Book Chapter 3
Concurrency: concurrent execution 1
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Chapter 3
Concurrent Execution
Concurrency: concurrent execution 2
©Magee/Kramer 2nd Edition
Concurrent execution
Concepts: processes – concurrent execution
and interleaving.
process interaction.
Models: parallel composition of asynchronous processes
– interleaving
interaction – shared actions
process labeling, and action relabeling and hiding
structure diagrams
Practice: Multithreaded Java programs
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Definitions
Concurrency
Logically simultaneous processing.
Does not imply multiple processing
elements (PEs). Requires
interleaved execution on a single PE.
Parallelism
Physically simultaneous processing.
Involves multiple PEs and/or
independent device operations.
A
Time
B
C
Both concurrency and parallelism require controlled access to
shared resources . We use the terms parallel and concurrent
interchangeably and generally do not distinguish between real and
pseudo-concurrent execution.
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3.1 Modeling Concurrency
How should we model process execution speed?
arbitrary speed
(we abstract away time)
How do we model concurrency?
arbitrary relative order of actions from different processes
(interleaving but preservation of each process order )
What is the result?
provides a general model independent of scheduling
(asynchronous model of execution)
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parallel composition – action interleaving
If P and Q are processes then (P||Q) represents the
concurrent execution of P and Q. The operator || is
the parallel composition operator.
ITCH = (scratch->STOP).
CONVERSE = (think->talk->STOP).
||CONVERSE_ITCH = (ITCH || CONVERSE).
Disjoint
alphabets
think talk scratch
think scratch talk
scratch think talk
Possible traces as
a result of action
interleaving.
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parallel composition – action interleaving
(0,0) (0,1) (0,2) (1,2) (1,1) (1,0)
from CONVERSEfrom ITCH
2 states 3 states
ITCH
scratch
0 1
CONVERSE
think talk
0 1 2
CONVERSE_ITCH
scratch
think
scratch
talk scratch
talk think
0 1 2 3 4 5
2 x 3 states
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parallel composition – algebraic laws
Commutative: (P||Q) = (Q||P)
Associative: (P||(Q||R)) = ((P||Q)||R)
= (P||Q||R).
Clock radio example:
CLOCK = (tick->CLOCK).
RADIO = (on->off->RADIO).
||CLOCK_RADIO = (CLOCK || RADIO).
LTS? Traces? Number of states?
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modeling interaction – shared actions
If processes in a composition have actions in common,
these actions are said to be shared. Shared actions are
the way that process interaction is modeled. While
unshared actions may be arbitrarily interleaved, a
shared action must be executed at the same time by all
processes that participate in the shared action.
MAKER
synchronizes
with USER
when ready.
MAKER = (make->ready->MAKER).
USER = (ready->use->USER).
||MAKER_USER = (MAKER || USER).
Non-disjoint
alphabets
LTS? Traces? Number of states?
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modeling interaction – handshake
A handshake is an action acknowledged by another:
MAKERv2 = (make->ready->used->MAKERv2).
USERv2 = (ready->use->used ->USERv2).
||MAKER_USERv2 = (MAKERv2 || USERv2).
3 states
3 states
3 x 3
states?
make ready use
used
0 1 2 3
4 states
Interaction
constrains
the overall
behaviour.
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modeling interaction – multiple processes
MAKE_A = (makeA->ready->used->MAKE_A).
MAKE_B = (makeB->ready->used->MAKE_B).
ASSEMBLE = (ready->assemble->used->ASSEMBLE).
||FACTORY = (MAKE_A || MAKE_B || ASSEMBLE).
Multi-party synchronization:
makeA
makeB makeA ready assemble
used
makeB
0 1 2 3 4 5
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composite processes
A composite process is a parallel composition of primitive
processes. These composite processes can be used in the
definition of further compositions.
||MAKERS = (MAKE_A || MAKE_B).
||FACTORY = (MAKERS || ASSEMBLE).
