CS360 Lecture notes -- Thread #4


In this lecture, we go over a simulation program that uses the producer/consumer paradigm with a bounded buffer, that requires monitors/condition variables.

The Printer Simulation Problem

This lecture revolves around a simulation that we will write. This is a simulation of a system that has users and printers and a fixed-size buffer that connects them. In particular, we have nusers users, and nprinters printers. We'll assume that all the printers are identical (e.g. in a machine room) so that when a user wants to print something out, it doesn't matter which printer it comes out on.

Now, in our simulation, every so often, a user will decide to print something. When this happens, the print job will be submitted, and if any printer is available, it will print the job, taking 4 seconds a page. If all the printers are printing something, then the job will be queued until one of the printers is ready. The print queue will be of a fixed size (this is what makes it a ``bounded buffer'' problem). If the queue is full, then the user must wait until the queue is not full to submit the job.

We are going to use threads for this simulation. Each user will have its own thread, and each printer will have its own thread. The threads will communicate through shared memory.


printqsim

This program has a specific format, which the threads lab will share. There is be a header file, in this case printqsim.h, that defines some data structures plus some subroutine prototypes. In the labs, these subroutines are the ones that you have to write to make the lab work.

There is also a driver program, in this case printqsim.c. This defines a main() routine which sets up the threads. Together with your definitions of the subroutines, the driver program will solve the problem.

You are not allowed to change either the header or driver files. Instead, you are to provide a C file that defines the subroutines in the header file, and when this is compiled with the driver program, the resulting program solves the problem.

In this case, our job is to define initialize_v(), submit_job() and get_print_job() so that together with printqsim.c, our program performs the user/printer simulation correctly.

Ok, let's look at printqsim.c. It takes 6 arguments:

  1. nusers: The number of users.
  2. nprinters: The number of printers.
  3. arrtime: The average time that users will take between submitting print jobs.
  4. maxpages: The maximum size of a print job (in pages).
  5. bufsize: The size of the print queue.
  6. nevents: The number of print jobs that each user will make.
The main() routine sets up a Spq struct. This is defined in printqsim.h. Each user and printer thread will receive a pointer to one of these structs as its argument, and all information that the thread needs will be in this struct. In this way, we won't need to use any global variables. Each of the command line arguments has a field in the Spq struct, plus there are the following extra fields: After setting up one Spq struct (including a call to initialize_v()), the main() thread sets up a random number generator, and then creates nusers user threads, and nprinters printer threads. Each thread gets its own Spq struct as its argument. The only thing that differs in each Spq struct is the id. Everything else (including the pointer to v) is the same. Finally, the main thread exits, leaving only the user and printer threads.

User threads

Each user thread does the same thing. It iterates for nevents iterations. In each iteration, it sleeps for a random period of time (between 1 and arrtime*2 -- this yields a mean waiting time of arrtime), and then submits a print job. This job is represented by a Job struct, which has three fields -- the user's id, a job id (which is i), and the number of pages, which is a random number between 1 and maxpages. The job is then submitted with submit_job.

After submitting nevents jobs, the user thread exits. The user thread prints out when it sleeps, and when it submits a job.

Printer threads

Each printer thread does the same thing. It iterates forever, first getting a job using get_print_job(), and then printing that job. It simulates printing the job by sleeping for 4 seconds for each page. After printing, it repeats the process. The printer thread prints out when it asks for a job, and when it prints a job.

A dummy solution

All that's left is to write initialize_v(), submit_job(), and get_print_job(). To reiterate, were this a lab, your job would be to write these three subroutines so that they work with printqsim.h and printqsim.c You would not be allowed to modify printqsim.h and printqsim.c

Now, look at ps1.c. This is one solution to the problem. It's not a working solution, but it is one that will compile and run. What it does is set s->v to NULL, ignore print jobs when they are submitted, and force the printer threads to exit.

Try running it:

This created a simulation with 5 users, 3 printers, an average of 5 seconds between print jobs, a max page size of 5, a print queue size of 5, and three print jobs per user.

You'll note that the simulation did run, but not correctly. Why? Well, the printers never printed anything, for starters. Moreover, more than 5 print jobs were submitted and ostesibly queued, and the subsequent print jobs were still allowed to be submitted.

This may seem like a boneheaded example, but it illustrates something important -- solutions to a problem may compile and run, but you have to check their output for correctness. I will provide "solutions" like this one for your thread labs that will be incorrect, but give you a starting point.


Starting on a real solution

To actually solve this problem, it's pretty clear how to start. You need to set up a queue of print jobs in your v pointer. This queue will have bufsize elements. When a user submits a job, if there are less than bufsize elements in the queue, you will put the job there. Otherwise, you'll have to wait for a printer to remove one of the jobs.

Since you have multiple threads accessing the buffer, you'll need to protect it with a mutex. The above is all done in ps2.c. First, it defines a Buffer struct that uses an array as a circular queue (with head/tail/njobs) defining the state of the queue. It also has a mutex.

In initialize_v(), the buffer is allocated, and v is set to be the buffer. Moreover, now submit_job inserts the job into the buffer if there's room. If there's not room, the user thread exits. Also, nothing is done with get_print_job(). This is an example of programming incrementally -- you try one thing and test it to make sure it works before going on.

When we call this with the same arguments as before, we see that 5 jobs get submitted, and then the users all exit. This is what we expect, so the code is working:


A semi-working solution

Now the question is -- what should we do when the queue is full? Moreover, when we start writing get_print_job(), what do we do when the queue is empty and there are no jobs to print? Well, ps3.c provides one solution. It is not a good solution, but it works. When submit_job() is called and the queue is full, the mutex is released, and sleep(1) is called. Then the queue is checked again. In this way, if a printer thread calls get_print_job() during that second, then it can take a job off the queue, and then user's job may be submitted. Similarly, when the queue is empty and a printer calls get_print_job() it sleeps for a second an checks again. note, it has to release the mutex when it sleeps so that a user thread can actually put a job on the queue.

