CSE451 Section 3:Winter 2k7Welcome to the world of

concurrency!

Kurtis Heimerl(kheimerl@cs)Aaron Kimball (ak@cs)

Threads & synchronization A rite of passage

All undergrad OS courses do

The most fun part Will feel like being a GURU

The craziest part Will be sick of synchronization

Start early & read thoroughly Grab a big picture first!

Discuss your design with Fellow students TAs

User-level threads: to do First submission(Feb 2nd 10:00am)

user-level thread manager common synchronization primitives simple synchronization problem

Second submission(Feb 14 10:00am) Preemption Multithreaded web server to test your thread package Analyze your design and report test results

Dependency

Part 1:Thread manager

Part 3:Synchronization problem

Part 4:Preemption

Part 5:Multithreaded web server

Part 6:Report

Part 2:Synchronization primitives

You can work concurrently! pthread compatible

Simplethreads code structure

include/sthread.h

Other apps

Web 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

From Section 3 – Winter 2006

Sthread: thread interface void sthread_init(void)

Initialize thread system(or manager) sthread_t sthread_create(sthread_start_func_t

start_routine,void *arg,int joinable) Spawn a new thread The new thread runs start_routine with given arg

void sthread_exit(void *ret) Exit caller thread with ret as exit value ret is actually return value of start_routine

void sthread_yield(void) Yield execution of caller thread

void *sthread_join(sthread_t t) Join thread t which is created as joinable Return value is the return value of start_routine of t

Refer sthread.h

Synchronization Problem: Controlling access from threads

to limited resources. Solutions?

Synchronization Problem: Controlling access from threads to

limited resources. Solutions?

Disable Interrupts (hardware) Atomic Instructions (hardware) Semaphores (OS) Condition Variables (OS) Monitors (PL)

We will be implementing the OS level primitives with the support of the hardware level primitives.

Disabling/Enabling Interrupts

Prevents context-switches during execution of CS Sometimes necessary

E.g. to prevent further interrupts during interrupt handling

Problems?

Thread A:disable_interrupts()critical_section()enable_interrupts()

Thread B:disable_interrupts()critical_section()enable_interrupts()

Hardware support Atomic instructions:

Test and set Swap Compare-exchange (x86) Load-linked store conditional (MIPS, Alpha, PowerPC)

Use these to implement higher-level primitives E.g. test-and-set on x86 (given to you for part 5) is written using

compare-exchange. compare_exchange(lock_t *x,int y,int z):

if(*x == y)*x = z;return y;

else return *x;

test_and_set(lock_t *l) {

return compare_exchange(l,0,1);}

Sthread: mutex interface sthread_mutex_t sthread_mutex_init()

Create a new-unlocked mutex

void sthread_mutex_free(sthread_mutex_t lock) Release resources held by given mutex

void sthread_mutex_lock(sthread_mutex_t lock) Returned thread is guaranteed to acquire lock

void sthread_mutex_unlock(sthread_mutex_t lock) Release lock

Refer sthread.h

Sthread: condition variable sthread_cond_t sthread_cond_init()

Create a new condition variable void sthread_cond_free(sthread_cond_t cond)

Release resources held by given condition variable void sthread_cond_signal(sthread_cond_t cond)

Wake-up one waiting thread if there is any void sthread_cond_broadcast(sthread_cond_t cond)

Wake-up all waiting threads void sthread_cond_wait(sthread_cond_t cond,

sthread_mutex_t lock) Wait for given condition variable Returning thread is guaranteed to hold the lock

Refer sthread.h

Sample multithreaded programint 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)

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

How sthread_switch worksXsthread_switch:

pusha

movl %esp,(%eax)

movl %edx,%esp

popa

ret

Thread 1 TCB … SP

Thread 2 TCB … SP

ESP

CPU

Thread 1 running Thread 2 ready

Want to switch to thread 2…

Thread 2registers

Thread 1 regs

From Section 3 – Winter 2006

Push old contextXsthread_switch:

pusha

movl %esp,(%eax)

movl %edx,%esp

popa

ret

Thread 1 TCB … SP

Thread 2 TCB … SP

ESP

CPU

Thread 1 running Thread 2 ready

Thread 2registers

Thread 1registers

Thread 1 regs

From Section 3 – Winter 2006

Save old stack pointerXsthread_switch:

