Principles of Computer Systems

Spring 2019

Stanford University

Computer Science Department

Lecturer: Chris Gregg

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Lecture 13: An Ice Cream Store

• Today, we will focus on a single, rather involved program that demonstrates how threads can communicate with each other through the use of mutexes and semaphores (which, as you will recall, is based on a conditional_variable_any).
• We are going to discuss five primary ideas:
  1. The binary lock
  2. A generalized counter
  3. A binary rendezvous
  4. A generalized rendezvous
  5. Layered construction
• ​Using these ideas, we will construct an ice cream store simulation that involves customers, clerks, managers, and cashiers. The original program was created as a final exam question by Julie Zelenski in CS 107 when CS 107 also taught multithreading.
• There is a handout with all of the code (on paper in class), which you can download here.
• You can download the runnable code here.

Lecture 13: An Ice Cream Store

• We have already discussed binary locks in some detail
  • Using a mutex, we construct a single-owner lock.
  • When created, a mutex is unlocked and brackets critical regions of code that are matched with lock  and unlock calls on the mutex. (We can also use a lock_guard<mutex> if the situation calls for it and our lock can go out of scope without further non-locked code being run).
  • This concurrency pattern locks down sole access to some shared state or resource that only one thread can be manipulating at a time.

Lecture 13: An Ice Cream Store, idea 1: the binary lock

• When we use a semaphore, we can track the use of a resource, be it empty buffers, full buffers, available network connection, or what have you.
• The semaphore is essentially an integer count, capitalizing on its atomic increment, decrement, and the efficient blocking when a decrement is levied against a zero.
• A semaphore is constructed to the initial count on the resource (sometimes 0, sometimes N—it depends on the situation).
• As threads require a resource, they wait on the semaphore to transactionally consume it.
• Other threads (possibly, but not necessarily the same threads that consume) signal the semaphore when a new resource becomes available.
• This sort of pattern is used to efficiently coordinate shared use of a limited resource that has a discrete quantity. It can also be used to limit throughput (such as in the Dining Philosophers problem) where unmitigated contention might otherwise lead to deadlock.

Lecture 13: An Ice Cream Store, idea 2: a generalized counter

• When we use a semaphore, we can coordinate cross-thread communication.
• Suppose thread A needs to know when thread B has finished some task before it itself can progress any further.
• Rather than having A repeatedly loop (e.g. busy wait) and check some global state, a binary rendezvous can be used to foster communication between the two.
• The rendezvous semaphore is initialized to 0. When thread A gets to the point that it needs to know that another thread has made enough progress, it can wait on the rendezvous semaphore.
• After completing the necessary task, B will signal it. If A gets to the rendezvous point before B finishes the task, it will efficiently block until B's signal. If B finishes the task first, it signals the semaphore, recording that the task is done, and when A gets to the wait, it will be sail right through it.
• A binary rendezvous semaphore records the status of one event and only ever takes on the value 0 (not-yet-completed or completed-and-checked) and 1 (completed-but-not-yet-checked).
• This concurrency pattern is sometimes used to wakeup another thread (such as disk reading thread that should spring into action when a request comes in), or to coordinate two dependent actions (a print job request that can't complete until the paper is refilled), and so forth.
• If you need a bidirectional rendezvous where both threads need to wait for the other, you can add another semaphore in the reverse direction (e.g. the wait and signal calls inverted).
• Be careful that both threads don’t try to wait for the other first and signal afterwards, else you can quickly arrive at deadlock!

Lecture 13: An Ice Cream Store, idea 3: a binary rendezvous

• The generalized rendezvous is a combination of binary rendezvous and generalized counter, where a single semaphore is used to record how many times something has occurred.

• For example, if thread A spawned 5 thread Bs and needs to wait for all of them make a certain amount of progress before advancing, a generalized rendezvous might be used.

• The generalized rendezvous is initialized to 0. When A needs to sync up with the others, it will call wait on the semaphore in a loop, one time for each thread it is syncing up with. A doesn't care which specific thread of the group has finished, just that another has. If A gets to the rendezvous point before the threads have finished, it will block, waking to "count" each child as it signals and eventually move on when all dependent threads have checked back.

• If all the B threads finish before A arrives at the rendezvous point, it will quickly decrement the multiply-incremented semaphore, once for each thread, and move on without blocking.

