concurrencycs-502 fall 20071 concurrency cs-502 operating systems fall 2007 (slides include...

ConcurrencyCS-502 Fall 2007 1

Concurrency

CS-502 Operating SystemsFall 2007

(Slides include materials from Operating System Concepts, 7th ed., by Silbershatz, Galvin, & Gagne and from Modern Operating Systems, 2nd ed., by Tanenbaum)


Concurrency

• Since the beginning of computing, management of concurrent activity has been the central issue

• Concurrency between computation and input or output

• Concurrency between computation and user• Concurrency between essentially independent

computations that take place at same time• Concurrency between parts of large computations

that are divided up to improve performance• …


Example – 1960s

• Programmers tried to write programs that would read from input devices and write to output devices in parallel with computing

• Card readers, paper tape, line printers, etc.

• Challenges• Keeping the buffers straight

• Synchronizing between read activity and computing

• Computing getting ahead of I/O activity

• I/O activity getting ahead of computing


Shared Computing Services

• Multiple simultaneous, independent users of large computing facilities

• E.g., Time Sharing systems of university computing centers

• Data centers of large enterprises• Multiple accounting, administrative, and data

processing activities over common databases

• …


Modern PCs and Workstations

• Multiple windows in personal computer doing completely independent things

• Word, Excel, Photoshop, E-mail, etc.

• Multiple activities within one application– E.g., in Microsoft Word

• Reading and interpreting keystrokes

• Formatting line and page breaks

• Displaying what you typed

• Spell checking

• Hyphenation

• ….


Modern Game Implementations

• Multiple characters in game• Concurrently & independently active

• Multiple constraints, challenges, and interactions among characters

• Multiple players


Technological Pressure

• From early 1950s to early 2000s, single processor computers increased in speed by 2 every 18 months or so

• Moore’s Law

• Multiprocessing was somewhat of a niche problem

• Specialized computing centers, techniques


Technological Pressure (continued)

• No longer!

• Modern microprocessor clock speeds are no longer increasing with Moore’s Law

• Microprocessor density on chips still is!

multi-core processors are now de facto standard

• Even on low-end PCs!


Traditional Challenge for OS

• Useful set of abstractions that help to – Manage concurrency– Manage synchronization among concurrent

activities– Communicate information in useful way among

concurrent activities– Do it all efficiently


Modern Challenge

• Methods and abstractions to help software engineers and application designers – Take advantage of inherent concurrency in

modern systems– Do so with relative ease


Fundamental Abstraction

• Process

• … aka Task

• … aka Thread

• … aka Task

• … aka [other terms]


Process (a generic term)

• Definition: A particular execution of a program.

• Requires time, space, and (perhaps) other resources

• Separate from all other executions of the same program

• Even those at the same time!

• Separate from executions of other programs


Process (a generic term – continued)

• …• Can be

• Interrupted• Suspended• Blocked• Unblocked• Started or continued

• Fundamental abstraction of all modern operating systems

• Also known as thread (of control), task, job, etc.• Note: a Unix/Windows “Process” is a heavyweight concept

with more implications than this simple definition


Process (a generic term – continued)

• Concept emerged in 1960s

• Intended to make sense out of mish-mash of difficult programming techniques that bedeviled software engineers

• Analogous to police or taxi dispatcher!


Background – Interrupts

• A mechanism in (nearly) all computers by which a running program can be suspended in order to cause processor to do something else

• Two kinds:–• Traps – synchronous, caused by running program

– Deliberate: e.g., system call

– Error: divide by zero

• Interrupts – asynchronous, spawned by some other concurrent activity or device.

• Essential to the usefulness of computing systems


Hardware Interrupt Mechanism

• Upon receipt of electronic signal, the processor• Saves current PSW to a fixed location• Loads new PSW from another fixed location

• PSW — Program Status Word• Program counter• Condition code bits (comparison results)• Interrupt enable/disable bits• Other control and mode information

– E.g., privilege level, access to special instructions, etc.

• Occurs between machine instructions• An abstraction in modern processors (see Silbershatz, §1.2.1

and §13.2.2)


Interrupt Handler

/* Enter with interrupts disabled */Save registers of interrupted computationLoad registers needed by handler

Examine cause of interruptTake appropriate action (brief)

Reload registers of interrupted computationReload interrupted PSW and re-enable interrupts

or

Load registers of another computationLoad its PSW and re-enable interrupts


Requirements of interrupt handlers

• Fast• Avoid possibilities of interminable waits• Must not count on correctness of interrupted

computation• Must not get confused by multiple interrupts in

close succession• …

• More challenging on multiprocessor systems


Result

• Interrupts make it possible to have concurrent activities at same time

• Don’t help in establishing some kind of orderly way of thinking

• Need something more


• Hence, emergence of generic concept of process – (or whatever it is called in a particular operating

system and environment)


Information the system needs to maintain about a typical process

• PSW (program status word)• Program counter• Condition codes• Control information – e.g., privilege level, priority,

etc

• Registers, stack pointer, etc.• Whatever hardware resources needed to compute

• Administrative information for OS• Owner, restrictions, resources, etc.