pusha

movl %esp,(%eax)

movl %edx,%esp

popa

ret

Thread 1 TCB … SP

Thread 2 TCB … SP

ESP

CPU

Thread 1 running Thread 2 ready

Thread 2registers

Thread 1registers

Thread 1 regs

From Section 3 – Winter 2006

Change stack pointersXsthread_switch:

pusha

movl %esp,(%eax)

movl %edx,%esp

popa

ret

Thread 1 TCB … SP

Thread 2 TCB … SP

ESP

CPU

Thread 1 ready Thread 2 running

Thread 2registers

Thread 1registers

Thread 1 regs

From Section 3 – Winter 2006

Pop off new contextXsthread_switch:

pusha

movl %esp,(%eax)

movl %edx,%esp

popa

ret

Thread 1 TCB … SP

Thread 2 TCB … SP

ESP

CPU

Thread 1 ready Thread 2 running

Thread 1registers

Thread 2 regs

From Section 3 – Winter 2006

Done; returnXsthread_switch:

pusha

movl %esp,(%eax)

movl %edx,%esp

popa

ret

Thread 1 TCB … SP

Thread 2 TCB … SP

ESP

CPU

Thread 1 ready Thread 2 running

Thread 1registers

What got switched? SP PC (how?) Other registers

Thread 2 regs

From Section 3 – Winter 2006

Adjusting the PCThread 1 TCB … SP

Thread 2 TCB … SP

ESP

CPU

Thread 2 running:switch(t2,...);

0x800: printf(“test 2”);

Thread 1registers

ret pops off the new return address!

ra=0x800

PC

Thread 1 (stopped):switch(t1,t2);

0x400: printf(“test 1”);

ra=0x400

From Section 3 – Winter 2006

Join! Exciting new construct! Wait until another thread terminates and

get its return value Convenient way to synchronize threads Assumption

A joinable thread is guaranteed to be joined by another thread (not threads!)

Join can be called only one time at any time in the life-cycle of a joinable thread

Behavior of multiple calls of join on a single joinable thread is undefined

Think about How do you start a new thread?

How do you pass an argument to the start function? How do you deal the main(or initial) thread? When & how do you reclaim resources of a

terminated thread? Where does sthread_switch() return? Who should call sthread_switch() and when? What should be in struct _sthread(TCB)? How do you identify current thread? How do you block a thread? What should be in struct _sthread_mutex? What should be in struct _sthread_cond?

Hints Read project descriptions & given codes

There are many hints already pthread manual pages may be helpful

Design first Answer questions in previous slides Write your algorithms and verify them

Program Don’t forget to comment your code!

Test, test, test, … Write your own test programs

Pick the right tree This project is not a lot of code. If you’re writing a lot of code, you’re probably

confused. Keep in mind, we might have different definitions of “a lot of code” This is an easy project to do, but a really f’n hard project to do right.

CVS Make your project concurrent just like your

programs Refer any CSE303 course material

http://www.cs.washington.edu/education/courses/cse303/05au/syllabus/11-14-files/

http://www.cs.washington.edu/education/courses/cse303/06wi/lec19.pdf

Scenario Your team has two members

dumb dumber

UNIX group of your team is cse451x dumber is going to maintain the

repository The name of repository is cse451x

Import source code Copy simplethreads-1.30.tar.gz Type following

% tar zxf simplethreads-1.30.tar.gz

% cd simplethreads-1.30

% cvs –d $USER@attu:/projects/.instr/CurrentQtr/cse451/cse451x import –m “initial skeleton code” simplethreads SIMPLETHREADS SIMPLETHREADS_1_30

% cd ..

% rm –rf simplethreads-1.30

Now you can check out using simplethreads as a module name

Set CVSROOT & CVS_RSH You want to access the repository remotely!

Bash export CVS_RSH=ssh export

CVSROOT=$USER@attu:/projects/.instr/CurrentQtr/cse451/cse451x Csh/tcsh

setenv CVS_RSH ssh setenv CVSROOT

$USER@attu:/projects/.instr/CurrentQtr/cse451/cse451x

To check your shell,% echo $SHELL