Operating Systems

04/24/23 CST 352 - Operating Systems 1

Operating Systems

CST 352Processes and Threads


Topics• Definitions• Communications

–Process to Process–Process to Thread–Thread to Thread

• Scheduling


Definitions - PrelimsConcurrency – The appearance

that threads are running simultaneously even though there is a single CPU.


Definitions - PrelimsContext – The “processor” state

of a block of executing code. This includes all registers required to uniquely identify this chain of execution.


Definitions - ProcessProcess – A group of instructions

along with the context defining the execution “state (s)” of those instructions.

Q: How does the concept of a “process” and the code that describes the process parallel the concept of a “class” and an “object”?


Definitions - ProcessA process is an abstraction defining

processor execution resource grouping.

In the days of “batch” processing, processes executed to completion before another process was loaded, then started.


Definitions - ProcessBatch Processing – What had to happen?1. Operator is waiting for input (reading newspaper)2. User writes a program on punch cards.3. Operator takes a tray of punch cards from a stack of jobs.4. Operator loads the cards into the system tray – computer

reads each card into memory.5. Operator starts the job.6. System compiles the code in the job.7. System jumps to the first machine instruction of compiled

code.8. Job runs to completion.9. Operator places output in out box. – Go to 1


Definitions - ProcessBatch Processing – What had to happen?

The computer is only involved in steps 4, 6,7 , and 8.

Where did the OS reside?


Definitions - ProcessSynchronous Processing – What had to

happen?1. System is waiting for input.2. User writes program and compiles it on the computer.3. User starts execution of the written program.4. System loads program into memory for execution.5. System jumps to the first instruction of program.6. Program runs to completion.7. System jumps back to state of waiting for input.


Definitions - ProcessPreemptive Multitasking – What has to

happen?1. System is waiting for input (running SETI program).2. User writes program and compiles it on the computer.3. User starts execution of the written program.

A. System loads next program into memory for execution.B. System jumps to the first instruction of program.C. Program runs to till time slice time-out or program done.D. System jumps back to “A”.


Definitions - ThreadIn an older OS, each process had:• Address space

– stack– heap

• Single “thread” of control. – Serial execution of instructions in the

executing program.


Definitions - ThreadA “thread” breaks the grouping

provided by a process of “resources and execution”.

A process main thread can “spawn” off threads. Each thread the process spawns will share the resources of the process.


Definitions - ThreadQ:

1.Define Process2.Define Thread3.Define Context


Processes and ThreadsProcesses:

Create Thread

Create primary thread1. Set up run-time stack.2. Set up memory segment.3. Create context.4. Set ready for execute.

Run-TimeStack

ProcessMemory

Primary Thread


Processes and ThreadsProcesses:• A process has a memory segment assigned to it.• A process has a primary run-time stack assigned

to it (used by the primary thread).• Processes must communicate with special

mechanism.– A process has protected code segment.– A process has a protected memory segment.– The OS must provide a special address space for

interprocess communication.


Processes and ThreadsProcesses and Threads:

Create Thread

Create Thread1. Set up run-time stack.2. Create context.3. Set ready for execute.

• Execution is controlled by parent process.

• Parent process must retain a leash on child threads.

• Threads may communicate through the process memory segment.

Run-TimeStack

ProcessMemory

Primary Thread

Run-TimeStack

Thread 2


Processes and ThreadsThreads:• Threads share the memory segment of a process

(data and code).• A Thread is assigned it’s own run-time stack by

the Operating System.• Threads can communicate with other threads

contained in the same process using data structures in the process memory segment.

• The data structures must be “thread-safe”.• The process and thread code must be written

“thread-safe”.


Processes and ThreadsA preemptive multitasking OS is

made up of several processes:1. Foreground processes – those that

require user interaction (shell, GUI, etc.).2. Background processes – those that run in

the background, performing their jobs without user intervention (mailers, network monitors, printing monitors, etc).


Processes and ThreadsBackground processes that help the

OS with some task are called daemons (not demons).

Daemon - an attentive benevolent entity. An intermediary between gods and men.


