程序代写代做代考 Java AI concurrency Book Chapter 5

Book Chapter 5

Concurrency: monitors & condition synchronization 1
©Magee/Kramer 2nd Edition

Chapter 5

Monitors &
Condition Synchronization

Concurrency: monitors & condition synchronization 2
©Magee/Kramer 2nd Edition

monitors & condition synchronization

Concepts: monitors:
encapsulated data + access procedures
mutual exclusion + condition synchronization
single access procedure active in the monitor

nested monitors

Models: guarded actions

Practice: private data and synchronized methods (exclusion).
wait(), notify() and notifyAll() for condition synch.
single thread active in the monitor at a time

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5.1 Condition synchronization

A controller is required for a carpark, which only permits
cars to enter when the carpark is not full and does not
permit cars to leave when there are no cars in the carpark.
Car arrival and departure are simulated by separate threads.

Concurrency: monitors & condition synchronization 4
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carpark model

♦ Events or actions of interest?

arrive and depart

♦ Identify processes.

arrivals, departures and carpark control

♦ Define each process and interactions (structure).

ARRIVALS CARPARK
CONTROL

DEPARTURESarrive depart

CARPARK

Concurrency: monitors & condition synchronization 5
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carpark model

CARPARKCONTROL(N=4) = SPACES[N],
SPACES[i:0..N] = (when(i>0) arrive->SPACES[i-1]

|when(iSPACES[i+1]
).

ARRIVALS = (arrive->ARRIVALS).
DEPARTURES = (depart->DEPARTURES).

||CARPARK =
(ARRIVALS||CARPARKCONTROL(4)||DEPARTURES).

Guarded actions are used to control arrive and depart.
LTS?

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carpark program

♦ Model – all entities are processes interacting by actions

♦ Program – need to identify threads and monitors

♦thread – active entity which initiates (output) actions

♦monitor – passive entity which responds to (input) actions.

For the carpark?

ARRIVALS CARPARK
CONTROL

DEPARTURESarrive depart

CARPARK

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carpark program – class diagram

Applet
Runnable

ThreadPanel

CarParkControl

Arrivals

Departures

DisplayCarParkCarParkCanvas

CarPark
arrivals,
departures

arrive()
depart()

carDisplay

carpark

disp

We have omitted
DisplayThread
and
GraphicCanvas
threads managed by
ThreadPanel.

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carpark program

Arrivals and Departures implement Runnable,
CarParkControl provides the control (condition synchronization).

Instances of these are created by the start() method of the
CarPark applet :

public void start() {
CarParkControl c =

new DisplayCarPark(carDisplay,Places);
arrivals.start(new Arrivals(c));
departures.start(new Departures(c));

}

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carpark program – Arrivals and Departures threads

class Arrivals implements Runnable {
CarParkControl carpark;

Arrivals(CarParkControl c) {carpark = c;}

public void run() {
try {

while(true) {
ThreadPanel.rotate(330);
carpark.arrive();
ThreadPanel.rotate(30);

}
} catch (InterruptedException e){}

}
}

Similarly Departures
which calls
carpark.depart().

How do we implement the control of CarParkControl?

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Carpark program – CarParkControl monitor

class CarParkControl {
protected int spaces;
protected int capacity;

CarParkControl(int n)
{capacity = spaces = n;}

synchronized void arrive() {
… –spaces; …
}

synchronized void depart() {
… ++spaces; …
}

}

condition
synchronization?

block if full?
(spaces==0)

block if empty?
(spaces==N)

mutual exclusion
by synch methods

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condition synchronization in Java

Java provides a thread wait set per monitor (actually per object)
with the following methods:
public final void notify()

Wakes up a single thread that is waiting on this object’s wait set.

public final void notifyAll()
Wakes up all threads that are waiting on this object’s wait set.

public final void wait()
throws InterruptedException

Waits to be notified by another thread. The waiting thread
releases the synchronization lock associated with the monitor.
When notified, the thread must wait to reacquire the monitor
before resuming execution.

