ceng 334 – operating systems 03- threads & synchronization asst. prof. yusuf sahillioğlu...

CENG 334 – Operating Systems

03- Threads & Synchronization

Asst. Prof. Yusuf Sahillioğlu

Computer Eng. Dept, , Turkey

Threads2 / 102

Threads. Parallel work on the same process for efficiency. Several activities going on as part of the same process. Share registers, memory, and other resources. All about data synchronization:

Threads3 / 102

Processes vs. Threads

One thread listens to connections; others handle page requests.

One thread handles GUI; other computations. One thread paints the left part, other the right part. ..

Thread State4 / 102

State shared by all threads in process: Memory content (global variables, heap, code, etc). I/O (files, network connections, etc). A change in the global variable will be seen by all other threads (unlike

processes).

State private to each thread: Kept in TCB (Thread Control Block).

CPU registers, program counter. Stack (what functions it is calling, parameters, local variables, return

addresses). Pointer to enclosing process (PCB).

Thread Behavior5 / 102

Single threaded main()

computePI(); //never finishprintf(“hi”); //never reach here

A process has a single thread of control: if it blocks on something nothing else can be done.

Multi-threaded main()

createThread( computePI() ); //never finish createThread( printf(“hi”) ); //reaches here main()

createThread( scanf() ); //not finish ‘till user enters (not in CPU)

createThread( autoSaveDoc() ); //reaches here while waiting on I/O

Thread Behavior6 / 102

Execution flow:

Threads on a Single CPU7 / 102

Still possible. Multitasking idea

Share one CPU among many processes (context switch). Multithreading idea

Share the same process among many threads (thread switch). Whenever this process has the opportunity to run in the CPU, OS can

select one of its many threads to run it for a while, and so on. One pid, several thread ids.

Schedulable entities increased.

Threads on a Single CPU8 / 102

If threads are all CPU-bound, e.g., no I/O or pure math, then we do not gain much by multithreading.

Luckily this is usually not the case, e.g., 1 thread does the I/O, ..

Select your threads carefully, one is I/O-bound, other is CPU-bound, ..

With multicores, we still gain big even if threads are all CPU-bound.

Multithreading Concept9 / 102

Multithreading concept: pseudo-parallel runs. (pseudo: interleaving switches on 1 CPU).

funct1() { .. }funct2() { .. }main() { .. createThread( funct1() ); .. createThread( funct2() ); .. createThread( funct1() ); ..}

thread1thread2thread3thread4

Single- vs. Multi-threaded Processes10 / 102

Shared and private stuff:

Benefits of Threads11 / 102

Responsiveness One thread blocks, another runs. One thread may always wait for the user.

Resources sharing Very easy sharing (use global variables; unlike msg queues, pipes,

shmget). Be careful about data synchronization tough.

Economy Thread creation is fast. Context switching among threads may be faster.

‘cos you do not have to duplicate code and global variables (unlike processes).

Scalability Multiprocessoers can be utilized better.

Process that has created 4 threads can use all 4 cores (single-threaded proc. utilize 1 core).

Fun Fact12 / 102

Why the hell we call it a thread anyway?

Execution flow of a program is not smooth, looks like a thread. Execution jumps around all over the place (switches) but integrity is

intact.

Multithreading Example: WWW13 / 102

Client (Chrome) requests a page from server (amazon.com).

Server gives the page name to the thread and resumes listening.

Thread checks the disk cache in memo; if page not there, do disk I/O; sends the page to the client.

Threading Support14 / 102

Thread libraries that provide us API for creating and managing threads. pthreads, java threads, win32 threads.

Pthreads. Common in Unix operating sytems: Solaris, Mac OS, Linux. No implemented in the standard C library; search the library named

pthread while compiling: gcc –o thread1 –lpthread thread1.c

Functions in pthread library are actually doing linux system calls, e.g., pthread_create() clone()

Pthreads15 / 102

int main(..){

..

