1 CSE451 Section 3

2 Reminders Start project 2! It’s long Read the assignment carefully Read it again Project 2 will be done in groups of 3 Today: Form project 2 groups Project 2 intro CVS

3 Project 2 You have to: Implement a user thread library Implement synchronization primitives Solve a synchronization problem Add Preemption Implement a multithreaded web server Get some results and write a (small) report Part a due: in two weeks (Thurs, April 28 12:00pm) Part b due: 11 days after part a (Monday, May 9 12:00pm) Part a Part b

4 Simplethreads We give you: Skeleton functions for thread interface Machine-specific code Support for creating new stacks Support for saving regs/switching stacks A generic queue When do you need one? Very simple test programs You should write more Singlethreaded web server

5 Simplethreads Code Structure include/sthread.h Other appsWeb server (web/sioux.c) test/*.c lib/sthread_user.h lib/sthread_user.c lib/sthread_ctx.c lib/sthread_ctx.h You write this sthread_switch_i386.h sthread_switch_powerpc.h lib/sthread_switch.S lib/sthread_queue.c lib/sthread_queue.h lib/sthread_preempt.c lib/sthread_preempt.h

6 Thread Operations void sthread_init() Initialize the whole system sthread_t sthread_create(func start_func, void *arg) Create a new thread and make it runnable void sthread_yield() Give up the CPU void sthread_exit(void *ret) Exit current thread What about the TCB? struct _thread { sthread_ctx_t *saved_ctx; ……… } Others?

7 Sample multithreaded program int main(int argc, char **argv) { int i; sthread_init(); if (sthread_create(thread_start, (void*)i) == NULL) { printf("sthread_create failed\n"); exit(1); } sthread_yield(); //yield main thread to our new thread printf("back in main\n"); return 0; } void *thread_start(void *arg) { printf("In thread_start, arg = %d\n", (int)arg); return 0; } Output? (assume no preemption)

8 Managing Contexts (given) Thread context = thread stack + stack pointer sthread_new_ctx(func_to_run) gives a new thread context that can be switched to sthread_free_ctx(some_old_ctx) Deletes the supplied context  sthread_switch(oldctx, newctx) Puts current context into oldctx Takes newctx and makes it current

9 How sthread_switch works Xsthread_switch: pusha movl %esp,(%eax) movl %edx,%esp popa ret Thread 1 TCB … SP Thread 2 TCB … SP ESP CPU Thread 1 runningThread 2 ready Want to switch to thread 2… Thread 2 registers Thread 1 regs

10 Push old context Xsthread_switch: pusha movl %esp,(%eax) movl %edx,%esp popa ret Thread 1 TCB … SP Thread 2 TCB … SP ESP CPU Thread 1 runningThread 2 ready Thread 2 registers Thread 1 registers Thread 1 regs

11 Save old stack pointer Xsthread_switch: pusha movl %esp,(%eax) movl %edx,%esp popa ret Thread 1 TCB … SP Thread 2 TCB … SP ESP CPU Thread 1 runningThread 2 ready Thread 2 registers Thread 1 registers Thread 1 regs

12 Change stack pointers Xsthread_switch: pusha movl %esp,(%eax) movl %edx,%esp popa ret Thread 1 TCB … SP Thread 2 TCB … SP ESP CPU Thread 1 readyThread 2 running Thread 2 registers Thread 1 registers Thread 1 regs

13 Pop off new context Xsthread_switch: pusha movl %esp,(%eax) movl %edx,%esp popa ret Thread 1 TCB … SP Thread 2 TCB … SP ESP CPU Thread 1 readyThread 2 running Thread 1 registers Thread 2 regs

14 Done; return Xsthread_switch: pusha movl %esp,(%eax) movl %edx,%esp popa ret Thread 1 TCB … SP Thread 2 TCB … SP ESP CPU Thread 1 readyThread 2 running Thread 1 registers What got switched? SP PC (how?) Other registers Thread 2 regs

Adjusting the PC Thread 1 TCB … SP Thread 2 TCB … SP ESP CPU Thread 2 running: switch(t2,...); 0x800:printf(“test 2”); Thread 1 registers ret pops off the new return address! ra=0x800 PC Thread 1 (stopped): switch(t1,t2); 0x400: printf(“test 1”); ra=0x400

16 Things to think about Who will call sthread_switch? Where does sthread_switch return? How do we delete a thread? Can a thread free its stack itself? Starting up a thread When you create a new stack with sthread_new_ctx(), you initialize it to run some function foo sthread_new_ctx doesn’t pass parameters to foo But in sthread_create, you give a function and an arg! Bottom line: how do you pass arguments to a function with no arguments?

17 Preemption Initially, you will build a non-preemptive thread library. In part 4, you add preemption Much more realistic model We give you: 1.Timer interrupts 2.Primitives to enable/disable interrupts 3.atomic_test_and_set/atomic_clear for synchronization You must: Do something on each timer interrupt. (What?) Synchronize all of your code using 2 and 3 above. More on this later…

18 Last note You can compare your implementation against pthreads (which is preemptive kernel-threads)../configure --with-pthreads

19 What is CVS Version control system for source files Multiple users can work on the same file simultaneously

20 Why use CVS The other way: Keep every version of code, all with different names: Project2good Project2_10_13_04 Project2working Project2_Feb_2_alex Project2_old Send s back and forth with new changes Merge different versions by hand The CVS way: One version, saved in the CVS repository Multiple people can work on the same file concurrently CVS merges the edited versions automatically as you put them back in the repository

21 Setting up CVS Set up CVS root environment var Tells CVS where to find your repository setenv CVSROOT /cse451/yourgroup/cvs (bash) export CVSROOT=/cse451/yourgroup/cvs Initialize a repository (only one person per group) Create a dir in your group’s dir to hold repository (master copy of code) cd /cse451/yourgroup mkdir cvs Initialize repository cvs init You now have an empty repository

22 Setting up CVS (2) Add/Import the simplethreads distribution to your repository tar xvfz simplethreads-1.20.tar.gz cd simplethreads-1.20 cvs import –m “initial code” simplethreads SIMPLETHREADS SIMPLETHREADS_1_20 cd.. rm –fr simplethreads-1.20

23 CVS Commands Check out a project (sandbox, or local copy) to your home directory to work on: CVS creates a dir simplethreads/ and puts copy of all source files in repository into that dir Also adds CVS dir where it stores data about what you check out cd ~ cvs checkout simplethreads cd simplethreads Merge in new changes from repository (update): cvs update [files…] Save your edited files into the repository so others can use them: cvs commit –m “fixed annoying bugs” [files…]

24 CVS Commands 2 Add a new file (source files.c or.h) to the repository cvs add [files…] Check status of a file cvs status file.c Check differences between your file and one in the repository: cvs diff file.c cvs diff –r 1.1 file.c (specifies version) View log of changes for a file cvs log file.c Use emacs for cvs commit log editing (default is vi): setenv VISUAL emacs More info: cvs --help-commands

25 CVS Remote Access Access CVS from another machine besides spinlock/coredump?: setenv CVSROOT coredump.cs.washington.edu:/cse451/yourgroup/cvs setenv CVS_RSH ssh (for CVS to know how to access repository – use ssh) (add to ~/.login (csh) or ~/.profile (bash) to avoid typing…) Warning: be wary of CVS on windows, OS X is ok