1. CS 3214 Computer Systems
Lecture 22
Godmar Back

2. Announcements Exercise 10 due Apr 19 Project 5 out later today
CS 3214 Spring 2010
7/22/2018

3. MULTI-THREADING
CS 3214 Spring 2010
7/22/2018

4. Rules for Easy Locking
Every shared variable must be protected by a lock
Establish this relationship with code comments
/* protected by … <lock>*/
Acquire lock before touching (reading or writing) variable
Release when done, on all paths
One lock may protect more than one variable, but not too many
If in doubt, use fewer locks (may lead to worse efficiency, but less likely to lead to race conditions or deadlock)
If manipulating multiple variables, acquire locks assigned to protecting each
Acquire locks always in same order (doesn’t matter which order, but must be same)
Release in opposite order
Don’t release any locks before all have been acquired (two-phase locking)
CS 3214 Spring 2010
7/22/2018

5. Mapping Locks To Variables
Choosing which lock should protect which shared variable(s) is not easy – must weigh:
Whether all variables are always accessed together (use one lock if so)
Whether there is an atomicity requirement if multiple variables are accessed in related sequence (must hold single lock if so)
Cost of multiple calls to lock/unlock (advantage of increased parallelism may be offset by those costs)
Whether code inside critical section may block (if not, no throughput gain from fine-grained locking on uniprocessor)
CS 3214 Spring 2010
7/22/2018

6. Race Detection Tools
A number of tools help to detect race conditions (Helgrind, Intel Thread Checker)
Dynamic analysis tools
Typically based one or both of these approaches:
Locksets: detect if no lock is consistently held when a given variable x is accessed
“Happens-before” relationship: (intuition) if we can’t prove that access A must happen before B due to the synchronization the programmer used, then it’s a race; example:
All accesses before a thread is started “happen before” all accesses by a thread; and these accesses “happen before” all accesses done after the thread is joined
Typically do not detect atomicity violations
CS 3214 Spring 2010
7/22/2018

7. Coordinating Multiple Threads
Aside from coordinating access to shared items, thread may need to communicate about events
“has event A already happened in another thread?”
aka “precedence constraint”, or “scheduling constraint”
Do B after A
Must do so
Correctly (never miss that event A has occurred when in fact it has)
Efficiently
Don’t waste resources in the process
Don’t unnecessarily delay notification of event A
CS 3214 Spring 2010
7/22/2018

8. int coin_flip;
static void *
thread1(void *_) {
coin_flip = rand() % 2;
printf("Thread 1: flipped coin %d\n",
coin_flip);
return NULL; }
thread2(void *_)
printf("Thread 2: flipped coin %d\n",
int
main() {
int i, N = 2;
pthread_t t[N];
srand(getpid());
pthread_create(&t[1], NULL, thread2, NULL);
pthread_create(&t[0], NULL, thread1, NULL);
for (i = 0; i < N; i++)
pthread_join(t[i], NULL);
return 0; }
Q.: How can thread2 make sure that ‘coin_flip’ has occurred before printing its outcome?
CS 3214 Spring 2010
7/22/2018

9. Thread 2 could “busy-wait” – spin until thread 1 completes the coin flip. Exceptions not withstanding, this is practically never an acceptable solution.
int coin_flip;
volatile bool coin_flip_done;
static void * thread1(void *_) {
coin_flip = rand() % 2;
coin_flip_done = true;
printf("Thread 1: flipped coin %d\n", coin_flip);
return NULL; }
The somewhat less wasteful variant of busy-waiting:
while (!coin_flip_done) sched_yield();
is not acceptable, either.
wastes CPU cycles
is fragile (volatile needed when using –O)
does not document semantics
static void * thread2(void *_) {
/* Thread 2 spins, "busy-waits" until the coin flip is done.
* This is an unacceptable solution. Bad for the planet, too. */
while (!coin_flip_done)
continue;
printf("Thread 2: flipped coin %d\n", coin_flip);
return NULL; }
CS 3214 Spring 2010
7/22/2018

10. Semaphores Invented by Edsger Dijkstra in 1960s
Source: inter.scoutnet.org
Invented by Edsger Dijkstra in 1960s
Counter S, initialized to some value, with two operations:
P(S) or “down” or “wait” – if counter greater than zero, decrement. Else wait until greater than zero, then decrement
V(S) or “up” or “signal” or “post” – increment counter, wake up any threads stuck in P.
Semaphores don’t go negative:
#V + InitialValue - #P >= 0
Note: direct access to counter value after initialization is not allowed
Counting Semaphores vs Binary Semaphores
Binary: counter can only be 0 or 1
Simple to implement, yet powerful
Can be used for many synchronization problems
CS 3214 Spring 2010
7/22/2018