..pthread_create(&tid,…,runner,..);

pthread_join(tid);

printf (sum); }

runner (..){

..sum = ..pthread_exit();

}

thread1thread2

wait

Single- to Multi-thread Conversion16 / 102

In a simple world Identify functions as parallel activities. Run them as separate threads.

In real world Single-threaded programs use global variables, library functions

(malloc). Be careful with them. Global variables are good for easy-communication but need special

care.


Careful with global variable:


Global, local, and thread-specific variables. thread-specific: global inside the thread, but not for the whole

process, i.e., other threads cannot access it, but all the functions of the thread can (no problem ‘cos fnctns within a thread executed sequentially).

No language support for this variable type; C cannot do this.

Thread API has special functions to create such variables.


Use thread-safe (reentrant, reenterable) library routines.

Multiple malloc()s are executed sequentially in a single-threaded code.

Say one thread is suspended on malloc(); another process calls malloc() and re-enters it while the 1st one has not finished.

Library functions should be designed to be reentrant = designed to have a second call to itself from the same process before it’s finished.

To do so, do not use global variables.

Synchronization21 / 102

Synchronize threads/coordinate their activities so that when you access the shared data (e.g., global variables) you are not having a trouble.

Multiple processes sharing a file or shared memory segment also require synchronization (= critical section handling).


The part of the process that is accessing and changing shared data is called its critical section.

Change X

Change X

Change Y

Change Y

Change Y

Change X

Process 1 Code Process 2 Code Process 3 Code

Assuming X and Y are shared data.


Solution: No 2 processes/threads are in their critical section at the same time, aka Mutual Exclusion (mutex).

Must assume processes/threads interleave executions arbitrarily (preemptive scheduling) and at different rates. Scheduling is not under application’s control.

We control coordination using data synchronization. We restrict interleaving of executions to ensure consistency. Low-level mechanism to do this: locks, High-level mechanisms: mutexes, semaphores, monitors, condition

variables.


General way to achieve synchronization:


An example: race condition.

Criticalsection:Criticalsection:

critical section respected not respected


Another example: race condition. Assume we had 5 items in the buffer. Then

Assume producer just produced a new item, put it into buffer, and about to do count++

Assume consumer just retrieved an item from the buffer, and about to do count--

or

Producer Consumer

Producer Consumer


Another example: race condition.

Critical region: is where we manipulate count.

count++ could be implemented as (similarly, count--) register1 = count; //read value register1 += 1; //increase value count = register1; //write back


Then:

register2 = countregister2 = register2 – 1count = register2

register1 = countregister1 = register1 + 1count = register1

Countregister1

register2

5register1 = countregister1 = register1 + 1count = register1

register2 = countregister2 = register2 – 1count = register2

PRODUCER (count++)

CONSUMER (count--)

56

54

64

CPU

Main Memory


Another example: race condition.

2 threads executing their critical section codes

Although 2 customers withdrew 100TL, balance is 900TL, not 800TL

balance = get_balance(account);

balance -= amount;

balance = get_balance(account);

balance -= amount;

put_balance(account, balance);

put_balance(account, balance);

Execution sequence

as seen by CPU

Balance = 1000TL

Balance = 900TL

Balance = 900TL!

Local = 900TL

Local = 900TL


Solution: mutual exclusion. Only one thread at a time can execute code in their critical section. All other threads are forced to wait on entry. When one thread leaves the critical section, another can enter.

Critical Section

Thread 1

(modify account balance)



Thread 2

Critical Section

Thread 1

(modify account balance)2nd thread must waitfor critical section to clear



1st thread leaves critical section

2nd thread free to enter

Thread 2

Critical Section

(modify account balance)


Solution: mutual exclusion. pthread library provides us mutex variables to control the critical

section access. pthread_mutex_lock(&myMutex) .. //critical section stuff pthread_mutex_unlock(&myMutex) See this in action..


Critical section requirements. Mutual exclusion: at most 1 thread is currently executing in the

critical section.

Progress: if thread T1 is outside the critical section, then T1 cannot prevent T2 from entering the critical section.