Processes and ThreadsProcess/Thread States:• Suspend – Processes that are waiting for some

event to occur (i.e. I/O, Time of Day, etc.)• Active State – Process is ready to run. Waiting

for CPU.• Execute State – Switched into the CPU and has

control of the execution unit.• Blocked State – Process that are waiting for

access to a dynamic resource.


Processes and ThreadsProcess/Thread/Fiber States:

Keep in mind the processing hierarchy:Processes contain threadsThreads contain fibers

– Process state change implies thread state change– Thread state change implies fiber state change

The inverse is not true.

Processes and ThreadsProcess Creation:

Scheduler creates the process control block (TCB) and places it in the suspend list.

– Create process data segment.

– Create process code segment.

– Load op codes from disk into memory

– Build run-time stack.


Suspend

Active

System R

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Resource Available

Suspend Cleared - Process Still BlockedProcess Exit

Process State Transition Analysis:


Processes and ThreadsProcess Suspend -

Activation:

Process System Call is done and process is ready for execute.

Scheduler removes the TCB from the suspend list and places it on the active list.

Suspend

Active

System Req. Complete

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Processes and ThreadsProcess Dispatch:

TCB is switched into the CPU based on the scheduling algorithm.

– Round Robin– Priority Based– Etc.

Suspend

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Processes and ThreadsProcess Preempt:

TCB is switched out of the CPU. Scheduler uses the scheduling algorithm to determine next TCB to be switched in.

Suspend

Active

System R

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Logical Block

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Processes and ThreadsProcess Execute -

Block:

Process requests some unavailable resource.

TCB is switched out of the CPU and placed in the “blocked” list.

Suspend

Active

System R

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Processes and ThreadsProcess Execute -

Suspend:

Process makes a system call that requires a lengthy operation.

TCB is switched out of the CPU and placed in the “Suspend” list.

Suspend

Active

System R

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Execute

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Process Entry

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Resource Request

Logical Block

Resource Available



Processes and ThreadsProcess Suspend -

Blocked:

Process system call is done but the resource required to complete the call is unavailable.

TCB is moved from the “Suspend” list to the “Blocked” list.

Suspend

Active

System R

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Process Entry

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Logical Block

Resource Available

Suspend Cleared - Process StillBlockedProcess Exit


Processes and ThreadsProcess Block-

Activated:

Requested resource becomes available.

TCB is moved from the Blocked list back to the Active List.

Suspend

Active

System R

eq. Complete

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Execute

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Process Entry

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Logical Block

Resource Available



Processes and ThreadsProcess Active -

Block:

Currently dispatched process “Blocks” an active process.

TCB is moved from the Active list to the Blocked List.

Suspend

Active

System R

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Processes and ThreadsProcess Active -

Suspend:

Currently dispatched process “Suspends” and active process.

TCB is moved from the Active list to the Suspend List.

Suspend

Active

System R

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Logica

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Execute

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Blocked

Logical Suspend

Process Entry

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Logical Block

Resource Available



Processes and ThreadsProcess Terminate:

Process is terminated by another process. Process exits.

• Process resources are returned to the OS.

• Process memory segment is cleaned up.

• Process run-time stack is cleaned up.

Suspend

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Processes and ThreadsQ:

1. What states are required for implementation of a multitasking kernel?


Processes and Threads• Processes must be managed in kernel

space. Why is this the case?• It is possible to manage threads in

user space. Why is this the case?• What are advantages of managing

threads in user space vs. kernel space?


OS Requirements for Process ImplementationTo exist, a process needs:• Process Control Block – Contains the

context of the process main thread.• Process Code Segment – Contains the code

relocated to a physical address space.• Process Data Segment – Contains the

memory “heap” assigned to the process.• Process Run-time Stack – Contains the call

stack assigned to the process main thread.


Processes and ThreadsWhen a process gets suspended it can be

moved out of physical memory.

This activity is called “swapping”.

Don’t confuse swapping with virtual memory.


Processes and ThreadsSwapping of a process involves taking

all the process components (TCBs, Code Segment, Data Segment, Heap, and run-time stack(s)) and moving them from memory to disk.


Processes and ThreadsPaging of a process involves taking a

piece of a process (e.g. a page of memory) and moving it from memory to disk.