• Other stuff …


Process Control Block (PCB)(example data structure in an OS)


Switching from process to process


Result

• A very clean way of thinking about separate computations

• Processes can appear be executing in parallel

• Even on a single processor machine

• Processes really can execute in parallel• Multiprocessor or multi-core machine

• …


Process States


The Fundamental Abstraction

• Each process has its “virtual” CPU

• Each process can be thought of as an independent computation

• On a fast enough CPU, processes can look like they are really running concurrently


Questions?


Challenge

• How can we help processes synchronize with each other?

• E.g., how does one process “know” that another has completed a particular action?

• E.g, how do separate processes “keep out of each others’ way” with respect to some shared resource


Digression (thought experiment) static int y = 0;

int main(int argc, char **argv)

{ extern int y;

y = y + 1;

return y;}


Digression (thought experiment) static int y = 0;

int main(int argc, char **argv)

{ extern int y;

y = y + 1;

return y;}

Upon completion of main, y == 1


Process 1int main(int argc, char **argv){ extern int y;

y = y + 1;

return y;}

Process 2int main2(int argc, char **argv){ extern int y;

y = y - 1;

return y;}

Assuming processes run “at the same time,” what are possible values of y after both terminate?

Thought experiment (continued)

static int y = 0;


Another Abstraction – Atomic Operation

• An operation that either happens entirely or not at all

• No partial result is visible or apparent

• Non-interruptible

• If two atomic operations happen “at the same time”

• Effect is as if one is first and the other is second

• (Usually) don’t know which is which


Hardware Atomic Operations

• On (nearly) all computers, reading and writing operations of machine words can be considered as atomic

• Non-interruptible• It either happens or it doesn’t• Not in conflict with any other operation

• When two attempts to read or write the same data, one is first and the other is second

• Don’t know which is which!

• No other guarantees• (unless we take extraordinary measures)


Definitions

• Definition: race condition• When two or more concurrent activities are trying to

do something with the same variable resulting in different values

• Random outcome

• Critical Region (aka critical section)• One or more fragments of code that operate on the

same data, such that at most one activity at a time may be permitted to execute anywhere in that set of fragments.


Synchronization – Critical Regions


Class Discussion

• How do we keep multiple computations from being in a critical region at the same time?

• Especially when number of computations is > 2

• Remembering that read and write operations are atomic

example


Possible ways to protect critical section

• Without OS assistance — Locking variables & busy waiting– Peterson’s solution (p. 195-197)– Atomic read-modify-write – e.g. Test & Set

• With OS assistance — abstract synchronization operations

• Single processor

• Multiple processors


Requirements – Controlling Access to a Critical Section

– Only one computation in critical section at a time

– Symmetrical among n computations– No assumption about relative speeds– A stoppage outside critical section does not

lead to potential blocking of others– No starvation — i.e. no combination of timings

that could cause a computation to wait forever to enter its critical section


Non-solutionstatic int turn = 0;

Process 1

while (TRUE) {

while (turn !=0)

/*loop*/;

critical_region();

turn = 1;

noncritical_region1();

};

Process 2

while (TRUE) {

while (turn !=1)

/*loop*/;

critical_region();

turn = 0;


};


Non-solutionstatic int turn = 0;

Process 1

while (TRUE) {

while (turn !=0)

/*loop*/;

critical_region();

turn = 1;


};

Process 2

while (TRUE) {

while (turn !=1)

/*loop*/;

critical_region();

turn = 0;


};

What is wrong with this approach?


Peterson’s solution (2 processes)static int turn = 0;

static int interested[2];

void enter_region(int process) {int other = 1 - process;

interested[process] = TRUE;turn = process;while (turn == process &&

interested[other] == TRUE)/*loop*/;

};

void leave_region(int process) {interested[process] = FALSE;

};


Another approach: Test & Set Instruction(built into CPU hardware)

static int lock = 0; extern int TestAndSet(int &i);

/* sets the value of i to 1 and returns the previous value of i. */

void enter_region(int &lock) {while (TestAndSet(lock) == 1)

/* loop */ ;};

void leave_region(int &lock) {lock = 0;

};


Test & Set Instruction(built into CPU hardware)

static int lock = 0; extern int TestAndSet(int &i);

/* sets the value of i to 1 and returns the previous value of i. */

void enter_region(int &lock) {while (TestAndSet(lock) == 1)

/* loop */ ;};

void leave_region(int &lock) {lock = 0;

};

What about this solution?