No starvation: if T1 is waiting for the critical section, it’ll eventually enter. Assuming threads eventually leave critical sections.

Performance: the overhead of entering/exiting critical section is small w.r.t. the work being done within it.


Solution: Peterson’s solution to mutual exclusion. Programming at the application (sw solution; no hw or kernel

support). Peterson.enter //similar to pthread_mutex_lock(&myMutex) .. //critical section stuff Peterson.exit //similar to pthread_mutex_unlock(&myMutex)

Works for 2 threads/processes (not more). Is this solution OK?

Set global variable lock = 1. A thread that wants to enter critical section checks lock == 1.

If true, enter. Do lock--. if false, another thread decremented it so not enter.




Works for 2 threads/processes (not more). Is this solution OK?

Set global variable lock = 1. A thread that wants to enter critical section checks lock == 1.

If true, enter. Do lock--. if false, another thread decremented it so not enter.

This solution sucks ‘cos lock itself is a shared global variable. Just using a single variable without any other protection is not

enough. Back to Peterson’s algo..




Works for 2 threads/processes (not more). Assume that the LOAD and STORE machine instructions are atomic;

that is, 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.

turn = i means process Pi can execute (i=0,1). 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 (wants to enter).


Solution: Peterson’s solution to mutual exclusion. The variable turn indicates whose turn it is to enter the critical

section. turn = i means process Pi can execute (i=0,1).

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 (wants to enter).

Algorithm for Pi; the other process is Pj:

I want to enter but,

be nice to other process.

Busy wait:


Solution: Peterson’s solution to mutual exclusion.

do {

flag[i] = TRUE;

turn = j;

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

critical section..

flag[i] = FALSE;

remainder section..

} while (1)

do {

flag[j] = TRUE;

turn = i;

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

critical section..

flag[j] = FALSE;

remainder section..

} while (1)

PROCESS i (0) PROCESS j (1)

Shared Variables:flag[]turn i=0, j=1 are local.


Solution: hardware support for mutual exclusion. Kernel code can disable clock interrupts (context/thread

switches).

disable interrupts (no switch)

enable interrupts (schedulable)


Solution: hardware support for mutual exclusion. Works for single CPU. Multi-CPU fails ‘cos you’re disablin the interrupt only for your

processor. That does not mean other processors do not get interrupts.

Each processor has its own interrupt mechanism. Hence another process/thread running in another processor

can touch the shared data. Too inefficient to disable interrupts on all available

processors.


Solution: hardware support for mutual exclusion. Another support mechanism: Complex machine instructions

from hw that are atomic (not interruptible). Locks (not just simple integers).

How to implement acquire/release lock? Use special machine instructions: TestAndSet, Swap.

do {

acquire lock

critical section

release lock

remainder section

} while (TRUE);


Solution: hardware support for mutual exclusion. Complex machine instruction for hw synch: TestAndSet

TestAndSet is a machine/assembly instruction. You must write the acquire-lock portion (entry section code) of your

code in assembly. But here is a C code for easy understanding:

boolean TestAndSet (boolean *target)

{boolean rv = *target;

*target = TRUE; return rv:} //atomic (not interruptible)!!!!!!!!!!!!

--Definition of TestAndSet Instruction--



do { while ( TestAndSet (&lock ) ) ; //do nothing; busy wait

// critical section

lock = FALSE; //release lock

// remainder section

} while (TRUE);

entry section

exit section

We use a shared Boolean variable lock, initialized to false.



Can be suspended/interrupted b/w TestAndSet & CMP, but not during TestAndSet.


Advertisement: Writing assembly in C is a piece of cake.


Solution: hardware support for mutual exclusion. Complex machine instruction for hw synch: Swap

Swap is a machine/assembly instruction. You must write the acquire-lock portion (entry section code) of your

code in assembly. But here is a C code for easy understanding:

boolean Swap (boolean* a, boolean* b)

{boolean temp = *a;

*a = *b; *b = temp;} //atomic (not interruptible)!!!!!!!!!!!!

