 sold a ticket! (4294967292 more to be sold).
// carries on for a very, very, very long time

Analysis of Ticket Agents Example

Before we solve this problem, we should really understand why remainingTickets-- itself isn't even thread-safe.
- C++ statements aren't inherently atomic. Virtually all C++ statements—even ones as simple as remainingTickets--—compile to multiple assembly code instructions.
- Assembly code instructions are atomic, but C++ statements are not.
- g++ on the myths compiles remainingTickets-- to five assembly code instructions, as with:
- The first two lines drill through the ticketsRemaining reference to load a copy of the ticketsRemaining held in main into %rax. The third line decrements that copy, and the last two write the decremented copy back to the ticketsRemaining variable held in main.

0x0000000000401a9b <+36>:    mov    -0x20(%rbp),%rax
0x0000000000401a9f <+40>:    mov    (%rax),%eax
0x0000000000401aa1 <+42>:    lea    -0x1(%rax),%edx
0x0000000000401aa4 <+45>:    mov    -0x20(%rbp),%rax
0x0000000000401aa8 <+49>:    mov    %edx,(%rax)

Improvements to Ticket Agents Example

We need to guarantee that the code that tests for remaining tickets, the code that sells a ticket, and everything in between are executed as part of one large transaction, without interference from other threads. Restated, we must guarantee that at no other threads are permitted to even examine the value of ticketsRemaining if another thread is staged to modify it.
One solution: provide a directive that allows a thread to ask that it not be swapped off the CPU while it's within a block of code that should be executed transactionally.
- That, however, is not an option, and shouldn't be.
- That would grant too much power to threads, which could abuse the option and block other threads from running for an indeterminate amount of time.
The other option is to rely on a concurrency directive that can be used to prevent more than one thread from being anywhere in the same critical region at one time. That concurrency directive is the mutex, and in C++ it looks like this:

class mutex {
public:
  mutex();        // constructs the mutex to be in an unlocked state
  void lock();    // acquires the lock on the mutex, blocking until it's unlocked
  void unlock();  // releases the lock and wakes up another threads trying to lock it
};

Improvements to Ticket Agents Example

The name mutex is a contraction of the words mutual and exclusion. It's so named because its primary use it to mark the boundaries of a critical region—that is, a stretch of code where at most one thread is permitted to be at any one moment.
- Restated, a thread executing code within a critical region enjoys exclusive access.
The constructor initializes the mutex to be in an unlocked state.
The lock method will eventually acquire a lock on the mutex.
- If the mutex is in an unlocked state, lock will lock it and return immediately.
- If the mutex is in a locked state (presumably because another thread called lock but has yet to unlock), lock will pull the calling thread off the CPU and render it ineligible for processor time until it's notified the lock was released.
The unlock method will release the lock on a mutex. The only thread qualified to release the lock on the mutex is the one that holds it.

class mutex {
public:
  mutex();        // constructs the mutex to be in an unlocked state
  void lock();    // acquires the lock on the mutex, blocking until it's unlocked
  void unlock();  // releases the lock and wakes up another threads trying to lock it
};

Final Solution to Ticket Agents Example

We can declare a single mutex beside the declaration of remainingTickets in main, and we can use that mutex to mark the boundaries of the critical region.
This requires the mutex also be shared by reference with the ticketAgent thread routine so that all child threads compete to acquire the same lock.
The new ticketAgent thread routine looks like this:
When do you need a mutex?
- When there are multiple threads writing to a variable.
- When there is a thread writing and one or more threads reading
- Why do you not need a mutex when there are no writers (only readers)?

static void ticketAgent(size_t id, size_t& remainingTickets, mutex& ticketsLock) {
  while (true) {
    ticketsLock.lock();
    if (remainingTickets == 0) break;
    handleCall();
    remainingTickets--;
    cout << oslock << "Agent #" << id << " sold a ticket! (" << remainingTickets 
         << " more to be sold)." << endl << osunlock;
    ticketsLock.unlock();
    if (shouldTakeBreak()) 
      takeBreak();
  }
  ticketsLock.unlock();
  cout << oslock << "Agent #" << id << " notices all tickets are sold, and goes home!" 
       << endl << osunlock;
}

critical region

Lecture 13: Threads and Race Conditions

By Jerry Cain

Lecture 13: Threads and Race Conditions

1,847

CS110: Principles of Computer Systems

Introduction to Threads

Introduction to Threads

Introduction to Threads

Introduction to Threads

Analysis of Ticket Agents Example

Improvements to Ticket Agents Example

Improvements to Ticket Agents Example

Final Solution to Ticket Agents Example

Lecture 13: Threads and Race Conditions

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