Processes and ThreadsThe disk space where the process

components get written to disk is called the swap file (page file virtual memory systems).

• Windows 7 uses virtual memory. The page file is normally on the “C:” drive.

• UNIX sets aside a partition on the hard drive.


Processes and ThreadsAn OS that has a poorly tuned process

management scheme will do what is known as “thrashing”.

Thrashing – The OS spends more time moving process elements from disk to memory and back again than it does running processes.


Processes and ThreadsQ:1. Define Swapping2. Define Paging3. Does swapping require virtual

memory? Why or why not?


OS Requirements for Multitasking Implementation• Data Structures of TCBs for process

control:– Suspend List (a linked list)– Active Queue (priority queues, circular

linked list)– Blocked List (a table of queues keyed on

a resource ID)


OS Requirements for Multitasking Implementation• Interrupt Service routine to

handle:– I/O devices– CPU interrupts

» Interrupt asserted for task switching» This ISR calls the dispatcher


OS Requirements for Thread ImplementationKernel Space Threads: • Process needs to maintain some

reference to threads it owns in a thread table associated with the TCB.

• The Kernel schedules threads based on run-time activity of controlling processes.


OS Requirements for Thread ImplementationUser Space Threads: • Process needs to maintain some reference to threads it owns

in a thread table. • User space system calls are made to perform thread

management. These calls cause the process to change “thread” context.

• The kernel only handles the scheduling of processes.• User space thread management is analogous to

“Cooperative Multitasking”. If a thread in a process “runs amok”, the process threads will starve.


OS Requirements for Thread ImplementationHybrid Threads (one-scheme): • Processes live in kernel space.• Threads are managed in kernel space.• User space threads are provided, known

as “light-weight” threads (fibers).


OS Requirements for Thread ImplementationHybrid Threads (another scheme): • Processes live in kernel space.• Threads live in user space.• When a thread blocks, it gives the kernel

what is basically a pointer to a call-back routine. The kernel then calls the “call-back” when the thread resource become available.


Process SynchronizationRace Conditions:

Two or more threads are reading or writing to a shared resource. The final result depends on who writes at what precise time.


Process SynchronizationRace Conditions: - Example

1. Process Thread A writes to a memory area then gets switched out.

2. Process Thread B writes to the same memory area then gets switched out.

3. Process Thread A gets switched back in and uses the memory area to make some calculation.

4. Process Thread A has no knowledge that Process Thread B has changed the value used for the calculation.



Consider the function:

void swap( int& intOne, int& intTwo ){

static int temp;temp = intTwo;intTwo = intOne;intOne = temp;

}



1. Initial Conditions:Thread T1 – varOne = 5, varTwo = 8Thread T2 – varOne = 15, varTwo = 25

2. Thread T1 calls swap(varOne, varTwo);3. 8 gets put in temp in the swap procedure.4. Thread T1 gets switched out of the CPU.5. Thread T2 gets switched in and calls swap(varOne, varTwo);6. 25 gets put into temp. Thread T2 finishes swap.7. Thread T1 gets switched back in. swap finishes.8. In Thread T1, varOne now contains 25 and varTwo contains 5.

T1 has no idea there was a problem.



Race conditions are a real problem to track because the behavior changes based on CPU Load, current active processes, amount of memory, etc.



Necessary conditions for avoidance:1. No two threads may be simultaneously accessing a

shared resource.2. No assumptions should be made about the CPU as a

resource.3. Any threads busy accessing a shared resource cannot

be blocked by another thread (with the scheduler as an exception).

4. No thread should have to wait indefinitely for a shared resource.


Process SynchronizationCritical Regions:

Create an area in a program where a thread cannot be interrupted while it is executing.

This way, a thread will be guaranteed that it completes it’s task before any other thread has the chance to mess it up.



A critical region can be implemented by writing a function call such that:»first thread can enter the code»subsequent threads will be blocked

until the first thread is done



Consider the scenario:1. Thread 1 enters the critical region.2. Thread 2 will not get CPU cycles.3. Thread 1 GPFs inside the critcal

region.4. What now happens to Thread 2?