11. POSIX Semaphores int coin_flip;
sem_t coin_flip_done; // semaphore for thread 1 to signal coin flip
static void * thread1(void *_) {
coin_flip = rand() % 2;
sem_post(&coin_flip_done); // raise semaphore, increment, 'up'
printf("Thread 1: flipped coin %d\n", coin_flip); }
POSIX Semaphores
Notice the 3rd argument of sem_init() – it gives the initial value of the semaphore: ‘0’ means the semaphore is used to express scheduling constraint
static void *
thread2(void *_) {
// wait until semaphore is raised,
// then decrement, 'down'
sem_wait(&coin_flip_done);
printf("Thread 2: flipped coin %d\n", coin_flip); }
int main() { …
sem_init(&coin_flip_done, 0, 0);
pthread_create(&t[1], NULL, thread2, NULL);
pthread_create(&t[0], NULL, thread1, NULL); }
CS 3214 Spring 2010
7/22/2018

12. Implementing Mutual Exclusion with Semaphores
Semaphores can be used to build locks
Must initialize semaphore with 1 to allow one thread to enter critical section
This is not a recommended style, despite of what Bryant & O’Hallaron suggest – you should use a mutex instead [Cantrill & Bonwick 2008]
sem_t S;
sem_init(&S, 0, 1);
lock_acquire()
{ // try to decrement, wait if 0
sem_wait (S); }
lock_release()
{ // increment (wake up waiters if any)
sem_post(S);
Easily generalized to allow at most N simultaneous threads: multiplex pattern (i.e., a resource can be accessed by at most N threads)
CS 3214 Spring 2010
7/22/2018

13. Condition Variables - Intro
Besides (and perhaps more so) than semaphores, condition variables are another widely used form to implement ‘signaling’ kinds of coordination/synchronization
In POSIX Threads, Java, C#
Based on the concept of a Monitor
ADT that combines protected access to state and signaling
Confusing terminology alert:
Word ‘signal’ is overloaded 3 times
Semaphore signal (V(), “up”, “post”)
Monitor/Condition variable signal (“signal”, “notify”)
Unix signals
Word ‘wait’ is overloaded
Semaphore wait (P(), “down”)
Monitor/Condition variable wait
Unix wait() for child process
CS 3214 Spring 2010
7/22/2018

14. Monitors
A monitor combines a set of shared variables & operations to access them
Think of a Java class with no public fields & all public methods carrying the attribute ‘synchronized’
A monitor provides implicit synchronization (only one thread can access private variables simultaneously)
Single lock is used to ensure all code associated with monitor is within critical section
A monitor provides a general signaling facility
Wait/Signal pattern (similar to, but different from semaphores)
May declare & maintain multiple signaling queues
CS 3214 Spring 2010
7/22/2018

15. Monitors (cont’d)
Classic monitors are embedded in programming languages
Invented by Hoare & Brinch-Hansen 1972/73
First used in Mesa/Cedar Xerox PARC 1978
Adapted version available in Java/C#
(Classic) Monitors are safer than semaphores
can’t forget to lock data – compiler checks this
In contemporary C, monitors are a synchronization pattern that is achieved using locks & condition variables
Helps to understand monitor abstraction to use it correctly
CS 3214 Spring 2010
7/22/2018

16. Infinite Buffer w/ Monitor
monitor buffer {
/* implied: struct lock mlock;*/
private:
char buffer[];
int head, tail;
public:
produce(item);
item consume(); }
buffer::produce(item i)
{ /* try { lock_acquire(&mlock); */
buffer[head++] = i;
/* } finally {lock_release(&mlock);} */ }
buffer::consume()
return buffer[tail++];
Monitors provide implicit protection for their internal variables
Still need to add the signaling part
CS 3214 Spring 2010
7/22/2018

17. Condition Variables Used by a monitor for signaling a condition
a general (programmer-defined) condition, not just integer increment as with semaphores
Somewhat weird: the condition is actually not stored in the variable – it’s typically some boolean predicate of monitor variables, e.g. “buffer.size > 0”
the condition variable itself is better thought of as a signaling queue
Monitor can have more than one condition variable
Three operations:
Wait(): leave monitor, wait for condition to be signaled, reenter monitor
Signal(): signal one thread waiting on condition
Broadcast(): signal all threads waiting on condition
CS 3214 Spring 2010
7/22/2018

18. Condition Variables as Queues
Region of mutual exclusion
Enter
Exit
Wait
Signal
A condition variable’s state is just a queue of waiters:
Wait(): adds current thread to (end of queue) & block
Signal(): pick one thread from queue & unblock it
Broadcast(): unblock all threads
Note on style: best practice is to leave monitor only once, and near the procedure’s entry.
CS 3214 Spring 2010
7/22/2018

19. Bounded Buffer w/ Monitor
monitor buffer {
condition items_avail;
condition slots_avail;
private:
char buffer[];
int head, tail;
public:
produce(item);
item consume(); }
buffer::produce(item i) {
while ((tail+1–head)%CAPACITY==0)
slots_avail.wait();
buffer[head++] = i;
items_avail.signal(); }
buffer::consume()
while (head == tail)
items_avail.wait();
item i = buffer[tail++];
slots_avail.signal();
return i;
CS 3214 Spring 2010
7/22/2018

