Ricci, Alessandro

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Almae Matris Studiorum Campus

Basic conceptsSISCO LS - II Facoltà Ingegneria - Cesena 1
Sistemi Concorrenti e di Rete LS
II Facoltà di Ingegneria - Cesena
a.a 2008/2009
[module 1.1] 
BASIC CONCEPTS
v1.0
Basic conceptsSISCO LS - II Facoltà Ingegneria - Cesena
CONCURRENCY AND CONCURRENT 
SYSTEMS
• Concurrency as a main concept of many domains and systems
– operating systems, multi-threaded and multi-process programs, distributed 
systems, control systems, real-time systems,...
– “complex” software systems
• General definitions
– “In computer science, concurrency is a property of systems in which several 
computational processes are executing at the same time, and potentially 
interacting with each other.” [ROS-97]
– “Concurrency is concerned with the fundamental aspects of systems of 
multiple, simultaneously active computing agents, that interact with one 
another” [CLE-96]
• Common aspects
– systems with multiple activities or processes whose execution overlaps in 
time
– activities can have some kind of dependencies, therefore can interact
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Basic conceptsSISCO LS - II Facoltà Ingegneria - Cesena
CONCURRENT PROGRAMMING
• Concurrent programming
– building programs in which multiple computational activities overlap in 
time and typically interact in some way 
• Concurrent program 
– finite set of sequential programs that can be executed in parallel, i.e. 
overlapped in time 
• a sequential program specifies sequential execution of a list of statements
• the execution of a sequential program is called process 
• a concurrent program specifies two or more sequential programs that may be 
executed concurrently as parallel processes 
– the execution of a concurrent program is called concurrent computation 
or elaboration
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Basic conceptsSISCO LS - II Facoltà Ingegneria - Cesena
CONCURRENT PROGRAMMING VS. 
PARALLEL PROGRAMMING
• Parallel programming
– the execution of programs overlaps in time by running on separate 
physical processors
• Concurrent programming
– the execution of programs overlaps in time without necessarily running 
on separate physical processors, by sharing for instance the same 
processor
• potential or abstract parallelism
• Distributed programming
– when processors are distributed over a network
– no shared memory
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Basic conceptsSISCO LS - II Facoltà Ingegneria - Cesena
WHY CONCURRENCY AND CONCURRENT 
PROGRAMMING (1/2)
• Performance improvement
– increased application throughput 
• by exploiting parallel hardware
– increased application responsiveness 
• by optimizing the interplay among CPU and I/O activities
• Quantitative measurement for performance: speedup 
5
x = a0 +
1
a1 + 1a2+ 1a3+a4
(1)
S =
T1
TN
=
1
1− P + PN
(2)
N is the number of processors
T1 is the execution time of the sequential algorithm
TN is the execution time of the parallel algorithm with N processors
1
x = a0 +
1
a1 + 1a2+ 1a3+a4
(1)
S =
T1
TN
(2)
S =
1
1− P + PN
(3)
N is the number of processors
T1 is the execution time of the sequential algorithm
TN is the execution time of the parallel algorithm with N processors
1
Basic conceptsSISCO LS - II Facoltà Ingegneria - Cesena
AMDAHL’S LAW
• Maximum speedup parallelizing a system:
• Theoretically maximum for P = 1 (linear speedup)
– actually there are specific cases with S > N  (super-linear) speedup
6
x = a0 +
1
a1 + 1a2+ 1a3+a4
(1)
S =
T1
TN
(2)
S =
1
1− P + PN
(3)
N is the number of processors
T1 is the execution time of the sequential algorithm
TN is the execution time of the parallel algorithm with N processors
1
x = a0 +
1
a1 + 1a2+ 1a3+a4
(1)
S =
T1
TN
(2)
S =
1
1− P + PN
(3)
N is the number of processors
T1 is the execution time of the sequential algorithm
TN is the execution time of the parallel algorithm with N processors
P is the proportion of a program that can be made parallel
(1− P ) is the proportion that cannot be parallelized (remains serial)
1
Basic conceptsSISCO LS - II Facoltà Ingegneria - Cesena
WHY CONCURRENCY AND CONCURRENT 
PROGRAMMING (2/2)
• Abstraction and engineering
– more appropriate level of abstraction for programs which interact with 
the environment, control multiple activities and handle multiple events
– e.g. reactive systems
• Concurrency an a tool for software design and construction
– rethinking to the way in which we solve problems 
• parallel algorithms
– rethinking to the way in which we design and build systems
• new level of abstraction
– different kind of decomposition, modularization, encapsulation
• full engineering spectrum
– modelling, implementing, testing, ...