Process SynchronizationMutual Exclusion – Busy Waiting1. Disabling Interrupts

Dangerous because of the disabling thread dies, our system needs to be reset.

2. Using a locking variable.Does not work properly since there is a possibility that between checking the lock and setting it, the thread gets switched out.


Process SynchronizationMutual Exclusion – Busy Waiting3. Strict Alteration

Threads grant other threads by setting a “turn” variable in a cooperative manner. “turn” is a shared variable.Called “Strict Alteration” because thread code needs to be specifically altered for exclusion.

While (TRUE)

{

while (turn != 0);

criticalRegion( );

turn = 1;

nonCriticalRegion( );

};

While (TRUE)

{

while (turn != 1);

criticalRegion( );

turn = 0 ;

nonCriticalRegion( );

};

Thread 0: Thread 1:


Process SynchronizationMutual Exclusion – Busy Waiting

3. Strict AlterationWhile Thread 1 is waiting for Thread 0, it is “polling” the turn variable.This uses up processor CPU cycles.


Process SynchronizationMutual Exclusion – Busy Waiting4. Petersons Algorithm.

Similar to Strict Alteration.Keep a list of PIDs for processes that want a turn for use of a shared resource.


Process SynchronizationMutual Exclusion – Busy Waiting4. Petersons Algorithm - Globals

#define MAX_PROC 500 // Maximum number of processes

struct petersonVars{

int turn;int interested[MAX_PROC];

};


Process SynchronizationMutual Exclusion – Busy Waiting4. Petersons Algorithm.

void enterRegion(int PID, petersonVars* peteVar){

int other = peteVar->turn; // Capture who is currently // using the resourcepeteVar->interested[PID] = TRUE; // Set PID interest to truepeteVar->turn = PID; // Set turn to our PID

//// Loop until it is our turn and "we" are the interested process.//while( peteVar->turn == PID && peteVar->interested[other] == TRUE ) // spin-lock{

if (peteVar->turn != PID) // Somebody else got our turn{

other = peteVar->turn;// Get the current userpeteVar->turn = PID; // Set the turn to us

}}

}


Process SynchronizationMutual Exclusion – Busy Waiting4. Petersons Algorithm (cont’d) - Use

void thread( void *dummy ){

thrdArgs* threadArgs = (thrdArgs*)dummy;

petersonVars* pvars = threadArgs->pVars;

int PID = threadArgs->pid;

while( 1 ){

enterRegion(PID, pvars);

shared_var = PID;

cout << "Child thread " << PID << endl;

if ( shared_var != PID ){

cout << "OOPS!!!! ****** Child thread " << PID << endl;}

leaveRegion(PID, pvars);}

}


Process SynchronizationMutual Exclusion – Busy Waiting4. Petersons Algorithm (cont’d).

void leaveRegion(int pid, petersonVars* peteVar){

peteVar->interested[pid] = FALSE; // We are done. Set our interest to FALSE;};


Process SynchronizationQ:1 – What is the difference between Strict Alteration

and Peterson’s Algorithm?


Process SynchronizationMutual Exclusion – Busy Waiting4. TSL instruction.

This is a special instruction that sets a memory area to 1 and returns the previous value in one execution cycle.

Test and SetCheck the valueIf the value was 1, loop until it test and set returns a previous value of 0.


Process SynchronizationMutual Exclusion is a valid method for resource sharing.The major drawback with mutual exclusion is the requirement of a busy wait.Low priority processes (or threads) may never gain access to a resource since they will never be the process that gets the resource access flag.The inverse problem is once a low priority process (or thread) gains access to a resource, all higher priority processes must wait for the slow one to finish.


Process SynchronizationProducer – Consumer

Two processes (or threads) share a fixed size buffer.One process puts information into the buffer (producer).The other process takes information out of the buffer (consumer).The producer will block when the buffer is full.The consumer will block when the buffer is empty.Either will block if the other has access of the buffer.


