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Advanced Operating Systems - Fall 2009Lecture 6 – January 28, 2009

Dan C. MarinescuEmail: dcm@cs.ucf.eduOffice: HEC 439 B. Office hours: M, Wd 3 – 4:30.

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Last, Current, Next Lecture

Last time: Process handling by the kernel Inter-process communication Threads

Today More about threads Process synchronization

Next time: Atomic transactions

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Re-entrant and thread-safe code

Reentrant code safe to be executed concurrently. Conditions for code to be reentrant:

No static (global) non-constant data. Should not return the address to static (global) non-constant data. Must work only on the data provided to it by the caller. Must not rely on locks to resources. Must not call non-reentrant code.

I/O code is not reentrant because it requires access to resources that are shared.

Every reentrant function is thread-safe; not every thread-safe function is reentrant. To make a non-reentrant function reentrant one must change the external interface. To make a non thread-safe code, thread safe we need only change its internal implementation.

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Examples of non-reentrant code

int global_variable = 1;

int f() {

global_variable = global_variable + 2;

return g_var;

}

If multiple threads call f() concurrently the result is unpredictable

int g()

{

return f() + 2;

}

g() calls a non-reentrant function f().

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The corresponding reentrant code

int f(int i) { return i + 2; }

int g(int i) { return f(i) + 2; }

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Signal Handling

Signals used in UNIX systems to notify a process that a particular event has occurred

A signal handler is used to process signals

1. Signal is generated by particular event

2. Signal is delivered to a process

3. Signal is handled Options:

Deliver the signal to the thread to which the signal applies

Deliver the signal to every thread in the process Deliver the signal to certain threads in the process Assign a specific thread to receive all signals for the

process

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Thread Pools

Create a number of threads in a pool where they await work

Advantages: Usually slightly faster to service a request with an

existing thread than create a new thread Allows the number of threads in the application(s) to be

bound to the size of the pool

Thread Specific Data Allows each thread to have its own copy of data Useful when you do not have control over the thread creation

process (i.e., when using a thread pool)

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Pthreads

Common in UNIX operating systems (Solaris, Linux, Mac OS X) POSIX standard (IEEE 1003.1c) specifies the API for thread

creation and synchronization Implementation up to developers of the library.

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Windows XP Threads

One-to-one mapping Each thread contains

A thread id Register set Separate user and kernel stacks Private data storage area

The register set, stacks, and private storage area are known as the context of the threads

The primary data structures of a thread include: ETHREAD (executive thread block) KTHREAD (kernel thread block) TEB (thread environment block)

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Linux Threads and Java Threads

Linux refers to them as tasks rather than threads Thread creation clone() system call clone() allows a child task to share the address

space of the parent task (process) Java threads:

are managed by the JVM created by:

Extending Thread class Implementing the Runnable interface

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States of a Java Thread

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Process Synchronization

The Critical-Section Problem Peterson’s Solution Synchronization Hardware Semaphores Classic Problems of Synchronization Monitors Synchronization Examples Atomic Transactions

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Concepts

Concurrency multiple activities are carried out at the same time. E.g., concurrent processes

Busy waiting keep checking if a condition is satified. Deadlock two or more activities are waiting indefinitely for an

event that can be caused by only one of the waiting activities. Starvation an activity cannot get access to the resources it

needs

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Synchronization

Activity coordination critical in any environment where entities must cooperate with one another.

Synchronization requires the coordination of events

Applications in: Physics Communications Computer Science Multimedia Music/sports

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Dining-Philosophers

Shared data Bowl of rice (data set) Semaphore chopstick [5] initialized to 1

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Dining-Philosophers (Cont.)