• The current value of the generalized rendezvous semaphore gives you a count of the number of tasks that have completed that haven't yet been checked, and it will be somewhere between 0 and N at all times.

• The generalized rendezvous pattern is most often used to regroup after some divided task, such as waiting for several network requests to complete, or blocking until all pages in a print job have been printed.

• As with the generalized counter, it’s occasionally possible to use thread::join instead of semaphore::wait, but that requires the child threads fully exit before the joining parent is notified, and that’s not always what you want (though if it is, then join is just fine).

Lecture 13: An Ice Cream Store, idea 4: a generalized rendezvous

• Once you have the basic patterns down, you can start to think about how mutexes and semaphores can be layered and grouped into more complex constructions.
• Consider, for example, the constrained dining philosopher solution in which a generalized counter is used to limit throughput and mutexes are used for each of the forks.
• Another layered construct might be a global integer counter with a mutex lock and a binary rendezvous that can do something similar to that of a generalized rendezvous. As tasks complete, they can each lock and decrement the global counter, and when the counter gets to 0, a single signal to a rendezvous point can be sent by the last thread to finish.
• The combination of mutex and binary rendezvous semaphore could be used to set up a "race": thread C waits for the first of threads A and B to signal. threads A and B each compete to be one who signals the rendezvous. thread C only expects exactly one signal, so the mutex is used to provide critical-region access so that only the first thread signals, but not the second.

Lecture 13: An Ice Cream Store, idea 5: layered construction

• The program we will create simulates the daily activity in an ice cream store.

• The simulation’s actors are the clerks who make ice cream cones, the single manager who supervises, the customers who buy ice cream cones, and the single cashier who accepts payment from customers. A different thread is launched for each of these actors.

• Each customer orders a few ice cream cones, waits for them to be made, gets in line to pay, and then leaves.

• customers are in a big hurry and don’t want to wait for one clerk to make several cones, so each customer dispatches one clerk thread for each ice cream cone he/she orders.

• Once the customer has all ordered ice cream cones, he/she gets in line at the cashier and waits his/her turn. After paying, each customer leaves.

   

Lecture 13: An Ice Cream Store: the simulation

• Each clerk thread makes exactly one ice cream cone. The clerk scoops up a cone and then has the manager take a look to make sure it is absolutely perfect. If the cone doesn't pass muster, it is thrown away and the clerk makes another. Once an ice cream cone is approved, the clerk hands the gem of an ice cream cone to the customer and is then done.

• The single manager sits idle until a clerk needs his or her freshly scooped ice cream cone inspected. When the manager hears of a request for an inspection, he/she determines if it passes and lets the clerk know how the cone fared. The manager is done when all cones have been approved.

• The customer checkout line must be maintained in FIFO order. After getting their cones, a customer "takes a number" to mark their place in the cashier queue. The cashier always processes customers from the queue in order.

• The cashier naps while there are no customers in line. When a customer is ready to pay, the cashier handles the bill. Once the bill is paid, the customer can leave. The cashier should handle the customers according to number. Once all customers have paid, the cashier is finished and leaves.

Lecture 13: An Ice Cream Store: the simulation (continued)

• Let's look at the following in turn:

  • random time/cone-perfection generation functions

  • struct inspection

  • struct checkout

  • customer

    • ​browse

  • clerk

    • ​makeCone

  • manager

    • ​inspectCone

  • cashier

  • main

Lecture 13: An Ice Cream Store: the simulation (continued)

• Because we are modeling a "real" ice cream store, we want to randomize the times for each event. We also want to generate a boolean that says yay/nay about whether a cone is perfect. The following functions accomplished this task:

Lecture 13: An Ice Cream Store: random generation functions

• There are two global structs -- because threads share global address space, this is the easiest way to handle them. We could, of course, create the data structures in main and then pass them into each thread and function by reference or pointer, but this simplifies it (though it does pollute the global namespace).
• The first struct we will look at is the inspection struct:

Lecture 13: An Ice Cream Store: struct inspection

• This struct coordinates between the clerk and the manager.
• The available mutex ensures the manager's undivided attention, so the single manager can only inspect one cone cone at a time
• The requested and finished semaphores coordinate a bi-directional rendezvous between the clerk and the manager.
• The passed bool provides the approval for a single cone.
• Note that we declare a variable of the struct right after the definition (line 6). This is the global variable (the struct definition itself, while global too, does not take up memory).