Process SynchronizationProducer – Consumer – Code sample

// Shared resource – Bounded Bufferint buffer[MAX_ITEMS];int nextInsert = 0; // Next index to produce toint nextRemove = 0; // Next index to consume fromint count = 0; // Number of items in the buffer



// Producer…while (1){

bb.addItem(rand());}

// Bounded Buffer…void addItem (int itemToAdd){

if ( count = = MAX_ITEMS ) sleep( ); // Suspendbuffer[nextInsert] = itemToAdd; // Produce++nextInsert %= MAX_ITEMS; // Increment the insert pointcount++; // Increment the countif (count > 0 ) wakeup(consumer); // Activate the consumer

}



// Consumer…int consumerInt;while (1){

consumerInt = removeItem();}

// Bounded Buffer…int removeItem ( ){

int retVal;if ( count = = 0 ) sleep( ); // SuspendretVal = buffer[nextRemove]; // Consume++nextRemove %= MAX_ITEMS; // Increment the remove pointcount--; // Decrement the countif (count < MAX_ITEMS )

wakeup(producer); // Activate the producerreturn retVal;

}


Process SynchronizationProducer – ConsumerWe can get into trouble here because of the unconstrained

access to the shared resource variables.If the count is set to 0, the consumer sleeps, but the sleep state is not fully entered since the scheduler switches out the Consumer.Now the Producer puts a value in the buffer and increments the count. Count == 1, so the producer signals the consumer to wakeup.Since the consumer is not yet asleep, the signal is ignored. The consumer is then switched back in and finishes going to sleep, never to wake again.


Process SynchronizationProducer – ConsumerThe problem exist because the consumer lost the

wakeup signal since it was still in the process of going to sleep.

To avoid this problem, the “wakeup” signals must be saved until processes are in a state that will allow the signal to be correctly applied by the OS.


Process SynchronizationCounting Semaphores

A semaphore is a construct that allows the saving of “wakeup” signals.



Two operations:1. dec( ) – same as “down” in the

book.» If semaphore count == 0

Put “this” thread on “this” semaphore block list.

Remove “this” thread from the active thread list.

» Decrement count and continue on.



Two operations:2. inc( ) – same as “up” in the book.

» Increment semaphore count.» If a thread is suspended waiting for

“this” semaphore Choose a waiting thread and move it to the

active list.



For the semaphore to work properly, the checking of the count, incrementing it and performing some “suspend” or “activate” operation, must be done without interrupt.To do this, the OS must disable interrupts before the operation starts, then enable them when it is done.


Process SynchronizationQ:1. Write the pseudo code for a

counting semaphore.


Process SynchronizationCounting SemaphoresThe Producer/Consumer problem can be solved

using semaphores:• Semaphore called full – counts the number of

full slots.• Semaphore called empty – counts the number of

empty slots.• Semaphore called access – initialized to 1 to

control access to shared structures.


Process SynchronizationProducer – Consumer – semaphore impl.

class semaphore{private:

int count;TCB* tcb_Q;

public:int inc( ); // increment and activateint dec( ); // decrement and suspendint getCount( ); // return semaphore state

};


Process SynchronizationProducer – Consumer – semaphore impl.

// Shared resourcesclass BoundedBuffer{private:

int buffer[MAX_ITEMS];int nextInsert = 0; // Next index to produce toint nextRemove = 0; // Next index to consume from

semaphore mutex(1); // resource access controlsemaphore empty(MAX_ITEMS); // empty behavior controlsemaphore full(0); // full behavior control

Public:void AddItem(int newItem);int RemoveItem( void );

}



// Producerstatic BoundedBuffer bb;

while (1){

bb.AddItem(rand( )); // Produce}



// Add Itemvoid BoundedBuffer::AddItem (int newItem){

empty.dec( ); // Suspend if buffer is fullmutex.dec( ); // Wait if shared data is in usebuffer[nextInsert] = newItem; // Add to the buffer++nextInsert %= MAX_ITEMS; // Increment the insert pointmutex.inc( ); // Done with shared datafull.inc( ); // Kick off any consumers waiting

}



// Consumerextern BoundedBuffer bb;int consumerInt;…while (1){

consumerInt = bb.RemoveItem ( ); // Consume}



// Consumerint BoundedBuffer::RemoveItem(void){

int retVal;full.dec( ); // Suspend if buffer is emptymutex.dec( ); // Lock up the shared dataretVal = buffer[nextRemove]; // Remove an item++nextRemove %= MAX_ITEMS; // Increment the remove pointmutex.inc( ); // Unlock shared dataempty.inc( ); // Kick off any producersreturn (retVal);

