Betriebssysteme/hw8/simu2/QUESTIONS.md

QUESTIONS Synchronisation: CV

This homework lets you explore some real code that uses locks and condition variables to implement various forms of the producer/consumer queue discussed in the chapter. You’ll look at the real code, run it in various configurations, and use it to learn about what works and what doesn’t, as well as other intricacies.

The different versions of the code correspond to different ways to “solve” the producer/consumer problem. Most are incorrect; one is correct. Read the chapter to learn more about what the producer/consumer problem is, and what the code generally does.

The first step is to download the code and type make to build all the variants. You should see four:

• main-one-cv-while.c: The producer/consumer problem solved with a single condition variable.

• main-two-cvs-if.c: Same but with two condition variables and using an if to check whether to sleep.

• main-two-cvs-while.c: Same but with two condition variables and while to check whether to sleep. This is the correct version.

• main-two-cvs-while-extra-unlock.c: Same but releasing the lock and then reacquiring it around the fill and get routines.

It’s also useful to look at pc-header.h which contains common code for all of these different main programs, and the Makefile so as to build the code properly.

See the README.md for details on these programs.

Questions

Please explain your answers, even if not asked for it. Try to think first about the question, then run the code. Remember, in the exam you can’t run the code, but have to think about the question and finally answer it correctly.

Fix Source Code

When you compile the programs on your workstation, you get a lot of warnings which are turned into errors. You have to correct these errors first. Check, by which parameters a thread is created, and how the id is casted later on.

1. Explain the problem, and how you fixed it.

main-two-cvs-while.c

Our first questions focus on main-two-cvs-while.c (the working solution). First, study the code. Do you think you have an understanding of what should happen when you run the program?

1. Now run with one producer and one consumer, and have the producer produce a few values. Start with a buffer of size 1, and then increase it. How does the behavior of the code change when the buffer is larger? (or does it?)

2. What would you predict num full to be with different buffer sizes (e.g., -m 10) and different numbers of produced items (e.g., -l 100), when you change the consumer sleep string from default (no sleep) to -C 0,0,0,0,0,0,1?

3. If possible, run the code on different systems (e.g., Mac OS X , Linux, lab container and your notebook). Do you see different behavior across these systems?

4. Let’s look at some timings of different runs. How long do you think the following execution, with one producer, three consumers, a single-entry shared buffer, and each consumer pausing at point c3 for a second, will take?

   prompt> ./main-two-cvs-while -p 1 -c 3 -m 1 -C

         0,0,0,1,0,0,0:0,0,0,1,0,0,0:0,0,0,1,0,0,0 -l 10 -v -t
   

5. Now change the size of the shared buffer to 3(-m3). Will this make any difference in the total time?

6. Now change the location of the sleep to c6 (this models a consumer taking something off the queue and then doing something with it for a while), again using a single-entry buffer. What time do you predict in this case?

    prompt> ./main-two-cvs-while -p 1 -c 3 -m 1 -C
            0,0,0,0,0,0,1:0,0,0,0,0,0,1:0,0,0,0,0,0,1 -l 10 -v -t
   

7. Finally, change the buffer size to 3 again (-m 3). What time do you predict now?

main-one-cv-while.c

1. Now let’s look at main-one-cv-while.c. Can you configure a sleep string, assuming a single producer, one consumer, and a buffer of size 1, to cause a problem with this code? Explain why this happens!

2. Now change the number of consumers to two. Can you construct sleep strings for the producer and the consumers so as to cause a problem in the code? Explain why this happens!

main-two-cvs-if.c

1. Now examine main-two-cvs-if.c. Can you cause a problem to happen in this code? Again consider the case where there is only one consumer, and then the case where there is more than one. Explain the problematic code sequence.

main-two-cvs-while-extra-unlock.c

1. Finally, examine main-two-cvs-while-extra-unlock.c. What problem arises when you release the lock before doing a put or a get? Can you reliably cause such a problem to happen, given the sleep strings? What bad thing can happen?