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condition synchronization in Java

We refer to a thread entering a monitor when it acquires the mutual
exclusion lock associated with the monitor and exiting the monitor
when it releases the lock.
Wait() – causes the thread to exit the monitor,

permitting other threads to enter the monitor.

Thread A Thread B

wait()
notify()

Monitor

data

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monitor lock

wait()

Monitor

data

wait

Thread C

Thread E
Thread B

Thread F

Thread A

notify()

Thread B

Thread F
Thread E

Thread A

Thread C

Thread A

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condition synchronization in Java

FSP: when cond act -> NEWSTAT

Java: public synchronized void act()
throws InterruptedException

{
while (!cond) wait();
// modify monitor data

notifyAll()
}

The while loop is necessary to retest the condition cond to ensure that
cond is indeed satisfied when it re-enters the monitor.

notifyall() is necessary to awaken other thread(s) that may be
waiting to enter the monitor now that the monitor data has been changed.

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CarParkControl – condition synchronization
class CarParkControl {

protected int spaces;
protected int capacity;

CarParkControl(int n)
{capacity = spaces = n;}

synchronized void arrive() throws InterruptedException {
while (spaces==0) wait();
–spaces;
notifyAll();

}

synchronized void depart() throws InterruptedException {
while (spaces==capacity) wait();
++spaces;
notifyAll();

}
}

Is it safe to use notify()
here rather than notifyAll()?

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models to monitors – summary

Active entities (that initiate actions) are implemented as threads.
Passive entities (that respond to actions) are implemented as monitors.

Each guarded action in the model of a monitor is
implemented as a synchronized method
which uses a while loop and wait() to
implement the guard. The while loop condition is
the negation of the model guard condition.

Changes in the state of the monitor are signaled to
waiting threads using notify() or notifyAll().

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5.2 Semaphores

Semaphores are widely used for dealing with inter-process
synchronization in operating systems. Semaphore s is an
integer variable that can take only non-negative values.

down(s): if s >0 then
decrement s

else
block execution of the calling process

up(s): if processes blocked on s then
awaken one of them

else
increment s

The only
operations
permitted on
s are up(s)
and down(s).
Blocked
processes are
held in a
FIFO queue.

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modeling semaphores

To ensure analyzability, we only model semaphores that
take a finite range of values. If this range is exceeded
then we regard this as an ERROR. N is the initial value.

const Max = 3
range Int = 0..Max

SEMAPHORE(N=0) = SEMA[N],
SEMA[v:Int] = (up->SEMA[v+1]

|when(v>0) down->SEMA[v-1]
),

SEMA[Max+1] = ERROR.

LTS?

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modeling semaphores

Action down is only accepted when value v of the
semaphore is greater than 0.

Action up is not guarded.

Trace to a violation:
up up up up

up up

down

-1 0 1 2 3

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semaphore demo – model

Three processes p[1..3] use a shared semaphore mutex
to ensure mutually exclusive access (action critical) to
some resource.

LOOP = (mutex.down->critical->mutex.up->LOOP).

||SEMADEMO = (p[1..3]:LOOP
||{p[1..3]}::mutex:SEMAPHORE(1)).

For mutual exclusion, the semaphore initial value is 1. Why?
Is the ERROR state reachable for SEMADEMO?
Is a binary semaphore sufficient (i.e. Max=1) ?
LTS?

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semaphore demo – model
p.1.mutex.down

p.2.mutex.down

p.3.mutex.down p.3.critical

p.3.mutex.up

p.2.critical

p.2.mutex.up

p.1.critical

p.1.mutex.up

0 1 2 3 4 5 6

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semaphores in Java

public class Semaphore {
private int value;

public Semaphore (int initial)
{value = initial;}

synchronized public void up() {
++value;
notifyAll();

}

synchronized public void down()
throws InterruptedException {

while (value== 0) wait();
–value;

}
} Is it safe to use notify()

here rather than notifyAll()?