The code works. Try it out:

It all works fine. When all the user jobs are done, the printer threads keep sleeping and checking the queue, so you eventually have to cntl-c out of the program.

This is a workable solution, but it is not a good one. The technique of periodically checking the queue is called polling. It's not really what you want because you'd like for a printer thread to wake up and start printing as soon as a job is inserted into the queue, instead of up to a second afterward. Similarly, you'd like the user to complete submitting a job as soon as a printer thread empties a space in the queue instead of up to a second afterward.

In short, polling is ok, but not great. I show it to you because it's good for you to see, but I don't want to see any polling in your labs -- if you do it, you will get points taken off.


Monitors and condition variables

Monitors and condition variables together form a very convenient tool for synchronization. There are two ways to discuss monitors and condition variables -- as part of a threaded language, or as part of a threads library.

The CS361 textbook (Silberschatz & Galvin, Chapter 6 -- or at least that's what it was last time I checked) discusses them as part of a threaded language, but I'm going to discuss them as part of a threads library, since that's how you will use them.

A monitor is a data structure which a thread can "enter" and "exit". Only one thread may be in the monitor at a time. This is just like a mutex, and in pthreads, there is no entity called a "monitor". You just use a mutex. Condition variables allow you to do more sophisticated things with monitors. A condition variable must be associated with a specific monitor. There are three procedures that act on condition varaibles, and whenever you call them, you must have entered the relevant monitor (i.e. you must have locked the relevant mutex):

Let me advocate testing the return values of all monitor and condition variable calls. This is because you often make errors messing with these, and testing the return value can save you hours of debugging.

Now, here is an odd thing -- if you call pthread_cond_signal() or pthread_cond_broadcast(), then you should own the mutex (i.e. you should have locked the mutex). However, the thread that you are unblocking will have locked the mutex when it called pthread_cond_wait(). This at first appears to be a contradiction, but you must remember that the waiting thread unlocks the mutex while it is blocked. When it is unblocked, it must relock the mutex before returning from pthread_cond_wait().

As it turns out there are a few choices that the threads system has in implementing condition variables.

  1. The unblocked thread has to wait until the the thread calling pthread_cond_signal()/pthread_cond_broadcast() unlocks the mutex to run. I.e. the unblocking merely makes it block on the mutex instead of the condition variable.
  2. The unblocked thread automatically locks mutex and the thread calling pthread_cond_signal()/pthread_cond_broadcast() goes back to blocking on the mutex. When the mutex is free, the thread will reenter it and continue executing following the pthread_cond_signal()/pthread_cond_broadcast() call.
Believe it or not, there are arguments for both approaches. In pthreads, the former approach is taken. My personal philosophy on this is that you should program in such a way that either approach will work. One way to do this is to make sure that you unlock the mutex immediately after calling pthread_cond_signal() or pthread_cond_broadcast(). My code will always do this.

Read the CS361 textbook (chapter 6) for a further discussion of this.


Using condition variables

Now, adding condition variables to our program is straightforward. We need two condition variables -- one for when the queue is full and one for when the queue is empty. We'll call pthread_cond_wait() in submit_job() when the queue is full, and pthread_cond_signal() in get_print_job() when a printer thread removes a job from a full queue.

Likewise, we'll call pthread_cond_wait() in get_print_job() when the queue is empty, and pthread_cond_signal() in submit_job() when a user thread inserts a job into an empty queue.

Note that submit_job() and get_print_job() both use while loops because when pthread_cond_wait() returns, the queue may have become full/empty in the time between when the waiting thread unblocked and the time that it acquired the mutex. Therefore, it may have to wait again.

The code is in ps4.c. When you run it, everything seems to work just fine.


A bug

However, there is still one problem with this code. Suppose there are two printer threads waiting because the queue is empty. Moreover, there are two user threads that want to submit jobs at the same time. The first user thread puts the job on the queue and calls pthread_cond_signal(). This unblocks one of the printer threads, but it then blocks so that it can acquire the mutex. Now, the user thread releases the mutex, but the printer thread does not get it -- instead, the next user thread gets it. It puts a job into the queue, but since there is already a job there, it does not call pthread_cond_signal(). Therefore, even though there are two jobs to be printed, only one printer thread is awake. This means that we've lost one printer. This is a bug.

Look at ps4-bad.txt. This is exactly what happens. There are three user threads and five printer threads. Initially, all of the printer threads block. At the 3 second mark, two user threads submit jobs, but only one printer thread (0) is signalled. Then, more jobs are put onto the print queue, but since njobs is greater than 1, no more printers get awakened. This is a bug.

Fixing this bug is simple (in ps5.c) -- simply remove the if statements around the pthread_cond_signal() calls. This means that submit_job always signals the empty condition variable, and get_print_job always signals the full condition variable. This works fine -- if there are no blocked threads, pthread_cond_signal() does nothing, and if, for example, a user thread is unblocked and there is no room on the queue, it will simply call pthread_cond_wait() again. Try it out. If you look at ps5-good.txt, you'll see the same scenario as in ps4-bad.txt at the 27 second mark, and that it is handled just fine.


So

So, you've learned what monitors/condition variables are, and you've seen a detailed example of their use. You have also seen that synchronization problems can be subtle, and you have to examime your program's output carefully to make sure that it is working like you think it should.