Philosopher i:While (true) { wait (chopstick[i] );

wait ( chopStick[ (i + 1) modulo 5] ); // eat signal (chopstick[i] ); signal (chopstick[ (i + 1) modulo 5] );

// think}

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Bounded buffer

Shared state between producer and consumer Count – the number of items written by the

producer and yet to be read by the consumer

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Producer

while (true) { /* produce an item and put in nextProduced */

while (count == BUFFER_SIZE); // do nothing

buffer [in] = nextProduced; in = (in + 1) modulo BUFFER_SIZE; count++;

}

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Consumer

while (true) { while (count == 0)

; // do nothing nextConsumed = buffer[out]; out = (out + 1) modulo BUFFER_SIZE;

count--;/* consume the item in nextConsumed

}

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Race conditions

Compiler translates producer’s process P instruction count++ as: R1 = count

R1 = R1 +1 count = R1

Compiler translates consumer’s process C instruction count-- as: R2 = count R2 = R2 - 1 count = R2

Initially count = 5 P is scheduled to run

P executes R1 = count (R1 = 5)P : executes R1 = R1 + 1 (R1 = 6)

An interrupt occurs and P is suspended and C is dispatched C executes R2 = count (R2 = 5) C executes R2 = R2-1 (R2 = 4) An interrupt occurs and C is suspended and P is dispatched

P executes count = R1 (count = 6) P writes the new item in buffer(12);

P has finished, it is suspended, and C is dispatched C executes count = R2 r(count = 4} C read the new item from buffer(7);

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Critical Section

Important concept in concurrent programming  A piece of code that accesses a shared resource (data

structure or device) that must not be concurrently accessed by more than one thread of execution.

Critical sections may occur in: User’s programs user’s program may be affected by the

error System programs the systems may end up in an erroneous

state.

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Conditions for Critical-Section

1. Mutual Exclusion - If process Pi is executing in its critical section, then no other processes can be executing in their critical sections protecting the same resource

2. Progress - If no process is executing in its critical section and there exist some processes that wish to enter their critical section, then the selection of the processes that will enter the critical section next cannot be postponed indefinitely

3. Bounded Waiting - A bound must exist on the number of times that other processes are allowed to enter their critical sections after a process has made a request to enter its critical section and before that request is grantedAssume that each process executes at a nonzero speed No assumption concerning relative speed of the N processes

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Peterson’s Solution

Two process solution: Pi and Pj Assume that the LOAD and STORE instructions are

atomic cannot be interrupted. The two processes share two variables:

int turn; Boolean flag[2]

The variable turn indicates whose turn it is to enter the critical section.

The flag array is used to indicate if a process is ready to enter the critical section. flag[i] = true implies that process Pi is ready!

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Algorithm for Process Pi

while (true)

{

flag[i] = TRUE;

turn = j;

while ( flag[j] && turn == j);

CRITICAL SECTION

flag[i] = FALSE;

NON-CRITICAL SECTION

}

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Another version of Peterson’s Solution

#define FALSE 0#define TRUE 1#define N 2 /* we have two processes 0 and 1 */ int turn; /* variable indicating whose turn it is: process 0 or process 1 */ int interested(N) /* initially set to FALSE shows if the process wishes to enter CS */ void enter_CR (int process); { int other; /* id of the other process) other = 1 – process; interested(process) = TRUE; turn = process; while (turn = process && interested(other) = TRUE) } void leave_CS(int process); { interested(process) = FALSE; }

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Synchronization Hardware

Disable interrupts do not allow preemption of the currently running process Works only on uniprocessors Not scalable

Atomic hardware instructions. Atomic = non-interruptable TestAndSet test memory word and set value Swap swap contents of two memory words

The two instructions allows synchronization among several processors.

A process may enter the critical section only if a lock acted upon these instructions allows it. Shared Boolean variable initially set to unlocked position: lock=FALSE Locked when a process enters the critical section: lock=TRUE Once finished working the process unlocks the lock: lock=FALSE

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TestAndSet Instruction

TestAndSet first checks the lock and returns its state and then sets it to locked state.

boolean TestAndSet (boolean *target)

{ boolean rv = *target; *target = TRUE; return rv: }

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TestAndSet use with a shared variable “lock”

If the lock=FALSE then TestAndSet: sets lock=TRUE allows the calling process to enter the critical sectionUpon exiting critical section the process resets lock: lock= TRUE.

while (true) { while (TestAndSet (&lock ); /* do nothing busy wait // critical section lock = FALSE; // remainder section

}

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Swap Instruction

Each process has its own key and the critical section is protected by a lock initially set to FALSE.

void Swap (boolean *a, boolean *b)

{ boolean temp = *a; *a = *b; *b = temp: }

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Solution using Swap

A process may enter the critical section only if the lock is open (lock=FALSE) and its key can lock the lock (key=TRUE)

while (true) { key = TRUE; while ( key == TRUE) busy wait again Swap (&lock, &key ); // critical section

lock = FALSE;

// remainder section }

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Two ways of a process to wait

Busy waiting the CPU running the process keeps testing a condition.