Semaphores are
passive objects,
therefore
implemented as
monitors.
(In practice,
semaphores are a
low-level mechanism
often used in
implementing the
higher-level monitor
construct.)

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SEMADEMO display

current
semaphore
value

thread 1 is
executing
critical
actions.

thread 2 is
blocked
waiting.

thread 3 is
executing
non-critical
actions.

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SEMADEMO

What if we adjust the time that each thread spends in its
critical section ?

♦large resource requirement – more conflict?

(eg. more than 67% of a rotation)?

♦ small resource requirement – no conflict?

(eg. less than 33% of a rotation)?

Hence the time a thread spends in its critical
section should be kept as short as possible.

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SEMADEMO program – revised ThreadPanel class

public class ThreadPanel extends Panel {
// construct display with title and rotating arc color c
public ThreadPanel(String title, Color c) {…}
// hasSlider == true creates panel with slider
public ThreadPanel
(String title, Color c, boolean hasSlider) {…}
// rotate display of currently running thread 6 degrees
// return false when in initial color, return true when in second color
public static boolean rotate()

throws InterruptedException {…}
// rotate display of currently running thread by degrees
public static void rotate(int degrees)

throws InterruptedException {…}
// create a new thread with target r and start it running
public void start(Runnable r) {…}
// stop the thread using Thread.interrupt()
public void stop() {…}

}

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SEMADEMO program – MutexLoop

class MutexLoop implements Runnable {
Semaphore mutex;

MutexLoop (Semaphore sema) {mutex=sema;}

public void run() {
try {

while(true) {
while(!ThreadPanel.rotate());
mutex.down(); // get mutual exclusion
while(ThreadPanel.rotate()); //critical actions
mutex.up(); //release mutual exclusion

}
} catch(InterruptedException e){}

}
} ThreadPanel.rotate() returns

false while executing non-critical
actions (dark color) and true otherwise.

Threads and
semaphore are
created by the
applet
start()
method.

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5.3 Bounded Buffer

A bounded buffer consists of a fixed number of slots.
Items are put into the buffer by a producer process and
removed by a consumer process. It can be used to smooth
out transfer rates between the producer and consumer.
(see car park example)

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bounded buffer – a data-independent model

PRODUCER BUFFER CONSUMERput get

BOUNDEDBUFFER

LTS:

The behaviour of BOUNDEDBUFFER is independent of
the actual data values, and so can be modelled in a data-
independent manner.

put put

get

put

get

put

get

put

get get

0 1 2 3 4 5

bounded buffer – a data-independent model

BUFFER(N=5) = COUNT[0],
COUNT[i:0..N]

= (when (iCOUNT[i+1]
|when (i>0) get->COUNT[i-1]
).

PRODUCER = (put->PRODUCER).
CONSUMER = (get->CONSUMER).

||BOUNDEDBUFFER =
(PRODUCER||BUFFER(5)||CONSUMER).

bounded buffer program – buffer monitor
public interface Buffer {…}

class BufferImpl implements Buffer {
…

public synchronized void put(E o)
throws InterruptedException {

while (count==size) wait();
buf[in] = o; ++count; in=(in+1)%size;
notifyAll();

}
public synchronized E get()

throws InterruptedException {
while (count==0) wait();
E o =buf[out];
buf[out]=null; –count; out=(out+1)%size;
notifyAll();
return (o);
}

}

We separate the
interface to
permit an
alternative
implementation
later.

Is it safe to use notify()
here rather than notifyAll()?

bounded buffer program – producer process

class Producer implements Runnable {
Buffer buf;
String alphabet= “abcdefghijklmnopqrstuvwxyz”;

Producer(Buffer b) {buf = b;}

public void run() {
try {

int ai = 0;
while(true) {

ThreadPanel.rotate(12);
buf.put(alphabet.charAt(ai));
ai=(ai+1) % alphabet.length();
ThreadPanel.rotate(348);

}
} catch (InterruptedException e){}

}
}

Similarly Consumer
which calls buf.get().

