1 last time: locks & semaphores implementing locks test & set busy waiting block waiting...

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Last Time: Locks & Semaphores

Implementing locks Test & Set Busy waiting Block waiting

Semaphores Generalization of locks

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This Time: More Synch Primitives

Reader-Writer Locks Monitors Condition Variables

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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?

RW R W R

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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?

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Readers/Writers Optimization

Single lock: safe, but limits concurrency Only one thread at a time, but…

Safe to have simultaneous readers! Allow only one writer at a time Must guarantee mutual exclusion for

writers How to implement this?

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Reader-Writer Locks

New synchronization operator:reader-writer lock Multiple processes acquire reader-writer

lock in reader mode One process acquires reader-writer lock in

writer mode

Can be built with standard synch primitives E.g., semaphores

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Readers/Writers Algorithm

As long as there are no writers Let readers in

If no readers or writers Let one writer in

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Readers/Writers Algorithm

As long as there are no writers Let readers in

If no readers or writers Let one writer in

reader

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Readers/Writers Algorithm

As long as there are no writers Let readers in

If no readers or writers Let one writer in

reader

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Readers/Writers Algorithm

As long as there are no writers Let readers in

If no readers or writers Let one writer in

reader

reader

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Readers/Writers Algorithm

As long as there are no writers Let readers in

If no readers or writers Let one writer in

reader

reader

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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

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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

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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

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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

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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

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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

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Readers/Writers Algorithm

As long as there are no writers Let readers in

If no readers or writers Let one writer in

writer

writer

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Readers/Writers Algorithm

As long as there are no writers Let readers in

If no readers or writers Let one writer in

writer

writer

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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

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Readers/Writers Algorithm

As long as there are no writers in critical section Let readers in

If no readers or writers in critical section Let one writer in

writer

writer readerreader

What happens next?

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Starvation Problem Two variants:

1. Give reader priority: No reader waits unless writer is already in

2. Give writer priority: Waiting writer always gets served first

Both variants may lead to starvation First: writers may starve Second: readers may starve

Concurrent control with “readers” and “writers”, by P. J. Courtois, F. Heymans, D. L. Parnas

Concurrent reading and writing, by Leslie Lamport, w/o mutual exclusion

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Implementing Give-reader-priority

Can implement with two semaphores “readerSemaphore”: protect number of

readers “writerSemaphore”: control scheduling

of writers

Control access to a “database” int getValue() void setValue (int n)

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Implementing Give-reader-priority

class RWDatabase { private Database db; private int readers; private Semaphore readerSemaphore; private Semaphore writerSemaphore; RWDatabase() { readers = 0; readerSemaphore = new Semaphore (1); writerSemaphore = new Semaphore (1); } int read() { … } void write (int n) { … }};

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Write value

Implementing Give-reader-priorityvoid RWDatabase::write (int v) { writerSemaphore.wait(); db.setValue(v); // Write the value writerSemaphore.signal();};

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Read, remove reader

Add a reader

Implementing Give-reader-priorityint RWDatabase::read() { int v; readerSemaphore.wait(); readers++; if (readers == 1) { // I’m first reader writerSemaphore.wait(); } readerSemaphore.signal(); v = db.getValue(); readerSemaphore.wait(); readers--; if (readers == 0) { // I’m last reader writerSemaphore.signal(); } readerSemaphore.signal(); return v;}; Who can starve?

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2-dinning-philosopher Problem

Shared resource: chopsticks Totally 2 philosophers & 2 chopsticks

Philosopher picks up chopstick in increasing order

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3-dinning-philosopher Problem

Shared resource: chopsticks Totally 3 philosophers & 3 chopsticks

Philosopher picks up chopstick in increasing order

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Problems withSemaphores & Locks

Much better than load/store Still many drawbacks

Access to semaphores may be anywhere

Not structured Not tied to data they control access

to Difficult to detect error

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This Time: More Synch Primitives

Reader-Writer Locks Monitors Condition Variables

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Monitors

Invented by C.A.R. “Tony” Hoare Also invented quicksort, “Hoare triples”

{a > 16} a = sqrt(a); { a > 4 }

Monitor: High-level synchronization construct Private data with public methods

operating on data All methods synchronized

Access of data inside monitor is mutual exclusive

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Monitors syntax

monitor monitor-name {// shared variable

declarationsprocedure P1 (…) { ….

