Module Concurrent Programming

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1 March 2012
(C) Hochschule für Technik
Fachhochschule Nordwestschweiz
1
Lecturer:
Contact:
Daniel Kröni
[email protected]
Time:
Room:
Friday: 8:15-9:00 / 9:10-9:55 / 10:05-10:50
4.415
Course Material (Created by Prof. Dr. D. Gruntz and D. Kröni)
Active Directory: E1862_Unterrichte_I\E1862_4Ia\conpr
Web Page:
http://web.fhnw.ch/plattformen/conpr/
Examination:
1 March 2012
27.04.2012 (Room 1.047) 9:00 - 10:30
2
The Free Lunch is Over!1
• Past 30 years
– Clock speed
– Execution optimization
as pipelining / branch pred.
(more work per cycle)
– Caches
• Now
– Multicore
– Hyperthreading
– Cache
But hyperthreading and multicore CPUs will have nearly no
impact on most current
applications !
1 March 2012
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http://www.gotw.ca/publications/concurrency-ddj.htm
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Multicore CPUs are here!
Quad Core!
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Amdahl's Law
• Formula to predict the maximum speedup using multiple
processors (N) based on the proportion of parallelizable (p) and
serial (1-p) component of a program.
Speedup ≤
1
(1-p) +
p
N
• Speedup is limited by the time needed for the sequential fraction
of the program
All concurrent applications have some sources of serialization!
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1
Amdahl's Law: Example (p = 0.5)
0.2
0.5
0.3
S=
N=1
1
p
N
=1
(1 - 0.5) + 0.5
t
1
(1-p) +
0.25
N=2
0.2
0.3
S=
1
= 1.33
(1 - 0.5) + 0.25
t
N=∞
0.2
0.3
S=
1
=2
(1 - 0.5) + 0
t
1 March 2012
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1
Amdahl's Law
p
N
Source: http://en.wikipedia.org/wiki/File:AmdahlsLaw.svg
(1-p) +
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Concurrent Programming
Programs
Concurrent Programs
Parallel Programs
1 March 2012
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Concurrency: Omnipresence
• Threads are everywhere
– Even if a programmer does not explicitly create threads, frameworks may
create threads, and code called from these threads must be thread-safe!
• Examples in Java
– Threads are used by every Java application
 GC-Thread
 Finalization-Thread
– AWT/Swing applications:
 AWT-EventQueue Thread
 AWT-Windows Thread
 AWT-Shutdown Thread
– Servlets: different requests are invoked by different threads
– java.util.Timer
1 March 2012
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Consequences
• Applications need to be concurrent if you want to exploit
multicores
– Existing applications will be redesigned
• Good support of programming languages/libraries
– Java > 1.5 [java.util.concurrent]
 Biased towards shared memory and the required coordination mechanisms
– Special language concepts become mainstream
 Functional Programming (Immutability, side effect avoidance)
 Actor model (Erlang, Scala)
 Software transactions (STM: Clojure, Scala)
The biggest sea change in software development since the
OO revolution is knocking at the door, and its name is
Concurrency.
[Herb Sutter]
1 March 2012
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Risks / Problems
• Safety hazards
– Race conditions
• Lifeness hazards
@NotThreadSafe
public class Counter {
private int value;
public int getNext(){ return value++; }
}
– Deadlock, Livelock, Starvation
• Performance hazards
– Over-Synchronization: Amdahl's Law
– Context switches are not free: Saving and restoring execution context
• Testing is not reliable
– Scheduling is not deterministic
• Debugging is often impossible
– Heisenbugs (bug disappears when trying to observe it)
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Goals
• The Students
… know typical problems and hassles in the context of concurrent programs
… know and can apply abstract solution patterns for such problems
… know different paradigms to solve concurrency problems such as actors
and software transactions
… know the memory model used by Java programs
… can write correct concurrent programs
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• Reference
– Java Concurrency in Practice
Brian Goetz, Tim Peierls, Joshua Bloch,
Joseph Bowbeer, David Holmes, Doug Lea
– Addison-Wesley Professional (May 19, 2006)
– ISBN-13: 978-0321349606
Writing correct programs is hard;
writing correct concurrent programs is harder
1 March 2012
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Schedule
1
2
3
4
5
6
7
8
9
10
11
12
13
14
24.02.2012
02.03.2012
09.03.2012
16.03.2012
23.03.2012
30.03.2012
06.04.2012
13.04.2012
20.04.2012
27.04.2012
04.05.2012
11.05.2012
18.05.2012
25.05.2012
01.06.2012
08.06.2012
15.06.2012
1 March 2012
Intro, Threads
Locks
Condition Synchronization
Java Memory Model
Sharing objects
Synchronizers
Ostern
Task based concurrency
Non Blocking Algorithms / Atomics
Prüfung
Scala Intro
Projektwoche
Auffahrt
Scala Concurrency
Actor Model
Software Transactional Memory
Performance, Review, Questions
14

Module Concurrent Programming

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