Substituting the definition for MAKERS in FACTORY and applying the
commutative and associative laws for parallel composition results in
the original definition for FACTORY in terms of primitive processes.
||FACTORY = (MAKE_A || MAKE_B || ASSEMBLE).
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process instances and labeling
a:P prefixes each action label in the alphabet of P with a.
SWITCH = (on->off->SWITCH).
||TWO_SWITCH = (a:SWITCH || b:SWITCH).
Two instances of a switch process:
a:SWITCH
a.on
a.off
0 1
b:SWITCH
b.on
b.off
0 1
||SWITCHES(N=3) = (forall[i:1..N] s[i]:SWITCH).
||SWITCHES(N=3) = (s[i:1..N]:SWITCH).
An array of instances of the switch process:
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process labeling by a set of prefix labels
{a1,..,ax}::P replaces every action label n in the
alphabet of P with the labels a1.n,…,ax.n. Further,
every transition (n->X) in the definition of P is
replaced with the transitions ({a1.n,…,ax.n} ->X).
Process prefixing is useful for modeling shared resources:
RESOURCE = (acquire->release->RESOURCE).
USER = (acquire->use->release->USER).
||RESOURCE_SHARE = (a:USER || b:USER
|| {a,b}::RESOURCE).
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process prefix labels for shared resources
How does the model ensure
that the user that acquires
the resource is the one to
release it?
a:USER
a.acquire a.use
a.release
0 1 2
b:USER
b.acquire b.use
b.release
0 1 2
{a,b}::RESOURCE
a.acquire
b.acquire
a.release
b.release
0 1
RESOURCE_SHARE
a.acquire
b.acquire b.use
b.release
a.use
a.release
0 1 2 3 4
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action relabeling
Relabeling functions are applied to processes to change
the names of action labels. The general form of the
relabeling function is:
/{newlabel_1/oldlabel_1,… newlabel_n/oldlabel_n}.
Relabeling to ensure that composed
processes synchronize on particular actions.
CLIENT = (call->wait->continue->CLIENT).
SERVER = (request->service->reply->SERVER).
Note that both newlabel and oldlabel can be sets of labels.
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action relabeling
||CLIENT_SERVER = (CLIENT || SERVER)
/{call/request, reply/wait}.
CLIENT
call reply
continue
0 1 2
SERVER
call service
reply
0 1 2
CLIENT_SERVER call service reply
continue
0 1 2 3
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action relabeling – prefix labels
An alternative formulation of the client server system is
described below using qualified or prefixed labels:
SERVERv2 = (accept.request
->service->accept.reply->SERVERv2).
CLIENTv2 = (call.request
->call.reply->continue->CLIENTv2).
||CLIENT_SERVERv2 = (CLIENTv2 || SERVERv2)
/{call/accept}.
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action hiding – abstraction to reduce complexity
When applied to a process P, the hiding operator \{a1..ax}
removes the action names a1..ax from the alphabet of P
and makes these concealed actions “silent”. These silent
actions are labeled tau. Silent actions in different
processes are not shared.
Sometimes it is more convenient to specify the set of
labels to be exposed….
When applied to a process P, the interface
operator @{a1..ax} hides all actions in the
alphabet of P not labeled in the set a1..ax.
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action hiding
USER = (acquire->use->release->USER)
\{use}.
USER = (acquire->use->release->USER)
@{acquire,release}.
The following definitions are equivalent:
acquire tau
release
0 1 2
Minimization removes hidden
tau actions to produce an
LTS with equivalent
observable behavior.
acquire
release
0 1
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structure diagrams – systems as interacting processes
P a
b
Process P with
alphabet {a,b}.
P a b Qm Parallel Composition
(P||Q) / {m/a,m/b,c/d}
P Qa
c dc
x xx
S
yx
Composite process
||S = (P||Q) @ {x,y}
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structure diagrams
We use structure diagrams to capture the structure
of a model expressed by the static combinators:
parallel composition, relabeling and hiding.
range T = 0..3
BUFF = (in[i:T]->out[i]->BUFF).
||TWOBUF = ?
a:BUFF b:BUFF
a.out
TWOBUFF
outin
inoutin out
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structure diagrams
Structure diagram for CLIENT_SERVER ?