}…

procedure Pn (…) {……}

Initialization code ( ….) { … }

…}

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Condition Variables

Another synchronisation primitive condition x; Two operations on a condition

variable: x.wait () – a process that invokes

the operation is suspended. x.signal () – resumes one of

processes (if any) that invoked x.wait ()

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Condition Variable in Linux

Wait for some condition to be true Then do some action. If condition not true:

Release lock temporarily Waiting inside critical sections

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Condition Variable in Linux: Syntax

pthread_cond_t cond; pthread_cond_init (&cond, NULL); //initialize condition variable

...pthread_mutex_lock(&my_mutex);while(!certain_condition) pthread_cond_wait(&cond, &mutex); //waitpthread_mutex_unlock(&my_mutex);...pthread_cond_signal(&cond); //signal

pthread_cond_t cond; pthread_cond_init (&cond, NULL); //initialize condition variable

...pthread_mutex_lock(&my_mutex);while(!certain_condition) pthread_cond_wait(&cond, &mutex); //waitpthread_mutex_unlock(&my_mutex);...pthread_cond_signal(&cond); //signal

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Condition Variables Example: Reader-writer Problem

A single reader and writer Share a buffer which holds one

item (an integer) Reader: only read when there is

item in buffer Writer: only write when there is

space in buffer

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Condition Variable Example: Reader-writer Problem

#include <pthread.h>

#include <stdio.h>

#include <stdio.h>

int buffer_has_item = 0;

int buffer;

pthread_mutex_t mutex;

pthread_cond_t cond;

void * reader_function(void *)

{

while(1){

pthread_mutex_lock( &mutex );

while(buffer_has_item == 0)pthread_cond_wait(&cond, &mutex);

if ( buffer_has_item == 1) {

printf("reader consumes one item: %d.\n", buffer);

buffer_has_item = 0;

}

pthread_mutex_unlock( &mutex );

}

}

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Condition Variable Example: Reader-writer Problemvoid writer_function(void){ while(1) { pthread_mutex_lock( &mutex ); if( buffer_has_item == 0 ){

buffer = rand() ;printf("writer produces one item: %d\n", buffer);buffer_has_item = 1;pthread_cond_signal(&cond);

} pthread_mutex_unlock( &mutex ); }}

int main(){ pthread_t reader;

pthread_mutex_init(&mutex, NULL);

if (pthread_cond_init (&cond, NULL) == -1){ printf("Error setting up semaphore condition signal"); exit(-1); } pthread_create( &reader, NULL, reader_function, NULL); writer_function(); return 1;}

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Condition Variables Example: Dining-philosophers Problem

Shared resource: chopsticks Totally 5 philosophers & 5 chopsticks

Philosopher may pick up chopsticks only when both available

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Monitor Example: Dining-philosophers Problem

Shared resource: chopsticks Totally 5 philosophers & 5 chopsticks

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Monitor Example: Dining-philosophers Problem

monitor DP {

enum { THINKING; HUNGRY, EATING) state [5] ;condition self [5];

void pickup (int i) { state[i] = HUNGRY; test(i); if (state[i] != EATING) self [i].wait;}

void putdown (int i) { state[i] = THINKING; // test left and right neighbors test((i + 4) % 5); test((i + 1) % 5);

} void test (int i) {

if ( (state[(i + 4) % 5] != EATING) && (state[i] == HUNGRY) && (state[(i + 1) % 5] != EATING) ) { state[i] = EATING ; self[i].signal () ; }

} initialization_code() {

for (int i = 0; i < 5; i++) state[i] = THINKING;}

}

• Philosopher: 0 … 4

• Philosopher may pick up chopsticks only when both available

• Philosopher i

dp.pickup(i)

eat;

dp.putdown(i)

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Summary

Multi-Reader-Writer problem Permit concurrent reads Implementable with semaphores

Dinning philosopher problem Monitors

Tie data, methods with synchronization Condition Variables

Release lock temporarily Waiting inside critical sections