Lecture 13: An Ice Cream Store: struct checkout

• The second struct we will look at is the checkout struct:

• This struct coordinates between the customers and the cashier.
• The nextPlaceInLine variable is a new, atomic variable that guarantees that ++ and -- work correctly without any data races.
• The customers array-based queue of semaphores allows the cashier to tell the customers that they have paid.
• The waitingCustomers semaphore informs the cashier that there are customers waiting to pay.
• Again, we define the global variable checkout on line 6.

• Customers in our ice cream store, order cones, browse while waiting for them to be made, then wait in line to pay, and then leave. The customer function handles all of the details of the customer's ice cream store visit:

Lecture 13: An Ice Cream Store: customer function

• The customer needs one clerk for each cone. The customer browses and then must join all of the threads before checking out.

• The customers line up by signaling for checkout.

• The customers wait in line until they are checked out.

• Note that the customer starts a clerk thread, and clerks are not waiting around like the manager or cashier.

• The browse function is straightforward:

Lecture 13: An Ice Cream Store: browse function

• The sleep_for function pushes the thread off the processor, so it is not busy-waiting.

• A clerk has multiple duties: make a cone, then pass it to a manager and wait for it to be inspected, then check to see if the inspection passed, and if not, make another and repeat until a well-made cone passes inspection:

Lecture 13: An Ice Cream Store: clerk function

• The clerk and the manager use the inspection struct to pass information -- note that there is only a single inspection struct, but that is okay because there is only one manager doing the inspecting.

  • This does not mean that we can remove the available lock -- it is critical because there are many clerks trying to get the manager's attention.

• Note that we only acquire the lock after making the cone -- don't over-lock.

• Note also that we signal the manager that we have a cone ready for inspection -- this wakes up the manager if they are sleeping. If the manger is in the middle of an inspection, they will immediately go to the next cone after the inspection.

• The makeCone function is straightforward:

Lecture 13: An Ice Cream Store: makeCone function

• The manager (somehow) starts out the day knowing how many cones they will have to approve (we could probably handle this with a global "all done!" flag)

• The manager waits around for a clerk to hand them a cone to inspect.For each cone that needs to be approved, the manager inspects the cone, then updates the number of cones approved (locally) if it passes. If it doesn't pass, the manger waits again. When the manager has passed all necessary cones, they go home.

Lecture 13: An Ice Cream Store: manager function

• The manager signals the waiting clerk that the cone has been inspected (why can there only be one waiting clerk?)

• The inspectCone function updates the inspection struct:

Lecture 13: An Ice Cream Store: inspectCone function

• Why aren't there any locks needed here? This is a global struct!

• The cashier (somehow) knows how many customers there will be during the day. Again, we could probably handle telling the cashier to go home with a global variable.

• The cashier first waits for a customer to enter the line, and then signals that particular customer that they have paid.

Lecture 13: An Ice Cream Store: cashier function

• Could we have handled the customer/cashier as we handled the clerks/manager, without the array?

• The cashier (somehow) knows how many customers there will be during the day. Again, we could probably handle telling the cashier to go home with a global variable.

• The cashier first waits for a customer to enter the line, and then signals that particular customer that they have paid.

Lecture 13: An Ice Cream Store: cashier function

• Could we have handled the customer/cashier as we handled the clerks/manager, without the array?

  • We must ensure that customers get handled in order (otherwise, chaos) -- not so for the clerks, who can fight for the manager's attention.

• Finally, we can look at the main function.

• The main function's job is to set up the customers, manager, and cashier. Why not the clerks? (they are set up in the customer function)

Lecture 13: An Ice Cream Store: main function

• main must wait for all of the threads it created to join before exiting.

• Now we see how the manager and cashier know how many cones / customers there are -- we let everyone in at the beginning, ask them how many cones they want, and off we go.

• There is a lot going on in this program!
• Managing all of the threads, locking, waiting, etc., takes planing and foresight.
• This isn't the only way to model the ice cream store
  • How would you modify the model?
  • What would we have to do if we wanted more than one manager?
  • Could we create multiple clerks in main, as well? (sure)
• This example prepares us for the next idea: ThreadPool.
  • Our manager and cashier threads are just waiting around much of the time, but they are created before needing to do their work.
  • ​It does take time to spin up a thread, so if we have the threads already waiting, we can use them quickly. This is similar to farm, except that now, instead of processes, we have threads.

Lecture 13: An Ice Cream Store: takeaways