1. Project Part I Background Tom Roeder CS415 2005sp

2. You need to implement minithreads  yield  fork/create/initialize  stop/start synchronization  P/V  create/initialize/destroy FIFO non-preemptive scheduler Test cases

3. Threads Each thread must have its own control block Contains  top pointer  base pointer  thread identifier  … whatever else you find useful To switch between threads, must change esp

4. Context Switching thread 1 call stack ESP thread 1 thread 2 call stack thread 2 ret addr 2 thread 2 registers context_switch(new, old) ??

5. Context Switching thread 1 call stack ESP thread 1 thread 2 call stack thread 2 ret addr 2 thread 2 registers thread 1 registers ret addr 1 context_switch(new, old)

6. Context Switching thread 1 call stack ESP thread 1 thread 2 call stack thread 2 ret addr 2 thread 2 registers thread 1 registers ret addr 1 context_switch(new, old)

7. Context Switching thread 1 call stack ESP thread 1 thread 2 call stack thread 2 thread 1 registers ret addr 1 context_switch(new, old)

8. Frequently Asked Questions How do I bootstrap into a minithreads environment? (a.k.a. How do I context switch into my first thread?)  Use the initial kernel thread  Wrap it in a minithread structure The stack sometimes grows downwards and sometimes up. What’s going on?  Normally grows down  We normally talk (and think) about it growing up

9. Frequently Asked Questions How do I clean up my thread when I’m still on its stack?  You can’t  add a garbage collection thread. What’s the difference between create and fork?  create results in a new not running thread  fork results in a new running thread

10. Scheduler Take the first element off the run queue and run it (FIFO) Need to have a garbage collection thread Need to have a system idle thread  see the System Idle Process in Ctl-Shift-Esc  Should run when nothing else is running When all your threads exit, your program should stay around

11. Software Engineering Need to check for valid values in functions  Use AbortOnCondition  Might want to define something like: #define AbortOnNull(var) AbortOnCondition(var == NULL, #var “ should not be NULL”); Use macros to your advantage Check return values of functions with AbortOnError(func_call());

12. Software Engineering Comments  Clear and understandable  Document why not what  We should not have to guess the code’s purpose Naming  give variables simple functional names  single letters are in general discouraged Modularization  remember to refactor every once in a while

13. Testing Write a test program  Create many threads  Create no threads  try our tests (test[1-3].c, sieve.c, buffer.c)  unit test your functions try unreasonable values for parameters figure out invariants and check them The key is to systematically try as many cases as possible

14. Testing Best to automate testing:  Have a test harness generate lots of tests  Especially ones that are easy to figure out what the right answer should be Test each other’s code in the group  When you look at code for long enough, you don’t always notice the bugs Code review  especially the tricky parts of the code

15. Questions?

16. Synchronization Recall the problem posed in 414  Operations that appear atomic to C are not always  Even in C, we often don’t want to be interrupted  Need a Do Not Disturb sign Enter locks  allow implementation of mutual exclusion  lets N threads in at a time  Everyone else must wait outside We will use semaphores for now

17. Synchronization: Semaphores Two operations: P (proberen) and V (verhogen)  P (test) does the following atomically: decrement the semaphore if it is less than 0, block else continue  V (increment) does the following atomically: increment the semaphore if the value of the semaphore is now greater than or equal to 0 and there is someone waiting, let them in Note that we have not solved atomicity

18. Synchronization: Semaphores Fortunately, there is no preemption for now  Easy to get mutual exclusion  Just don’t yield in P or V! Ranveer will explain in 414 what to do in the case of preemption  Don’t need to know this for this part of the project

19. Synchronization Example semaphore s; int x = 0; thread_1 () { while(true) { P(s); x = x + 10; V(s); } thread_2 () { while(true) { P(s); x = x - 10; V(s); }