c laus b rabrand © c oncurrency (q3,’06) j an 19, 2006 c oncurrency claus brabrand...

CLAUS BRABRAND © CONCURRENCY (Q3,’06) JAN 19, 2006

CONCURRENCY

Claus Brabrand

[email protected]

[ http://www.daimi.au.dk/~brabrand/ ]


WELCOME !

CLAUS BRABRAND © CONCURRENCY (Q3,’06)[ 3 ]

JAN 19, 2006

Agenda (Course Presentation)

Course Presentation (from Homepage): Aims & Goals Exam (assessment & format) Motivation: ”Why bother learning about Concurrency!?!” Roles (students & teachers) Program (course schedule & weekly schedule) Materials (book & notes)

Week 1: ”Processes and Threads” Concepts Models (FSP) Practice (Java) Models (FSP) (Java) Practice


JAN 19, 2006

Weekly Schedule

Lectures: Mondays 12:15-15:00 (sometimes a bit less)

Practical Programming Labs (aka. “Lab”): Wednesdays 11:15-13:00 [ zuse.127 ]

Theoretical Exercise Classes (aka. “TØ”): Wednesdays 13:15-15:00 [ codd-s.121 ]

Studying: Exercises, Reading, and Thinking(!)

Compulsory homework: Hand-ins (Sunday deadlines), Mini-project

3 hrs/week

2 hrs/week

2 hrs/week

? hrs/week

? hrs/week

Expected total: 15 hrs/week


JAN 19, 2006

Concurrency: Aims & Goals Exam

Motivation Roles Program Project Exam Materials Classes Webboard

Homepage [ http://www.daimi.au.dk/~brabrand/concurrency/ ]


YOUR ROLE (CONT’D)

Active Learning (!)


JAN 19, 2006

Consider the following “alphabet” (60 secs):

Exercise: Now write my office phone#: 89425771 ?

Exercise 1

1. 2. 3. 4. 5.

6. 7. 8. 9. 0.


JAN 19, 2006

Exercise 1 (cont’d)

Suppose I now showed you…:

Exercise: Now write my office phone#: 89425771 ?

1 2 3

4 5 6

7 8 9


JAN 19, 2006

Exercise 1 (cont’d)

So what is the point? Random information is really hard to remember We learn (efficiently) by associating (building)

new unknown information / with (on) old known information

Knowledge is constructed as a result of: Learner’s activity (!)

In short: “You need to study”: i.e. read, make exercises, and think(!)


COLLABORATION

Teaching / Learning Collaboration


JAN 19, 2006

Roles (Teaching/Learning Collaboration)

During learning period (i.e., before the exam): Teaching/Learning collaboration:

You = “Learner” Learning responsibility

Me = “Teacher” (and Teaching Assistant) (To the best of my ability) aid you in learning

During the exam period: You = “Performer”

Demonstrate what you have learned

Me = “Evaluator” (Neutrally) assess your level of understanding


MOTIVATION

“Why bother learning about Concurrency?!?”


JAN 19, 2006

What is a Concurrent Program?

A sequential program has a single thread of control

A concurrent program has multiple threads of control:

Perform multiple computations in parallel Control multiple external activities which

occur at the same time


JAN 19, 2006

Why Concurrent Programming? More appropriate program structure

Concurrency reflected in program

Performance gain from multiprocessing HW Parallelism

Increased application throughput An I/O call need only block one thread

Increased application responsiveness High-priority thread for user requests


JAN 19, 2006

Concurrency vs. Sequentiality

Sequential programming: Inherently deterministic Describe computation as a “reduction” of expressions to values Termination often desirable Resulting value is of primary interest and focus

Concurrent programming: Inherently non-deterministic Describe execution as “process evolution” Describe possible executions (aka. execution traces) Describe possible interactions during execution Describe interaction with an environment Non-termination often desirable (e.g. Op.Sys., Control sys, Cell-phone, …) Resulting “value” is not (necessarily) interesting


JAN 19, 2006

Concurrency is much Harder

Harder than sequential programming: Huge number of possible executions Inherently non-deterministic Parallelism conceptually harder

Consequences: Programs are harder to write(!) Programs are harder to debug(!) Errors are not always reproducible(!) New kinds of errors possible(!):

Deadlock, starvation, priority inversion, interference, …


JAN 19, 2006

Example Concurrency Problems

Therac-25 Radiation Therapy ’85-’87

Massive overdoses (6 deaths / amputations)!

Mars Pathfinder July ’97

Periodic resets (on mars)!

Win95/98 w/ 3rd-Party Device Drivers late ’90es

Dysfunction (“blue screen of death”)!


JAN 19, 2006

Concurrency Problems (cont’d)

Mobile Phones ’00-…

Freeze and odd behaviors (really annoying)!

Cruise Control System Model ’86 [Grady Booch]

Accellerated after car ignition (car crashes)!


JAN 19, 2006

…and what about?

Air Plane Control System

Dysfunction (plane crash)!

Nuclear Powerplant Control System

Core melt-down (“China-syndrome”)!