}


Process SynchronizationMutexes

A semaphore provides a general purpose method to control multiple access to resources.When simple process exclusion is all that is required, a mutex can be used.A Mutex is a counting semaphore with count initialized to “1”;


Process SynchronizationMutexesA Mutex has two methods:1. lock( ) – decrement the mutex value

to 0. If already 0, put the process to sleep;

2. unlock( ) – increment the mutex value from 0 to 1;


Process SynchronizationQ:1. What is the difference between a

Semaphore and a Mutex?


Process SynchronizationMonitorsA Monitor is a programming

language construct providing the functionality of a semaphore, however, a monitor is a programming language construct.


Process SynchronizationMonitorsE.g. a monitor requires support of a

compiler.

It has been implemented in various flavors of Pascal, Modula, and Java.


Process SynchronizationMonitorsA monitor is an encapsulation of

procedures and data. Entry into the monitor is done through a public procedure.


Process SynchronizationMonitorsCharacteristics:

All data variables are private.Access to the monitor is done only through public methods.Only one process may be active in the monitor at any given time.


Process SynchronizationMonitors

Once a process/thread is inside the monitor, it must engage in cooperative multitasking using

cwait(c) – suspend process based on some condition ccsignal(c) – inform OS that any processes waiting on c may resume.



A Monitor provides an abstraction where data and a set of associated procedures are protected through compiler generated signaling primitives.

Monitor

Condition c1 Q

Condition c2 Q

Condition cn Q

cwait(c1)

cwait(c2)

cwait(cn)

Entry Q

Local Data

Condition Variables

Procedure 1

Init Code

Exit

Procedure n

Monitor WaitingArea



Remember, what we are dealing with here is defined entry points to data.The interface defines the points where a process may gain access.The process has no control over when the monitor will block it.



A process (thread) enters the monitor by calling one of the public functions.The process (thread) will end up in the waiting area if it calls cwait(c) and c is a condition not yet met.As soon as the condition c is met, if there are processes (threads) queued waiting on it, the next one in the queue will be unblocked.A process (thread) will cause a condition to be met by calling csignal(c).


Process SynchronizationMessage Passing

Two processes or threads will use “send( )” and “receive( )” to pass an agreed upon message.



send(message msg) – send a message to a shared message area. Block if the buffer is full.message receive( ) – receive a message from a shared message buffer. Block if the buffer is empty.



Process 1 Process 2

Message Queue

Send Receive



The OS manages the shared memory area containing message queues.The OS manages the blocking and activating processes using the message queue.



What is the quality of service you are capable of providing?

Guaranteed delivery?Ack/Nak protocol?How can you guarantee delivery for deadlock avoidance?

How will you provide security?


Process SynchronizationBarriers

Barriers apply to a group of processes or threads within a process.A Barrier is used to make sure a group of processes (or threads) will all wait for a particular synchronization point.This concept is useful for problems that are divided into parallel computations.


Process SynchronizationBarriers1. A process spawns off threads to

perform a computation.2. The OS assigns threads to run on one

of 32 processors.3. The parent process then waits for all

children threads to finish before continuing on using the results of the computation.


Communication - IPCShared Memory

A segment of memory that can be accessed from two different process context.The OS must manage access to this memory segment.Address of a shared memory segment will not resolve to the process heap.


Communication - IPCShared Memory

There are usually no synchronization primitives built into a shared memory segment.The shared memory must be protected through the use of some synchronization primitive (see above).


Communication - IPCNamed Pipes

A named pipe is a message passing mechanism.The message queue (the pipe) is usually implemented as a file on a disk.

This allows IPC to have a persistent behavior (communication could occur across process life).Disk access is slower than memory


Communication - IPCNamed Pipes

There are also implementations that can cross network boundaries._pipe( ), _popen( ), _wpopen( ) in the MSDN to see Microsoft implementation.