7
Basic conceptsSISCO LS - II Facoltà Ingegneria - Cesena
BASIC JARGON OF CONCURRENT 
PROGRAMMING
• Processes ~  a sequential program in execution 
– abstract / general concept
• different kind of incarnation depending on the specific context
• the basic unit of a concurrent system, single thread of control
– synonim: task 
• sequence of instructions operating together as a group, unit of work
– process execution is meant to be completely asynchronous with each other
• different speed in executing statements ==> non-determism 
• Process interaction
– any non trivial concurrent program is based on multiple processes that need 
to interact in some way in order to achieve the objective of the system
– basic kinds of interaction: 
• competition / contention, cooperation, interferences
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Basic conceptsSISCO LS - II Facoltà Ingegneria - Cesena
PROCESS INTERACTION: 
CONTENTION (OR COMPETITION)
• Refers to interactions which are expected and necessary, but not 
wanted
– typically concerns the need of coordinating the access by multiple 
processes to shared resources
• Two basic class of problems
– mutual exclusion
• ruling the access to shared resources by distinct processes
– critical sections
• ruling the concurrent execution of blocks of actions by distinct processes
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Basic conceptsSISCO LS - II Facoltà Ingegneria - Cesena
PROCESS INTERACTION: 
COOPERATION
• Refers to interactions which are expected and wanted
– they are part of the semantics of the concurrent program
• Two basic kinds
– synchronization 
• concerns the explicit definition or presence of temporal relationships or 
dependencies among processes and among actions of distinct processes
• introduction of specific supports for the exchange of temporal signals
– communication
• concerns the need of realising an information flow among processes, typically 
realised in terms of messages
• introduction of specific supports for the exchange of messages
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Basic conceptsSISCO LS - II Facoltà Ingegneria - Cesena
SYNCHRONIZATION VS. MUTUAL 
EXCLUSION
• Profoundly different - even if related - concepts 
– “synchronization = mutual exclusion urban legend” [BUH-05]
• false story, still present in textbooks / research papers 
– synchronization defines a timing relationship among processes
• mantaining time-relationships which includes  actions happening at the same 
time or happening at the same relative rate or simply some action having to 
occur before another (precedence relationships)
– mutual-exclusion defines a restriction on access to shared data
• mutual-exclusion is meaningless if no shared data is involved
• Relationships
– mutual-exclusion typically require some forms of implicit synchronization
• blocking some actions, waiting for other actions to complete
– synchronization does not necessarily require any kind of shared data 
and the mutual exclusion 
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Basic conceptsSISCO LS - II Facoltà Ingegneria - Cesena
ON THE DIFFICULTY OF SYNCHRONIZATION: 
EXAMPLE 1: “BUY-THE-MILK” PROBLEM
• “Alice and Bob live together, happily without cell-phones. Both are 
responsible to buy the milk when it finishes...”
12
Time Alice Bob
5:00 Arrive home
5:05 Look in the fridge; no milk
5:10 Leave for a grocery
5:15 Arrive home
5:20 Look in the fridge; no milk
5:25 Buy milk Leave for grocery
5:30 Arrive home; put milk in fridge
5:40 Buy milk
5:45 Arrive home; oh no!
Basic conceptsSISCO LS - II Facoltà Ingegneria - Cesena
A SOLUTION: NOTES IN THE FRIDGE (1/2)
• Looking for a solution to ensure that:
– only one person buys the milk, when there is no milk
– someone always buys the milk, when there is no milk
• Tentative solution: using notes on the fridge!
