Emery Berger

1
Operating SystemsCMPSCI 377Lecture 9
Synchronization III

• Emery Berger
• University of Massachusetts, Amherst

2
Last Time Locks Semaphores

• More on hardware support
• Implementing locks
• Test Set
• Busy waiting
• Semaphores
• Generalization of locks

3
This Time More Synch Primitives

• Reader-Writer Locks
• Monitors
• Condition Variables

4
Reader/Writers Problem

• Suppose one object shared among many threads
• Each thread is either a reader or a writer
• Readers only read data, never modify
• Writers read modify data
• How should we control access to this object?
• Which synchronization primitive?

R
W
R
W
R
5
Single Lock
thread A Lock.acquire() Read data Lock.release()
thread B Lock.acquire() Modify
data Lock.release()
thread C Lock.acquire() Read data Lock.release()
thread D Lock.acquire() Read data Lock.release()
thread E Lock.acquire() Read data Lock.release()
thread F Lock.acquire() Modify
data Lock.release()

• Drawbacks of this solution?

6
Readers/Writers Optimization

• Single lock safe, but limits concurrency
• Only one thread at a time, but
• Safe to have simultaneous readers!
• But only one writer at a time
• Must guarantee mutual exclusion for writers

7
Readers/Writers Example
thread A Lock.acquire() Read data Lock.release()
thread B Lock.acquire() Modify
data Lock.release()
thread C Lock.acquire() Read data Lock.release()
thread D Lock.acquire() Read data Lock.release()
thread E Lock.acquire() Read data Lock.release()
thread F Lock.acquire() Modify
data Lock.release()

• Maximizes concurrency
• Great! But how do we implement this?

8
Reader-Writer Locks

• New synchronization operatorreader-writer lock
• Multiple readers, just one writer
• Can be built with standard synch primitives
• E.g., semaphores

9
Readers/Writers Algorithm

• As long as there are no writers
• Let readers in
• If no readers or writers
• Let one writer in

10
Readers/Writers Algorithm

• As long as there are no writers
• Let readers in
• If no readers or writers
• Let one writer in

reader
11
Readers/Writers Algorithm

• As long as there are no writers
• Let readers in
• If no readers or writers
• Let one writer in

reader
12
Readers/Writers Algorithm

• As long as there are no writers
• Let readers in
• If no readers or writers
• Let one writer in

reader
reader
13
Readers/Writers Algorithm

• As long as there are no writers
• Let readers in
• If no readers or writers
• Let one writer in

reader
reader
14
Readers/Writers Algorithm

• As long as there are no writers
• Let readers in
• If no readers or writers
• Let one writer in

reader
reader
reader
15
Readers/Writers Algorithm

• As long as there are no writers
• Let readers in
• If no readers or writers
• Let one writer in

reader
reader
reader
16
Readers/Writers Algorithm

• As long as there are no writers
• Let readers in
• If no readers or writers
• Let one writer in

reader
reader
reader
writer
17
Readers/Writers Algorithm

• As long as there are no writers
• Let readers in
• If no readers or writers
• Let one writer in

reader
reader
writer
18
Readers/Writers Algorithm

• As long as there are no writers
• Let readers in
• If no readers or writers
• Let one writer in

reader
reader
writer
writer
19
Readers/Writers Algorithm

• As long as there are no writers
• Let readers in
• If no readers or writers
• Let one writer in

reader
writer
writer
20
Readers/Writers Algorithm

• As long as there are no writers
• Let readers in
• If no readers or writers
• Let one writer in

writer
writer
21
Readers/Writers Algorithm

• As long as there are no writers
• Let readers in
• If no readers or writers
• Let one writer in

writer
writer
22
Readers/Writers Algorithm

• As long as there are no writers
• Let readers in
• If no readers or writers
• Let one writer in

writer
writer
reader
23
Readers/Writers Algorithm

• As long as there are no writers
• Let readers in
• If no readers or writers
• Let one writer in

writer
writer
reader

• What happens next?