Suspend the process and allow the CPU to execute another process until someone sends a signal that the condition has been

set.

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No busy waiting for producer-consumer

producer { while(TRUE} { produce item; if (count=N) sleep(); write_Item; count=count++; if (count=1) wakeup(consumer); } }

consumer { while(TRUE) { if (count=0) sleep(); read_Item; count--; if (count = N-1) wakeup(producer) })

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Problems with this solution

A wakeup signal sent to a process that is not yet sleeping is lost. Scenario

The buffer is empty and the consumer finds that count=0. The consumer is suspended by the scheduler. The scheduler activates the producer. The producer adds an item to the buffer and sets count=1. The producer believing that prior to this the count was 0 and thus the

consumer has put itself to sleep sends a sends a wakeup signal to consumer.

The wakeup signal is lost because the consumer is not in a sleep state.

When the consumer runs next it will test again the value of the counter it read previously, find it to be 0 and go back to sleep.

Eventually the producer will fill up the buffer and go to sleep.

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Semaphore

Introduced by Dijkstra in 1965 Does not require busy waiting Semaphore S – integer variable Two standard operations modify S: wait() and signal()

Originally called P() and V() Less complicated Can only be accessed via two indivisible (atomic) operations

wait (S) { while S <= 0

; // no-op S--; } signal (S) { S++; }

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Semaphore as General Synchronization Tool

Counting semaphore – integer value can range over an unrestricted domain

Binary semaphore mutex locks– integer value can range only between 0 and 1; simpler to implement.

Can implement a counting semaphore S as a binary semaphore Provides mutual exclusion

Semaphore S; // initialized to 1 wait (S);

Critical Section

signal (S);

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Semaphore Implementation

Must guarantee that no two processes can execute wait () and signal () on the same semaphore at the same time

Implementation becomes the critical section problem where the wait and signal code are placed in the critical section. Could now have busy waiting in critical section implementation

But implementation code is short Little busy waiting if critical section rarely occupied

Applications may spend lots of time in critical sections and therefore this is not a good solution.

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Semaphore Implementation with no Busy waiting

With each semaphore there is an associated waiting queue. Each entry in a waiting queue has two data items: value (of type integer) pointer to next record in the list

The two operations on semphore S, Wait(S) and Signal(S) are implemented using: block – place the process invoking the operation on

the appropriate waiting queue. wakeup – remove one of processes in the waiting

queue and place it in the ready queue.

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Semaphore Implementation with no Busy waiting

Implementation of wait: wait (S){

value--; if (value < 0) {

add this process to waiting queue block(); }

} Implementation of signal: Signal (S){

value++; if (value <= 0) {

remove a process P from the waiting queue wakeup(P); }

}

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Deadlock and Starvation

Deadlock two or more processes are waiting indefinitely for an event that can be caused by only one of the waiting processes

Let S and Q be two semaphores initialized to 1

P0 P1

wait (S); wait (Q); wait (Q); wait (S);

. .

. .

. . signal (S); signal (Q); signal (Q); signal (S);

Starvation – indefinite blocking. A process may never be removed from the semaphore queue in which it is suspended.

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Semaphores for Classical Synchronization

Problems

Bounded-Buffer Readers and Writers Dining-Philosophers

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Semaphores for Bounded-Buffer Problem

N buffers, each can hold one item Semaphores:

mutex (binary semaphore) initially set to 1 (allow access) full (counting semaphore - counts the number of full

buffers) initially set to 0. empty (counting semaphore - counts the number of

empty buffers) initially set to N.

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The producer process

while (true) {

produce_Item

wait (empty); /* decrement the count of empty slots */

wait (mutex); /* wait for permission to access buffer */

AddItemToBuffer;

signal (mutex); /* signal that the buffer can be accessed */

signal (full); /* increment count of full slots */

}

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The consumer process

while (true) {

wait (full); /* decrement the count of full slots */

wait (mutex); /* wait for permission to access buffer */

removeItemFromBuffer

signal (mutex); /* signal that the buffer can be accessed */

signal (empty); /* increment the count of empty slots */

}

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Readers-Writers

A data set is shared among a number of concurrent processes

Readers – only read the data set; they do not perform any updates

Writers – can both read and write. Problem – allow multiple readers to read at the same time.