--Definition of Swap Instruction--


Solution: hardware support for mutual exclusion. Complex machine instruction for hw synch: Swap

We use a shared Boolean variable lock, initialized to false. Each process also has a local Boolean variable key.

do { key = TRUE; while (key == TRUE) Swap (&lock, &key ); // critical section

lock = FALSE;

// remainder section

} while (TRUE);

entry sect

exit sect


Solution: hardware support for mutual exclusion. A comment on TestAndSwap & Swap.

Although they both guarantee mutual exclusion, they may make one process (X) wait a lot:

A process X may be waiting, but we can have the other process Y going into the critical region repeatedly.

One toy/bad solution: keep the remainder section code so long that scheduler kicks Y out of the CPU before it reaches back to the entry section.


Solution: Semaphores (= shared integer variable). Idea: avoid busy waiting: waste of CPU cycles by waiting in a

loop ‘till the lock is available, aka spinlock. Example1: while (flag[i] && turn == i); //from Peterson’s algo. Example2: while (TestAndSet (&lock )); //from TestAndSet algo.

How to avoid? If a process P calls wait() on a semaphore with a value of zero, P is

added to the semaphore’s queue and then blocked. The state of P is switched to the waiting state, and control is transferred

to the CPU scheduler, which selects another process to execute (instead of busy waiting on P).

When another process increments the semaphore by calling signal() and there are tasks on the queue, one is taken off of it and resumed.

wait() = P() = down(). //modify semaphore s via these functions. signal() = V() = up(). //modify semaphore s via these functions.


Solution: Semaphores. wait() = P() = down(). //modify semaphore s via these functions. signal() = V() = up(). //modify semaphore s via these functions. These functions can be implemented in kernel as system calls. Kernel makes sure that wait(s) & signal(s) are atomic.

Less complicated entry & exit sections.


Solution: Semaphores. Operations (kernel codes).

Busy-waiting vs. Efficient

More formally, s->value--; s->list.add(this); etc.


Solution: Semaphores. Operations. wait(s):

if s positive

s-- and return

else

block/wait (‘till somebody wakes you up;

then return)


Solution: Semaphores. Operations. signal(s):

if there’s 1+ process waiting (new s<=0)

wake one of them up and return

//wake = change state from waiting to ready

else

s++ and return


Solution: Semaphores. Types. Binary semaphore

Integer value can range only between 0 and 1; can be simpler to implement; aka mutex locks.

Provides mutual exclusion; can be used for the critical section problem.

Counting semaphore Integer value can range over an unrestricted domain. Can be used for other synchronization problems; for example for resource

allocation. Example: you have 10 instances of a resource. Init semaphore s to 10 in this case.


Solution: Semaphores. Usage. An integer variable s that can be shared by N processes/threads. s can be modified only by atomic system calls: wait() & signal(). s has a queue of waiting processes/threads that might be

sleeping on it.

Atomic: when process X is executing wait(), Y can execute wait() if X finished executing wait() or X is blocked in wait().

When X is executing signal(), Y can execute signal() if X finished.

typedef struct {int value; struct process *list;

} semaphore;


Solution: Semaphores. Usage. Binary semaphores (mutexes) can be used to solve critical

section problems.

A semaphore variable (lets say mutex) can be shared by N processes, and initialized to 1.

Each process is structured as follows: do {wait (mutex);

// Critical Sectionsignal (mutex);// remainder

section} while (TRUE);


Solution: Semaphores. Usage.

do {wait (mutex);



do {wait (mutex);



Semaphore mutex; //initialized to 1Kernel

Process 0 Process 1

wait() {…} signal() {…}


Solution: Semaphores. Usage. Kernel puts processes/threads waiting on s in a FIFO queue. Why

FIFO?


Solution: Semaphores. Usage other than critical section. Ensure S1 definitely executes before S2 (just a synch problem).

…S1; ….

…S2; ….

P0 P1


Solution: Semaphores. Usage other than critical section. Ensure S1 definitely executes before S2 (just a synch problem).