Classic IPCDining Philosophers

1. Five philosophers are at a table

2. Each plate has two forks next to it.

3. For a philosopher to eat, he/she must have two forks.

4. The life of a philosopher consists of eating and thinking.



5. When a philosopher gets hungry, he first tries to acquire his left fork, then his right fork.

6. If he is successful, he eats for a while, then puts down the forks and continues to think.

Write a program that will allow all philosophers to live a well fed and well thought life.



Try #1:1. Take left fork. If unavailable, block until it is available.2. Take right fork. If unavailable, block until it is available.3. Eat.4. Put down right then left fork.5. Think.6. Repeat (go to 1).

Why does this fail?



Try #2:1. Take left fork. If right fork is unavailable, put down the left

fork and wait for a while. Try again.2. Take right fork.3. Eat.4. Put down right then left fork5. Think6. Repeat (go to 1).

Why does this fail?



Try #2:If each philosopher waited a random

amount of time, the chances of a deadlock would be reduced.

This is what CSMA/CD does in the Ethernet protocol. (Carrier Sense Multiple Access/Collision Detection)



Try #3:Use an array of state information to keep track of the state each philosopher is in – Eating, Thinking or Hungry (trying to acquire forks to eat).A Philosopher can only move into the Eating state if neither neighbor is eating.



Try #3:Both neighbor philosophers must be checked before allowing the philosopher to eat.This analogy applies to processes in the CPU

An eating philosopher is a process in the CPU. The forks are shared resources requiring exclusive access for a process to run.



User space solution:Each fork is protected by a semaphore.The semaphore is implemented by the OS.


Classic IPCReaders and Writers

Many processes and contained threads are attempting access to the same information.At any given time, any number of processes and contained threads may read the information source.Only one thread may write to the information source at any time.


Classic IPCReaders and Writers

Any reader can gain access.A writer must wait until all readers are done before it can be allowed to write to the information source.This problem is typical of DBMS systems.


Classic IPCSleeping Barber

There is a barber shopone barber.one barber chair.“n” chairs for waiting customers.



If there are no customers present, the barber sits down in the barber chair and sleeps.When a customer arrives, he wakes up the barber to get a haircut.If an additional customer arrives, he will sit down if there are empty chairs. If there are no chairs, the customer will leave.



The problem is to allow the customers to get their hair cut without getting into race conditions.



One solution – use three semaphoresA semaphore to count the number of waiting customers.A semaphore to keep track of the number of barbers.A Mutex (binary semaphore) to keep track of when a customer is waiting for for the barber to start cutting.Waiting – a counter to check the number of waiting customers.


Communication - PTCVia Shared Heap

Process-to-Thread communication can be done through the shared heap.The shared heap acts as a shared memory segment, however, it does not require intervention of the OS.


Communication - PTCVia Shared Heap1. Main thread creates shared data

structures in the shared heap.2. Main thread spawns child threads and

passes the address to the created data structures to the child threads.

3. Child threads use the shared memory to post information to the parent thread.


Communication - TTCVia Shared Heap1. Parent thread sets up shared heap data

structures.2. Parent thread passes address of

structures to child threads.3. Threads then use shared structures to

pass information.4. Shared structures must be protected.


Communication - TTCQ:1. What is the difference between

inter-process communication and thread to thread communication?

…yes, one is between processes and the other is between threads….but what else?


SchedulingGoals

Maximize CPU utilization.Minimize process waiting.Enforce system processing priorities.Provide all system resident processes with fair resource allocation.


SchedulingThe area of

scheduling impact is shown in blue.

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SchedulingDefinitions

Scheduler – The OS process or thread that is responsible for moving a TCB from the Active list to the an area where it will be dispatched next into the CPU.Scheduling Algorithm – The policy the scheduler uses to determine what TCB to move.


SchedulingDefinitions

Compute Bound Process – A process that spends the majority of it’s time waiting for the CPU.I/O Bound Process– A process that spends the majority of it’s time waiting for some I/O device.


SchedulingBounded Curve:

CPU Speed

Deg

ree

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ound

edne

ssCPU Bounded

I/O Bounded

As CPUs get faster, they outpace the increases in I/O devices, causing processes to spend more of their run-time waiting for I/O devices.


