1. CS 3214 Introduction to Computer Systems
Lecture 19
Godmar Back

2. Announcements Read Chapter 13 Exercise 10 due Nov 3 (today)
(ignore stuff about file web servers for now)
Exercise 10 due Nov 3 (today)
Exercise 11 is out – very structured exercise
Due Nov 12 (but should not take this long; don’t procrastinate)
Will give feedback on whether Project 3 meets minimum requirements before weekend
Will use the tests you provided
Project 4 out by Thursday
CS 3214 Fall 2009
9/12/2018

3. Threads and Processes Part 8
Some of the following slides are taken with permission from
Complete Powerpoint Lecture Notes for Computer Systems: A Programmer's Perspective (CS:APP)
Randal E. Bryant and David R. O'Hallaron
Part 8
Threads and Processes
CS 3214 Fall 2009
9/12/2018

4. 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 Fall 2009
9/12/2018

5. 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 Fall 2009
9/12/2018

6. 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 Fall 2009
9/12/2018

7. 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 Fall 2009
9/12/2018

8. 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 Fall 2009
9/12/2018

9. 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 Fall 2009
9/12/2018

10. 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 Fall 2009
9/12/2018

11. 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 Fall 2009
9/12/2018

12. 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 Fall 2009
9/12/2018

13. 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 Fall 2009
9/12/2018

14. 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 Fall 2009
9/12/2018

15. 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 Fall 2009
9/12/2018

16. 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 Fall 2009
9/12/2018

17. 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 Fall 2009
9/12/2018

18. 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 Fall 2009
9/12/2018

19. 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 Fall 2009
9/12/2018

20. Deadlock (Definition)
A situation in which two or more threads or processes are blocked and cannot proceed
“blocked” either on a resource request that can’t be granted, or waiting for an event that won’t occur
Possible causes: resource-related or communication-related
Cannot easily back out
CS 3214 Fall 2009
9/12/2018

21. Resource Deadlock Canonical Example
pthread_mutex_t A;
pthread_mutex_t B; …
pthread_mutex_lock(&A);
pthread_mutex_lock(&B);
pthread_mutex_unlock(&B);
pthread_mutex_unlock(&A);
pthread_mutex_lock(&B);
pthread_mutex_lock(&A); …
pthread_mutex_unlock(&A);
pthread_mutex_unlock(&B); A B
Thread 1
Thread 2
CS 3214 Fall 2009
9/12/2018

22. Using Lock Ordering to Avoid Deadlock
acquire locks in same order
void transferTo(account *this, account *that, int amount) {
if (this < that) {
pthread_mutex_lock(&this->lock);
pthread_mutex_lock(&that->lock);
} else { }
/* rest of function */
Tricky in Java when synchronized is used: System.identityHashcode provides ordering of arbitrary objects
CS 3214 Fall 2009
9/12/2018

23. Deadlocks, more formally
4 necessary conditions
Exclusive Access
Hold and Wait
No Preemption
Circular Wait
Aka Coffman conditions
Note that cond 1-3 represent things that are normally desirable or required
Strategies involve
Detect & break deadlocks
Prevent
Avoid
CS 3214 Fall 2009
9/12/2018

24. Deadlocks Detection Uses Resource Allocation Graph
For single-item resources cycle in this graph is necessary & sufficient for resource deadlock to exist
Some systems have deadlock detection built-in
But usually an option that must be turned on
Resource Allocation Graph
R → P Assignment
P → R Request R1 P1 P2 P3 P4 R2 R3 R4
CS 3214 Fall 2009
9/12/2018

25. Deadlock Recovery
Can attempt to revoke resources, kill some participants, or (worst case) kill all participants to break cycle
CS 3214 Fall 2009
9/12/2018

26. Deadlock Prevention Idea: remove a necessary condition
C1: (do not require) exclusive access
C2: (don’t) hold and (then) wait
C3: (allow) preemption/revocation
C4: (don’t) create cycles: lock ordering!
Deadlock is tricky because it’s not always clear/possible how to remove or eliminate these conditions, particularly C4
Distinct from “Deadlock Avoidance”
Well studied, but rarely used technique in which necessary conditions aren’t removed, but resource allocations are managed in a way that avoids deadlock
CS 3214 Fall 2009
9/12/2018

27. Deadlock vs. Starvation vs. Livelock
No matter which policy the scheduler chooses, there is no possible way for processes to make forward progress
Starvation:
There is a possible way in which threads can make possible forward progress, but the scheduler doesn’t choose it
Example: strict priority scheduler will never scheduler lower priority threads as long as higher-priority thread is READY
Example: naïve reader/writer lock: starvation may occur by “bad luck”
Livelock:
Threads are continually engaged in tasks that prevent them from making progress on their actual goals, such as reattempting to acquire locks, or responding to each other’s signals
CS 3214 Fall 2009
9/12/2018

28. Informal uses of term `deadlock’
4 Coffman conditions apply specifically to deadlock with definable resources
Term deadlock is sometimes informally used to also describe situations in which not all 4 criteria apply
See interesting discussion in Levine 2003, Defining Deadlock
Consider: When two trains approach each other at a crossing, both shall come to a full stop and neither shall start up again until the other has gone.
Does it meet the 4 conditions?
However, even under informal/extended view, not all “lack of visible progress” situations can reasonably be called deadlocked
e.g., an idle system is not usually considered deadlocked
CS 3214 Fall 2009
9/12/2018

29. Summary Deadlock: Strategies to deal with:
4 necessary conditions: mutual exclusion, hold-and-wait, no preemption, circular wait
Strategies to deal with:
Prevention: remove one of 4 necessary conditions
Detect & recover
Avoidance: if you can’t do that, avoid deadlock by being conservative
CS 3214 Fall 2009
9/12/2018