…S1; ….

…S2; ….

P0 P1

…S1;signal (x); ….

…wait (x);S2; ….

P0 P1

Solution via semaphores:Semaphore x = 0; //inited to 0


Solution: Semaphores. Usage other than critical section. Resource allocation (just another synch problem). We have N processes that want a resource that has 5 instances. Solution:


Solution: Semaphores. Usage other than critical section. Resource allocation (just another synch problem). We’ve N processes that want a resource R that has 5 instances. Solution:

Semaphore rs = 5; Every process that wants to use R will do wait(rs);

If some instance is available, that means rs will be nonnegative no blocking. If all 5 instances are used, that means rs will be negative block ‘till rs nonneg.

Every process that finishes with R will do signal(rs); A blocked processes will change state from waiting to ready.


Solution: Semaphores. Usage other than critical section. Enforce consumer to sleep while there’s no item in the buffer

(another synch problem).

do {// produce item..put item into buffer

..signal (Full_Cells);

} while (TRUE);

do {wait (Full_Cells);

//instead of busy-waiting, go to sleep mode and give CPU back to producer for faster production (efficiency!!).

..remove item from buffer..

} while (TRUE);Semaphore Full_Cells = 0; //initialized to 0 Kernel

Producer Consumer


Solution: Semaphores.


Solution: Semaphores. Consumer can never cross the producer curve. Difference b/w produced and consumed items can be <= BUFSIZE


Problems with seampahores: 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.



This code may sometimes (not all the time) cause a deadlock: P0 P1 wait(S); wait(Q); wait(Q); wait(S);

. .

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

When does the deadlock occur? How to solve?


Problems with seampahores: Deadlock and Starvation. Starvation.

Indefinite blocking: a process may never be removed from the semaphore queue in which it is susupended; it’ll always be sleeping; no service.

When does it occur? How to solve?

Another problem: Low-priority process may cause high-priority process to wait.


Classic Synchronization Problems to be solved with Semaphores. Bounded-buffer problem. Readers-Writers problem. Dining philosophers problem.


Classic Synchronization Problems to be solved with Semaphores. Bounded-buffer problem (aka Producer-Consumer problem).

Producer should not produce any item if the buffer is full: Semaphore full = 0; //inited

Consumer should not consume any item if the buffer is empty: Semaphore empty = N;

Producer and consumer should access the buffer in a mutually exc manner: mutex = 1;

Types of 3 semaphores above?

full = 4empty = 6

bufferprod cons


Classic Synchronization Problems to be solved with Semaphores.

Bounded-buffer problem. Producer should not produce any item if the buffer is full: Semaphore full =

0; //inited Consumer should not consume any item if the buffer is empty: Semaphore

empty = N; Producer and consumer should access the buffer in a mutually exc manner:

mutex = 1;

Think about the code of this?



Bounded-buffer problem. Producer should not produce any item if the buffer is full: Semaphore full =

0; //inited Consumer should not consume any item if the buffer is empty: Semaphore

empty = N; Producer and consumer should access the buffer in a mutually exc manner:

mutex = 1;



Readers-Writers problem. 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 (no reader/writer when writer is active).





Integer readcount initialized to 0. Number of readers reading the data at the moment.

Semaphore mutex initialized to 1. Protects the readcount variable (multiple readers may try to modify it).

Semaphore wrt initialized to 1. Protects the shared data (either writer or reader(s) should access data at a time).





Think about the code of this? Reader and writer processes running in (pseudo) parallel. Hint: first and last reader should do something special.




//acquire lock to shared data.

//release lock of shared data.


Classic Synchronization Problems to be solved with Semaphores. Readers-Writers problem.

Case1: First reader acquired the lock, reading, what happens if writer arrives?

Case2: First reader acquired the lock, reading, what happens if reader2 arrvs?

Case3: Writer acquired the lock, writing, what happens if reader1 arrives? Case4: Writer acquired the lock, writing, what happens if reader2 arrives?