Only one single writer can access the shared data at the same time.

Shared Data Data set Semaphore mutex initialized to 1. Semaphore wrt initialized to 1. Integer readcount initialized to 0.

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The writer process

while (true)

{

wait (wrt) ;

// writing is performed

signal (wrt) ;

}

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Reader process

while (true) { wait (mutex) ; readcount ++ ; if (readcount == 1) wait (wrt) ; signal (mutex) // reading is performed wait (mutex) ; readcount - - ; if (readcount == 0) signal (wrt) ; signal (mutex) ; }

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Problems with Semaphores

Correct use of semaphore operations: signal (mutex) …. wait (mutex)

What if Omit wait (mutex) or signal (mutex) (or both) wait (mutex) … wait (mutex)

What if someone does not stop at a traffic light and crosses an intersection on red?

Is there a way to enforce the traffic lights?

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Monitors

Programming languages constructs. The compiler handles calls to monitors differently than

other calls. Only one process may be active within the monitor at a

timemonitor monitor-name{

// shared variable declarationsprocedure P1 (…) { …. }

…procedure Pn (…) {……}

Initialization code ( ….) { … }…

}}

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Schematic view of a Monitor

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Blocking a process when it cannot proceed

Condition variables condition x, y;

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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Monitor with Condition Variables

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A Producer-Consumer Monitor

monitor ProducerConsumer;

condition full, empty; integer count; procedure enter; if count = N then wait(full); add_Item; count++; if count = 1 then signal(empty); end;

procedure remove; if count = 0 then wait(empty); remove_Item; count--; if count = N-1 then signal(full); end; count=0;end monitor;

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A Producer-Consumer Monitor (cont’d)

procedure producer;

begin

while true do

begin

produce_Item;

ProducerConsumer_enter;

end

end

procedure producer;

begin

while true do

begin

ProducerConsumer_remove;

consume_Item;

end

end

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Solution to Dining Philosophers

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) modulo 5); test((i + 1) modulo 5);

}

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Dining Philosophers (cont’d)

void test (int i) { if ( (state[(i + 4) modulo 5] != EATING) && (state[i] == HUNGRY) && (state[(i + 1) modulo 5] != EATING) ) { state[i] = EATING ;

self[i].signal () ; } }

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

}

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Dining Philosophers (cont)

Each philosopher I invokes the operations pickup() and putdown() in the following sequence:

dp.pickup (i)

EAT

dp.putdown (i)

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Monitor Implementation Using Semaphores

Variables semaphore mutex; // (initially = 1)semaphore next; // (initially = 0)int next-count = 0;

Each procedure F will be replaced bywait(mutex); …

body of F; …if (next-count > 0)

signal(next)else

signal(mutex); Mutual exclusion within a monitor is ensured.

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Monitor Implementation

For each condition variable x, we have:semaphore x-sem; // (initially = 0)int x-count = 0;

The operation x.wait can be implemented as:

x-count++;if (next-count > 0)

signal(next);

elsesignal(mutex);

wait(x-sem);

x-count--;

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Monitor Implementation

The operation x.signal can be implemented as:

if (x-count > 0) {next-count++;signal(x-sem);wait(next);next-count--;

}

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Synchronization Examples

Solaris Windows XP Linux Pthreads

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Solaris Synchronization

Implements a variety of locks to support multitasking, multithreading (including real-time threads), and multiprocessing

Uses adaptive mutexes for efficiency when protecting data from short code

segments condition variables and readers-writers locks when longer sections of

code need access to data turnstiles to order the list of threads waiting to acquire either an

adaptive mutex or reader-writer lock

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Windows XP Synchronization

To protect global resources uses interrupt masks on uniprocessor systems spinlocks on multiprocessor systems

Provides dispatcher objects which may act as either mutexes and semaphores

Dispatcher objects may also provide events An event acts much like a condition variable

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Linux Synchronization

Disables interrupts to implement short critical sections Linux provides:

semaphores spin locks

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Pthreads Synchronization

Pthreads API is OS-independent It provides:

mutex locks condition variables

Non-portable extensions include: read-write locks spin locks