– “if you find that there is no milk and there is no note on the door of the 
fridge, then leave a note on the fridge’s door, go and buy milk, put the 
milk in the fridge, and remove your note.”
• Does it work? Not always actually...
13
PROGRAM for Alice & Bob
1 if (no note) then
2   if (no milk) then
3     leave note
4     buy milk
5     remove note
6   fi
7 fi
Basic conceptsSISCO LS - II Facoltà Ingegneria - Cesena
A SOLUTION: NOTES IN THE FRIDGE (2/2)
(..NOT SO EASY, ACTUALLY..)
14
Time Alice Bob
5:00 Arrive home
5:05 Look at the fridge; no note
5:10 ...ops! need a toilet
5:15 ...still in the toilet... Arrive home
5:20 ...still in the toilet... Look at the fridge; no note
5:21 ...still in the toilet... Look in the fridge; no milk (argh)
5:22 ...still in the toilet... leave note
5:25 ...still in the toilet... go and buy milk
5:45 look in the fridge: no milk (*) ...
5:50 leave note...
[*] Alice does not realize that a note was put on the fridge (she is not really a 
good observer) and strictly follows the esablished program
Basic conceptsSISCO LS - II Facoltà Ingegneria - Cesena
ON THE DIFFICULTY OF SYNCHRONIZATION: 
EXAMPLE 2: “COORDINATED ATTACK” PROBLEM
• “Two camps of the same army in different locations need to decide 
on the exact time for a coordinated attack on the enemy camp”
• Communication between 1-2 through messangers
– each general: send a message (messanger), wait for an ack
• If  communication is not reliable, the problem has no solution
15
Enemy
1 2
Basic conceptsSISCO LS - II Facoltà Ingegneria - Cesena
PROCESS INTERACTION: 
INTERFERENCES
• Refers to interactions which are neither expected, not wanted
– producing bad effects only when the ratio among the process speeds 
assumes specific values (time-dependent errors)
– the “nightmare” of concurrent programming
• race condition or race hazard or simply  race
– whenever two or more processes concurrently access and update 
shared resources, and the result depends on the specific order occurring 
in process access
• Related to two main types of programming errors
– bad management of expected interactions 
– presence of spurious interactions not expected in the problem
• Interferences and errors in concurrent programs can lead to critical 
situations for the concurrent system in the overall. 
– deadlock, starvation, livelock
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Basic conceptsSISCO LS - II Facoltà Ingegneria - Cesena
CRITICAL SITUATIONS
• deadlock
– situation wherein two or more competing actions (processes)  are 
waiting for the other to finish, and thus neither ever does
– such actions typically concerns the release of a locked shared resource, 
the receiption of a temporal signal or a message
• starvation
– situation wherein a process is blocked in an infinite waiting
– resource starvation = the process is perpetually denied in accessing 
necessary resources. 
• without those resources, the program can never finish its task
• livelock
– a livelock is similar to a deadlock, except that the states of the processes 
involved in the livelock constantly change with regard to one another, 
none progressing
– livelock is a special case of resource starvation: the general definition 
only states that a specific process is not progressing
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Basic conceptsSISCO LS - II Facoltà Ingegneria - Cesena
BIBLIOGRAPHY
• [HAN-73] 
– Per Brinch Hansen - “Concurrent Programming Concepts”, ACM Computing 
Surveys, Vol. 5, No. 4, Dec. 1973
• [AND-83] 
– Gregory Andrews and Fred Schneider - “Concepts and Notations for Concurrent 
Programming”, ACM Computing Surveys, Vol. 15, No. 1, March 1983
• [CLE-96] 
– Rance Cleaveland, Scott Smolka et al - “Strategic Directions in concurrency 
Research”, ACM Computing Surveys, Vol. 28, No. 4, Dec. 1996
• [ROS-97]
– Roscoe, A. W. (1997). The Theory and Practice of Concurrency. Prentice Hall. 
ISBN 0-13-674409-5.
• [BUH-05]
– Peter Buhr and Ashif Harji.  “Concurrent Urban Legends”. Concurrency and 
Computation: Practice and Experience. 2005. 17:1133-1172.
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