…


JAN 19, 2006

Problem: System Development?

In the presence of all these errors: deadlock, starvation, priority inversion, interference,

anti-cooperation, un-intended execution traces, un-fairness, …

How to…: 1. …design a system that “works” ? 2. …verify that the system is “safe” ? 3. …verify that the system “meets its specification” ?

…and: What does “works”, “safe”, and “to meet a specification” mean ?!?


JAN 19, 2006

Solution: Modeling

“Models come to the rescue”: Create models (~ architecture, bridge construction, …)

Formal modeling (e.g., “FSP”) permits: (Offline) Reasoning understanding (Runtime) Testing confidence (C-time) Property Verification safety (C-time) Specification Verification correctness

Note: “Errors are much cheaper to commit in models than in implementations”

“Never send a human to do a machine’s job”-- A.Smith (’99)

auto-mate


JAN 19, 2006

Methodology: Model-based Design

Design abstract model

Decompose model

Reason/Test/Verify model individual parts and whole

Recompose insights make model safe

Impl. concrete program

concretize

REAL PROBLEM

SAFE MODEL

SAFE PROGRAM

abstract

? ?

?

?

MODEL

testreason

verify


JAN 19, 2006

Program worldModel world

ConcreteAbstract

P Mabstraction

concretization

Program vs. Model World

(Java) (FSP)

What discerns a really good programmer from one that is not so good is the capability of moving (consciously or unconsciously) between the concrete world of programs and the abstract world of semantic models (via abstraction and concretization).

Specifically, such a programmer is capable of (consciously or unconsciously): - 1) abstracting programs into models - 2) reasoning about the models - 3) concretizing the insights back into the world of programs


JAN 19, 2006


ConcreteAbstract

P M

1. P ? 3. M ?

4. validate

5. M !7. P !

2. abstract

6. concretize

Program / Model Verification

(Java) (FSP)


JAN 19, 2006


ConcreteAbstract

~

P

P’

M

M’

1. P ~ P’ ?2. abstract

3. M ~ M’ ?

4. relate

5. M ~ M’ !6. concretize7. P ~ P’ !

Program / Model Relationship

(Java) (FSP)


JAN 19, 2006

Glasses of Abstraction…

It’s Amazing…

Abstract away computational details: see only concurrency aspects

at appropriate level of abstraction!

Only$12,95

“See only the relevant!”

“The Glasses of Abstraction”

COMPLEX PROGRAMCOMPLEX PROGRAM

MODELMODEL

abstraction


JAN 19, 2006

Modeling: Level of Abstraction

Consider a client/server system:

Extremely abstract (high level of abstraction):

Appropriate (level of abstraction) for … :

Extremely concrete (low level of abstraction):

Universe = (event -> Universe).

NAND_Gate = ...Transistor = ...Accumulator = ......Client = ...

Server = (request->compute->reply->Server).Client = (calc->request->wait->reply->Client).Database = ...

high

er le

vel o

f ab

stra

ctio

n


JAN 19, 2006

Notes on Skill Acquisition

Your skill acquisition will progress according to the following stages of learning:

4. Unconscious competence 3. Conscious competence 2. Conscious incompetence 1. Unconscious incompetence

Course aim (and measurement): Level 3 Later in life, you probably mostly need level 4

…but you need to get there via level 3


JAN 19, 2006

Modeling Language (e.g. “FSP”)

Compositional: ||

Break things into (several) smaller things

Algebraic: ,

Intuitive ideal and eases automated verification

Syntactic:

Provide basis for programming languages

P || Q P Q

big

P || Q Q || P P || (Q || R)

USER =def (acquire -> use -> release -> USER).

(P || Q) || R


JAN 19, 2006

Parallel- vs. Concurrent Programming

Strategy: Optimal strategy for a particular goal Use available resources efficiently

Safety: Conceptually independent players Control interaction and “rules”

The Football Match Analogy:“An analogy that one can make is with football*; - the coach of the team is a parallel programmer while - the referee is a concurrent programmer” -- [P.Panangaden, ’96]

The Referee (~ the concurrent programmer):-- Make sure what is happening is a soccer match

The Trainer (~ the parallel programmer):-- Make sure my agents are performing “optimally”

*/ interpret appropriately on either side of the Atlantic


JAN 19, 2006

Course Aims & Goals

YourAims & Goals:

at huske udenad… at identificere brug af concurrency i (Java) programmer; at beskrive concurrency mekanismer i programmeringssprog (Java); at forklare fundamentale problemer, teknikker og løsninger indenfor concurrency; at forklare sammenhængen mellem modeller og programmer generelt; at relatere konkrete modeller og programmer; at sammenligne konkrete modeller; at ræsonnere om egenskaber af konkrete modeller og programmer; at implementere konkrete modeller i programmeringssprog (Java); at anvende ovenstående med henblik på at skabe konkrete modeller af systemer.

T

c laus b rabrand © c oncurrency (q3,’06) j an 19, 2006 c oncurrency claus brabrand...

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