reader
24
Starvation Problem

• Two possible policies
• No reader waits unless writer is already in
• Waiting writer always gets served first
• Both variants may lead to starvation
• First writers may starve
• Second readers may starve

25
Readers/Writers Algorithm

• As long as there are no writers
• Let readers in
• If no readers or writers
• Let one writer in

writer
variant 2
variant 1
writer
reader

• How to avoid starvation?

reader
26
Implementing R/W Locks

• Can implement with two semaphores
• Mutex protect number of readers
• Queue control scheduling of writers
• Control access to a database
• int getValue()
• void setValue (int n)

27
Implementing R/W Locks
class RWDatabase private Database db
private int readers private Semaphore mutex
private Semaphore writersSemaphore RWInt()
readers 0 mutex new Semaphore (1)
queue new Semaphore (1) int read()
void write (int n)
28
Implementing R/W Locks
void RWDatabasewrite (int v)
writersSemaphore.wait() db.setValue(v) //
Write the value writersSemaphore.signal()
Write value
29
Implementing R/W Locks
int RWDatabaseread() int v
mutex.wait() readers if (readers 1)
// Im first reader writersSemaphore.wait()
mutex.signal() v db.getValue()
mutex.wait() readers-- if (readers 0)
// Im last reader writersSemaphore.signal()
mutex.signal() return v
Add a reader
Read, remove reader

• Who can starve?

30
Problems withSemaphores Locks

• Much better than load/store
• Still many drawbacks
• Serve two purposes
• Mutual exclusion
• Scheduling constraints
• Effectively shared global variables
• Access to semaphores may be anywhere
• Not structured
• Not tied to data they control access to

31
Monitors

• Invented by C.A.R. Tony Hoare for Mesa
• Also invented quicksort, Hoare triples
• a gt 16 a sqrt(a) a gt 4
• monitor Java class with
• All data private
• All methods synchronized

32
Implementing Monitors in Java
class QueueMonitor private queue q public
void synchronized add (Object item) q.add
(item) public Object synchronized remove()
if (q.notEmpty()) Object o
q.remove() return o else //
what should we do here?
33
Remove Options

• Options for remove when queue empty
• Return special error value (e.g., NULL)
• Throw an exception
• Wait for something to appear in the queue
• Wait sleep()
• But sleep inside synchronized
• Holds lock
• Goes to sleep
• Never wakes up!

34
Condition Variables

• Queue of threads waiting in critical section
• Thread must hold lock when performing
  operations
• wait(Lock l)
• Atomically releases lock, goes to sleep
• Reacquires lock when awakened
• notify()
• Wakes up one waiting thread, if any
• notifyAll()
• Wakes up all waiting threads

35
Implementing Monitors in Java
class QueueMonitor private queue q public
void synchronized add (Object item) q.add
(item) notify() public Object
synchronized remove() while (q.empty())
wait() return q.remove()
36
R/W Locks in Java
class RWDatabase private Database db
private int readers private int writers
RWInt() readers 0 public
synchronized int read() public
synchronized void write (int n)
37
R/W Locks in Java
public int RWDatabaseread() preRead() int
v db.getValue() postRead() return
v private void synchronized preRead()
while (writers gt 0) wait()
readers private void synchronized
postRead() readers-- if (readers 0)
notify()
38
R/W Locks in Java
public synchronized void RWDatabasewrite (int
v) preWrite() db.setValue(v)
postWrite() private void preWrite()
writers while (readers gt 0) wait()
private void postWrite() writers--
notify()
39
Summary

• Reader-Writer Locks
• Permit concurrent reads
• Implementable with semaphores
• Monitors
• Classes tie data, methods with synchronization
• Condition Variables
• Release lock temporarily
• Waiting inside critical sections

40
Next Time

• Deadlock