5.4 Nested Monitors

Suppose that, in place of using the count variable and condition
synchronization directly, we instead use two semaphores full and
empty to reflect the state of the buffer.

class SemaBuffer implements Buffer {
…

Semaphore full; //counts number of items
Semaphore empty; //counts number of spaces

SemaBuffer(int size) {
this.size = size; buf =(E[])new Object[size];
full = new Semaphore(0);
empty= new Semaphore(size);

}
…
}

nested monitors – bounded buffer program

synchronized public void put(E o)
throws InterruptedException {

empty.down();
buf[in] = o;
++count; in=(in+1)%size;
full.up();

}
synchronized public E get()

throws InterruptedException{
full.down();
E o =buf[out]; buf[out]=null;
–count; out=(out+1)%size;
empty.up();
return (o);

}

Does this behave
as desired?

empty is decremented during a put operation, which is blocked
if empty is zero; full is decremented by a get operation, which
is blocked if full is zero.

nested monitors – bounded buffer model

const Max = 5
range Int = 0..Max

SEMAPHORE …as before…

BUFFER = (put -> empty.down ->full.up ->BUFFER
|get -> full.down ->empty.up ->BUFFER
).

PRODUCER = (put -> PRODUCER).
CONSUMER = (get -> CONSUMER).

||BOUNDEDBUFFER = (PRODUCER|| BUFFER || CONSUMER
||empty:SEMAPHORE(5)
||full:SEMAPHORE(0)

)@{put,get}. Does this behave
as desired?

nested monitors – bounded buffer model

LTSA analysis predicts a possible DEADLOCK:
Composing
potential DEADLOCK

States Composed: 28 Transitions: 32 in 60ms
Trace to DEADLOCK:

get

The Consumer tries to get a character, but the buffer is
empty. It blocks and releases the lock on the semaphore
full. The Producer tries to put a character into the
buffer, but also blocks. Why?

This situation is known as the nested monitor problem.

nested monitors – bounded buffer model

synchronized public Object get()
throws InterruptedException{

full.down(); // if no items, block!
…

}

full

empty

buffer

get down

waitbuffer

fullfull

put

nested monitors – revised bounded buffer program

The only way to avoid it in Java is by careful design. In this
example, the deadlock can be removed by ensuring that the monitor
lock for the buffer is not acquired until after semaphores are
decremented.

public void put(E o)
throws InterruptedException {

empty.down();
synchronized(this){
buf[in] = o; ++count; in=(in+1)%size;

}
full.up();

}

nested monitors – revised bounded buffer model

BUFFER = (put -> BUFFER
|get -> BUFFER
).

PRODUCER =(empty.down->put->full.up->PRODUCER).
CONSUMER =(full.down->get->empty.up->CONSUMER).

The semaphore actions have been moved to the producer
and consumer. This is exactly as in the implementation
where the semaphore actions are outside the monitor .
Does this behave as desired?
Minimized LTS?

5.5 Monitor invariants
An invariant for a monitor is an assertion concerning the variables
it encapsulates. This assertion must hold whenever there is no thread
executing inside the monitor i.e. on thread entry to and exit from a
monitor .

CarParkControl Invariant: 0 ≤ spaces ≤ N

Semaphore Invariant: 0 ≤ value

Buffer Invariant: 0 ≤ count ≤ size
and 0 ≤ in < size and 0 ≤ out< size and in = (out + count) modulo size Invariants can be helpful in reasoning about correctness of monitors using a logical proof-based approach. Generally we prefer to use a model-based approach amenable to mechanical checking . Concurrency: monitors & condition synchronization 40 ©Magee/Kramer 2nd Edition Summary Concepts monitors: encapsulated data + access procedures mutual exclusion + condition synchronization nested monitors Model guarded actions Practice private data and synchronized methods in Java wait(), notify() and notifyAll() for condition synchronization single thread active in the monitor at a time

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