Dining philosophers problem. A philosopher (process) needs 2 forks (resources) to eat. While a philosopher is holding a fork, it’s neighbor cannot have it.


Classic Synchronization Problems to be solved with Semaphores. Dining philosophers problem.

A philosopher (process) needs 2 forks (resources) to eat. While a philosopher is holding a fork, it’s neighbors cannot have it.

Not gay , just going for a fork.


Classic Synchronization Problems to be solved with Semaphores. Dining philosophers problem.

A philosopher (process) needs 2 forks (resources) to eat. While a philosopher is holding a fork, it’s neighbors cannot have it.

Philosopher in 2 states: eating (needs forks) and thinking (not need forks).

We want parallelism, e.g., 4 or 5 (not 1 or 3) can be eating while 2 is eating. We don’t want deadlock: waiting for each other indefinitely. We don’t want starvation: no philosopher waits forever (starves to death).



Dining philosophers problem. A philosopher (process) needs 2 forks (resources) to eat. While a philosopher is holding a fork, it’s neighbors cannot have it.

A solution that provides concurrency but not deadlock prevention:Semaphore forks[5]; //inited to 1 (assume 5 philosophers on table). do {

wait( forks[i] );wait( forks[ (i + 1) % 5] );

// eat

signal( forks[i] );signal( forks[ (i + 1) % 5] );

// think} while(TRUE);




A solution that provides concurrency but not deadlock prevention: How is deadlock possible?




A solution that provides concurrency but not deadlock prevention: How is deadlock possible?

Deadlock in a circular fashion: 5 gets the left fork, context switch (cs), 4 gets the left fork, cs, .. , 1 gets the left fork, cs, 5 now wants the right fork which is held by 1 forever. Unlucky sequence of cs’s not likely but possible.

A perfect solution w/o deadlock danger is possible with again semaphores.

Solution #1: put the left back if you cannot grab right. Solution #2: grab both forks at once (atomic).


Problems with semaphores. Careless programmer may do

signal(mutex); .. wait(mutex); //2+ threads in critical region (unprotected).

wait(mutex); .. wait(mutex); //deadlock (indefinite waiting). Forgetting corresponding wait(mutex) or signal(mutex); //unprotect

& deadlck

Need something else, something better, something easier to use:

Monitors.


Solution: Monitors. Idea: get help not from the OS but from the programming

language. High-level abstraction for process/thread synchronization. C does not provide monitors (use semaphores) but Java does. Compiler ensures that the critical regions of your code are

protected. You just identify the critical section of the code, put them into a

monitor, and compiler puts the protection code. Monitor implementation using semaphores.

Compiler writer/language developer has to worry about this stuff, not the casual application programmer.


Solution: Monitors. Monitor is a construct in the language, like class construct:

monitor construct guarantees that only one process may be active within the monitor at a time.

monitor monitor-name {// shared variable declarations

procedure P1 (..) { .. }..

procedure Pn (..) { .. }

Initialization code (..) { .. }..

}


Solution: Monitors. monitor construct guarantees that only one process may be

active within the monitor at a time. This means that, if a process is running inside the monitor (=

running a procedure, say P1()), then no other process can be active inside the monitor (= can run P1() or any other procedure of the monitor) at the same time.

Compiler is putting some locks/semaphores to the beginning/ending of these critical regions (procedures, shared variables, etc.). So it is not the programmer’s job anymore to insert these

locks/semaphores.


Solution: Monitors. Schematic view of a monitor.

All other processes that want to be active in the monitor (execute a monitor procedure) must wait in the queue ‘till current active P leaves.


Solution: Monitors. Schematic view of a monitor.

This monitor solution solves the critical section (mutual exc.) problem.

But not the other synchronization problems such as produc-consume,

dining philosophs.


Solution: Monitors. Condition variables to solve all the synchronization

problems. In previous model, no way to enforce a process/thread to wait

‘till a condition happens. Now we can

Using conition variables.

condition x, y;

Two operations on a condition variable: x.wait (): a process that invokes the operation is suspended.