SchedulingQ:1.What happens to a compute bound

process as CPU speed increases?2.What happens to an IO bound

process as CPU speed increases?


SchedulingWhen does the scheduler get invoked:1. Process/Thread Creation – A new process or

thread gets created by the OS. What process will run next?

2. Process/Thread Exit – What process or thread will run next?

3. I/O block or process/thread synchronization block – The current process or thread is suspended or blocked. What process or thread will run next?


SchedulingWhen does the scheduler get invoked:4. I/O interrupt – An I/O device is

now available. What process or thread will run next?

5. Clock interrupt – The current process or thread turn is over. Next?


SchedulingScheduling algorithms apply to both the

scheduling of CPU cycles from a “process” point of view (e.g. a collection of 1 or more threads), a “thread” point of view (e.g. how much “process” time should a contained thread get).


SchedulingFirst Come – First Serve

The first process and/or contained threads in the active queue will run in the CPU to completion.This algorithm defines a non-preemptive system.


SchedulingShortest Process First

Based on historical run-time information or job-time estimates.Run the “shortest” job first.This is a non-preemptive algorithm.Only works if all processes to run are pre-determined.Typically used for older batch processing systems.


SchedulingShortest Process First (preemptive

version):Run the process with the estimated “shortest” time left.Must keep a running estimate of how much time a job will take after each run.


SchedulingThree Level Scheduling

Keep three queues1. Entering processes2. Processes “swapped” out to disk

(which ones should come in next?)3. CPU scheduling – from memory to

the CPU (active dispatch to execute).


SchedulingThree Level Scheduling

This requires tracking of what types of processes are in the system

I/O bound processes (these may “thrash” the system).CPU bound processes (these may be process pigs).How much CPU time has the process used.How much I/O resource has the process used.Etc.


SchedulingRound Robin Scheduling

Keep a circular linked list of TCBs.Each TCB gets a turn in the CPU in a circular fashion.Must balance context switching overhead with process run-time.



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SchedulingPriority Scheduling

Each process or contained thread is assigned a priority.The process or contained threads with the highest priority get to run for a time slice.To prevent the highest priority process (thread) from stealing the CPU, the OS may decrease priority based on CPU time used.



I/O bound processes (threads) should get a higher priority when the I/O device becomes available.Simple algorithm – Increase the priority to 1/f where f = fraction of time slice used by the process.



Example:If in a time slice of 30 ticks, a process

uses 2 ticks, the priority will be increased by 15.

e.g. 1/(2/30) = 15


SchedulingPriority Queue Scheduling

TCBs are assigned to queues.Each queue has a priority.A TCB from a higher priority queue will get a larger time quantum multiple than a TCB from a lower priority queue.


SchedulingGuaranteed Scheduling

Guarantee a certain level of CPU time to a user process.Given the number of processes and contained threads running on the CPU, calculate the amount of time each process will get.



Example:• There are “n” processes (or

threads) running on a system.• Guarantee each (process or

thread) will receive 1/nth of the CPU time.



Example (cont’d):• The OS must keep track of how

much time in the CPU each process (and threads) uses.



Example (cont’d):For instance:

4 processes on a CPU.Each process gets ¼ (.25) of the cycles.2 processes blockedProcess “2” has used 10 of 100 possible CPU cycles -> 1/10 -> .10 -> .10/.25 -> ratio of .4



Example (cont’d):Process 1 has used 50/100 cycles = .50.Process 1 has used .25 excess cycles -> ratio of .50/.25 -> ratio of 2.0Dispatch process with the lowest ratio.


SchedulingLottery

Processes (threads) are chosen from a pool based on a random decision.Processes (threads) can be given more “tickets” to increase their chances of obtaining the right to be dispatched.


SchedulingFair Share

Base scheduling on not only the process but also the process origination.If user A starts up 1 process and user B starts up 9 User A process will get 50% of the CPU time User B process will divide the remaining 50%.


SchedulingReal-Time Policies

Hard real-time policiesConfigure a process to react in a given amount of time.Failure to meet this deadline is considered a failure.

Soft real-time systemsTime constraint must be met within certain tolerances.

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