Variations

• Compare & Swap (a, b)• temp = b• b = a• a = temp• return(a == b)

• …• A whole mathematical theory about efficacy of

these operations

• All require extraordinary circuitry in processor memory, and bus to implement atomically


With OS assistance

• Implement an abstraction:–– A data type called semaphore

• Non-negative integer values.

– An operation wait_s(semaphore &s) such that• if s > 0, atomically decrement s and proceed.• if s = 0, block the computation until some other

computation executes post_s(s).

– An operation post_s(semaphore &s):–• Atomically increment s; if one or more

computations are blocked on s, allow precisely one of them to unblock and proceed.


Critical Section control with Semaphorestatic semaphore mutex = 1;

Process 1

while (TRUE) {wait_s(mutex);

critical_region();

post_s(mutex);

noncritical_region1();};

Process 2


critical_region();

post_s(mutex);



Critical Section control with Semaphorestatic semaphore mutex = 1;

Process 1


critical_region();

post_s(mutex);


Process 2


critical_region();

post_s(mutex);


Does this meet the requirements for controlling access to critical sections?


Semaphores – History

• Introduced by E. Dijkstra in 1965.

• wait_s() was called P()• Initial letter of a Dutch word meaning “test”

• post_s() was called V()• Initial letter of a Dutch word meaning “increase”


Abstractions

• The semaphore is an example of a new and powerful abstraction defined by OS

• I.e., a data type and some operations that add a capability that was not in the underlying hardware or system.

• Any program can use this abstraction to control critical sections and to create more powerful forms of synchronization among computations.


Process and semaphoredata structures

class State {long int PSW;long int regs[R];/*other stuff*/

}class PCB {

PCB *next, prev, queue;State s;

PCB (…); /*constructor*/~PCB(); /*destructor*/

}

class Semaphore {int count;PCB *queue;

friend wait_s(Semaphore &s);friend post_s(Semaphore &s);

Semaphore(int initial);/*constructor*/

~Semaphore();/*destructor*/

}


Implementation

Ready queue PCB PCB PCB PCB

Semaphore Acount = 0

PCB PCB

Semaphore Bcount = 2


Implementation

• Action – dispatch a process to CPU• Remove first PCB from ready queue• Load registers and PSW• Return from interrupt or trap

• Action – interrupt a process• Save PSW and registers in PCB• If not blocked, insert PCB into ReadyQueue (in some order)• Take appropriate action• Dispatch same or another process from ReadyQueue



Implementation – Semaphore actions

• Action – wait_s(Semaphore &s)• Implement as a Trap (with interrupts disabled)if (s.count == 0)

– Save registers and PSW in PCB

– Queue PCB on s.queue– Dispatch next process on ReadyQueue

else– s.count = s.count – 1;

– Re-dispatch current process


Event wait


Implementation – Semaphore actions

• Action – post_s(Semaphore &s)• Implement as a Trap (with interrupts disabled)if (s.queue != null)

– Save current process in ReadyQueue– Move first process on s.queue to ReadyQueue– Dispatch some process on ReadyQueue

else– s.count = s.count + 1;– Re-dispatch current process


Semaphore Acount = 0

PCBPCB

Event completion


Interrupt Handling

• (Quickly) analyze reason for interrupt• Execute equivalent post_s to appropriate

semaphore as necessary• Implemented in device-specific routines• Critical sections• Buffers and variables shared with device managers

• More about interrupt handling later in the course


Timer interrupts

• Can be used to enforce “fair sharing”

• Current process goes back to ReadyQueue• After other processes of equal or higher priority

• Simulates concurrent execution of multiple processes on same processor


Definition – Context Switch

• The act of switching from one process to another

• E.g., upon interrupt or some kind of wait for event

• Not a big deal in simple systems and processors

• Very big deal in large systems such • Linux and Windows

• Pentium 4Many microseconds!


Complications for Multiple Processors

• Disabling interrupts is not sufficient for atomic operations

• Semaphore operations must themselves be implemented in critical sections

• Queuing and dequeuing PCB’s must also be implemented in critical sections

• Other control operations need protection

• These problems all have solutions but need deeper thought!


Summary

• Interrupts transparent to processes• wait_s() and post_s() behave as if they are

atomic• Device handlers and all other OS services

can be embedded in processes• All processes behave as if concurrently

executing• Fundamental underpinning of all modern

operations systems


Questions?

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