20. Bounded Buffer w/ Monitor
monitor buffer {
condition items_avail;
condition slots_avail;
private:
char buffer[];
int head, tail;
public:
produce(item);
item consume(); }
buffer::produce(item i) {
while ((tail+1–head)%CAPACITY==0)
slots_avail.wait();
buffer[head++] = i;
items_avail.signal(); }
buffer::consume()
while (head == tail)
items_avail.wait();
item i = buffer[tail++];
slots_avail.signal();
return i;
Q1.: How is lost update problem avoided?
lock_release(&mlock);
block_on(items_avail);
lock_acquire(&mlock);
Q2.: Why while() and not if()?
CS 3214 Spring 2010
7/22/2018

21. cond_signal semantics
cond_signal keeps lock, so it leaves signaling thread in monitor
waiter is made READY, but can’t enter until signaler gives up lock
There is no guarantee whether signaled thread will enter monitor next or some other thread will (who may be already waiting!)
so must always use “while()” when checking condition – cannot assume that condition set by signaling thread will still hold when monitor is reentered by signaled thread
This semantics is also referred to as “Mesa-Style” after the system in which it was first used
POSIX Threads, Java, and C# use this semantics
CS 3214 Spring 2010
7/22/2018

22. Condition Variables vs. Semaphores
Signals are lost if nobody’s on the queue (e.g., nothing happens)
Wait() always blocks
Semaphores
Signals (calls to V() or sem_post()) are remembered even if nobody’s current waiting
Wait (e.g., P() or sem_wait()) may or may not block
CS 3214 Spring 2010
7/22/2018

23. Monitors in C POSIX Threads as well as many custom environments
No compiler support, must do it manually
must declare locks & condition vars
must call pthread_mutex_lock/unlock when entering & leaving the monitor
must use pthread_cond_wait/pthread_cond_signal to wait for/signal condition
Note: pthread_cond_wait(&c, &m) takes monitor lock as parameter
necessary so monitor can be left & reentered without losing signals
pthread_cond_signal() does not
leaving room for programmer error!
CS 3214 Spring 2010
7/22/2018

24. Monitors in Java synchronized block means
class buffer {
private char buffer[];
private int head, tail;
public synchronized produce(item i) {
while (buffer_full())
this.wait();
buffer[head++] = i;
this.notifyAll(); }
public synchronized item consume() {
while (buffer_empty())
i = buffer[tail++];
return ;
synchronized block means
enter monitor
execute block
leave monitor
wait()/notify() use condition variable associated with receiver
Every object in Java can function as a condition variable (just like it can function as a lock)
More restrictive than Pthreads/C which allow multiple condition variables (signaling conditions) to be used in connection with a lock protecting state
CS 3214 Spring 2010
7/22/2018

25. import java.util.concurrent.locks.*;
class buffer {
private ReentrantLock monitorlock
= new ReentrantLock();
private Condition items_available
= monitorlock.newCondition();
private Condition slots_available
public /* NO SYNCHRONIZED here */ void produce(item i) {
monitorlock.lock();
try {
while (buffer_full())
slots_available.await();
buffer[head++] = i;
items_available.signal();
} finally {
monitorlock.unlock(); }
} /* consume analogous */
Monitors in Java, Take 2
Previous slide (bounded buffer) is actually an example of where Java’s built-in monitors suck
Needed “notifyAll()” to make sure one at least one of the right kind of threads was woken up
Unacceptably inefficient
Use java.util.concurrent.- locks.Condition instead in case where multiple condition queues are needed
CS 3214 Spring 2010
7/22/2018

26. A ReadWrite Lock Implementation
struct lock mlock; // protects rdrs & wrtrs
int readers = 0, writers = 0;
struct condvar canread, canwrite;
void read_lock_acquire() {
lock_acquire(&mlock);
while (writers > 0)
cond_wait(&canread, &mlock);
readers++;
lock_release(&mlock); }
void read_lock_release() {
if (--readers == 0)
cond_signal(&canwrite);
void write_lock_acquire() {
lock_acquire(&mlock);
while (readers > 0 || writers > 0)
cond_wait(&canwrite, &mlock);
writers++;
lock_release(&mlock); }
void write_lock_release() {
writers--;
ASSERT(writers == 0);
cond_broadcast(&canread);
cond_signal(&canwrite);
Note: this is a naïve implementation that may lead to livelock – no guarantees a reader or writer ever enters the locked section even if every threads eventually leaves it
CS 3214 Spring 2010
7/22/2018

27. Summary Semaphores & Condition Variables provide signaling facilities
Condition variables are loosely based on “monitor” concept
Java/C# provide syntactic sugar
Semaphores have “memory”
But require that # of signals matches # of waits
Good for rendez-vous, precedence constraints – if problem lends itself to semaphore, use one
Always use idiomatic “while (!cond) *_wait()” pattern when using condition variables (in C) or Object.wait() (in Java)
CS 3214 Spring 2010
7/22/2018