CLIENT call request SERVERcall
replywait reply servicecontinue
Structure diagram for CLIENT_SERVERv2 ?
CLIENTv2 call accept SERVERv2call
servicecontinue
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structure diagrams – resource sharing
a:USER
printer
b:USER
printer
printer:
RESOURCE
acquire
release
PRINTER_SHARE
RESOURCE = (acquire->release->RESOURCE).
USER = (printer.acquire->use
->printer.release->USER)\{use}.
||PRINTER_SHARE
= (a:USER||b:USER||{a,b}::printer:RESOURCE).
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3.2 Multi-threaded Programs in Java
Concurrency in Java occurs when more than one thread is alive.
ThreadDemo has two threads which rotate displays.
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ThreadDemo model
ROTATOR = PAUSED,
PAUSED = (run->RUN | pause->PAUSED
|interrupt->STOP),
RUN = (pause->PAUSED |{run,rotate}->RUN
|interrupt->STOP).
||THREAD_DEMO = (a:ROTATOR || b:ROTATOR)
/{stop/{a,b}.interrupt}.
b:ROTATOR
a.run
a.pause
a.rotate
b.run
b.pause
b.rotate
THREAD_DEMO
a:ROTATOR
stop
Interpret
run,
pause,
interrupt
as inputs,
rotate as
an output.
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ThreadDemo implementation in Java – class diagram
ThreadDemo creates two ThreadPanel displays when initialized.
ThreadPanel manages the display and control buttons, and delegates calls to
rotate() to DisplayThread. Rotator implements the runnable interface.
Applet
ThreadDemo ThreadPanel
rotate()
start()
stop()
A,B
init()
start()
stop()
Runnable
Rotator
run()
GraphicCanvas
Panel
Thread
DisplayThread
display
thread
target
rotate()
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Rotator class
class Rotator implements Runnable {
public void run() {
try {
while(true) ThreadPanel.rotate();
} catch(InterruptedException e) {}
}
}
Rotator implements the runnable interface, calling
ThreadPanel.rotate() to move the display.
run()finishes if an exception is raised by Thread.interrupt().
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ThreadPanel class
public class ThreadPanel extends Panel {
// construct display with title and segment color c
public ThreadPanel(String title, Color c) {…}
// rotate display of currently running thread 6 degrees
// return value not used in this example
public static boolean rotate()
throws InterruptedException {…}
// create a new thread with target r and start it running
public void start(Runnable r) {
thread = new DisplayThread(canvas,r,…);
thread.start();
}
// stop the thread using Thread.interrupt()
public void stop() {thread.interrupt();}
}
ThreadPanel
manages the display
and control buttons for
a thread.
Calls to rotate()
are delegated to
DisplayThread.
Threads are created by
the start() method,
and terminated by the
stop() method.
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ThreadDemo class
public class ThreadDemo extends Applet {
ThreadPanel A; ThreadPanel B;
public void init() {
A = new ThreadPanel(“Thread A”,Color.blue);
B = new ThreadPanel(“Thread B”,Color.blue);
add(A); add(B);
}
public void start() {
A.start(new Rotator());
B.start(new Rotator());
}
public void stop() {
A.stop();
B.stop();
}
}
ThreadDemo creates two
ThreadPanel displays
when initialized and two
threads when started.
ThreadPanel is used
extensively in later
demonstration programs.
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Summary
Concepts
concurrent processes and process interaction
Models
Asynchronous (arbitrary speed) & interleaving (arbitrary order).
Parallel composition as a finite state process with action
interleaving.
Process interaction by shared actions.
Process labeling and action relabeling and hiding.
Structure diagrams
Practice
Multiple threads in Java.