Execute wait() operation on the condition variable x.

x.signal(): resumes one of processes (if any) that invoked x.wait(). Usually the first one that is blocked is waken up (FIFO).


Solution: Monitors. condition x, y;

wait(Semaphore s); //you may or may not block depending on s.value

x.wait () //you (= process) definitely block.

No integer is attached to x (unlike s.value).


Solution: Monitors. Schematic view of a monitor w/ condition variables.

If currently active process wants to wait (e.g., empty buffer), it calls x.wait() and added to the queue of x, and it is no longer active.



New active process in the monitor (fetched from the entry queue), does x.signal() from a different/same procedure. Prev. process resumes from where it got blocked.



Now we may have 2 processes active: caller of x.signal & waken-up.

Solution: put x.signal() as the last statement in the procedure.



Now we may have 2 processes active: caller of x.signal & waken-up.

Solution: call x.wait() right after x.signal() to block the caller.


Solution: Monitors. An example: We have 5 instances of a resource and N

processes. Only 5 processes can use the resource simultaneously. Process code Monitor code


Solution: Monitors. An example: Dining philosophers.

monitor DP { enum { THINKING, //not holding/wanting resoursces HUNGRY, //not holding but wanting EATING} //has the resources state[5]; condition cond[5];

//no need for entry/exit code to pickup() ‘cos its in monitor void pickup (int i) { state[i] = HUNGRY; test(i); if (state[i] != EATING)

cond[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 + 1) % 5] != EATING) && (state[i] == HUNGRY)) { state[i] = EATING ;

cond[i].signal (); } }

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

} /* end of monitor */


Solution: Monitors. One philosopher/process doing this in an endless loop:

..DP DiningPhilosophers; ..while (1) {

//THINK..

DiningPhilosophters.pickup (i);

//EAT (use resources)

DiningPhilosophers.putdown (i);

//THINK..}

Philosopher i:


Solution: Monitors. First things first; what are the ID’s to access neighbors?

Processi

Process??

Process??.. ..

state[LEFT] = ? state[RIGHT] = ?state[i] = ?

#define LEFT ?#define RIGHT ?

THINKING?HUNGRY?EATING?


Solution: Monitors. General idea.

Processi

Process(i+1) % 5

Process(i+4) % 5

Test(i)

… …

Test((i+1) %5)Test((i+4) %5)

state[LEFT] = ? state[RIGHT] = ?state[i] = ?

#define LEFT (i+4)%5#define RIGHT (i+1)%5

THINKING?HUNGRY?EATING?


Solution: Monitors. An example: Allocate a resource to one of the several

processes. Priority-based: The process that will use the resource for the

shortest amount of time (known) will get the resource first if there are other processes that want the resource.

Resource

..Processes or Threads

that want to use the resource



processes. Assume we have condition variable implementation that can

enqueue sleeping/waiting processes w.r.t. a priority specified as a parameter to wait() call.

condition x; x.wait (priority);

10 20 45 70

Queue of sleeping processes waiting on condition x:

x

priority could be the time-duration to use the resource.



processes.monitor ResourceAllocator {

boolean busy; //true if resource is currently in use/allocated

condition x; //sleep the process that cannot acquire the resource

void acquire(int time) { if (busy) x.wait(time); busy = TRUE;

} void release() {

busy = FALSE; x.signal(); //wakeup the P at the head of the

waiting qu} initialization_code() {

busy = FALSE; } }



processes.

ResourceAllocator RA;

RA.acquire(10);

..use resource..

RA.release();


RA.acquire(30);

..use resource..

RA.release();

Process/Thread 1


RA.acquire(25);

..use resource..

RA.release();

..

Each process should use resource between acquire() and release() calls.

Process/Thread 2 Process/Thread N

ceng 334 – operating systems 03- threads & synchronization asst. prof. yusuf sahillioğlu...

Documents

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thread state

thread ids

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benefits of threads

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single thread of control

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