<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
concurrency	B-Architecture
is	O
the	O
ability	O
of	O
different	O
parts	O
or	O
units	O
of	O
a	O
program	B-Application
,	O
algorithm	O
,	O
or	O
problem	O
to	O
be	O
executed	B-General_Concept
out-of-order	O
or	O
in	O
partial	O
order	O
,	O
without	O
affecting	O
the	O
outcome	O
.	O
</s>
<s>
This	O
allows	O
for	O
parallel	B-Operating_System
execution	O
of	O
the	O
concurrent	B-Operating_System
units	O
,	O
which	O
can	O
significantly	O
improve	O
overall	O
speed	O
of	O
the	O
execution	O
in	O
multi-processor	B-Operating_System
and	O
multi-core	B-Architecture
systems	O
.	O
</s>
<s>
In	O
more	O
technical	O
terms	O
,	O
concurrency	B-Architecture
refers	O
to	O
the	O
decomposability	B-Algorithm
of	O
a	O
program	B-Application
,	O
algorithm	O
,	O
or	O
problem	O
into	O
order-independent	O
or	O
partially-ordered	O
components	O
or	O
units	O
of	O
computation	O
.	O
</s>
<s>
According	O
to	O
Rob	O
Pike	O
,	O
concurrency	B-Architecture
is	O
the	O
composition	O
of	O
independently	O
executing	O
computations	O
,	O
and	O
concurrency	B-Architecture
is	O
not	O
parallelism	B-Operating_System
:	O
concurrency	B-Architecture
is	O
about	O
dealing	O
with	O
lots	O
of	O
things	O
at	O
once	O
but	O
parallelism	B-Operating_System
is	O
about	O
doing	O
lots	O
of	O
things	O
at	O
once	O
.	O
</s>
<s>
Concurrency	B-Architecture
is	O
about	O
structure	O
,	O
parallelism	B-Operating_System
is	O
about	O
execution	O
,	O
concurrency	B-Architecture
provides	O
a	O
way	O
to	O
structure	O
a	O
solution	O
to	O
solve	O
a	O
problem	O
that	O
may	O
(	O
but	O
not	O
necessarily	O
)	O
be	O
parallelizable	O
.	O
</s>
<s>
A	O
number	O
of	O
mathematical	O
models	O
have	O
been	O
developed	O
for	O
general	O
concurrent	B-Architecture
computation	I-Architecture
including	O
Petri	B-Operating_System
nets	I-Operating_System
,	O
process	O
calculi	O
,	O
the	O
parallel	B-Operating_System
random-access	I-Operating_System
machine	I-Operating_System
model	O
,	O
the	O
actor	B-Application
model	I-Application
and	O
the	O
Reo	B-Application
Coordination	I-Application
Language	I-Application
.	O
</s>
<s>
As	O
Leslie	O
Lamport	O
(	O
2015	O
)	O
notes	O
,	O
"	O
While	O
concurrent	B-Architecture
program	I-Architecture
execution	O
had	O
been	O
considered	O
for	O
years	O
,	O
the	O
computer	B-General_Concept
science	I-General_Concept
of	O
concurrency	B-Architecture
began	O
with	O
Edsger	O
Dijkstra	O
's	O
seminal	O
1965	O
paper	O
that	O
introduced	O
the	O
mutual	B-Operating_System
exclusion	I-Operating_System
problem	O
.	O
</s>
<s>
The	O
ensuing	O
decades	O
have	O
seen	O
a	O
huge	O
growth	O
of	O
interest	O
in	O
concurrency	B-Architecture
—	O
particularly	O
in	O
distributed	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
Because	O
computations	O
in	O
a	O
concurrent	B-Operating_System
system	I-Operating_System
can	O
interact	O
with	O
each	O
other	O
while	O
being	O
executed	B-General_Concept
,	O
the	O
number	O
of	O
possible	O
execution	O
paths	O
in	O
the	O
system	O
can	O
be	O
extremely	O
large	O
,	O
and	O
the	O
resulting	O
outcome	O
can	O
be	O
indeterminate	B-Application
.	O
</s>
<s>
Concurrent	B-Operating_System
use	O
of	O
shared	O
resources	B-General_Concept
can	O
be	O
a	O
source	O
of	O
indeterminacy	O
leading	O
to	O
issues	O
such	O
as	O
deadlocks	B-Operating_System
,	O
and	O
resource	B-Operating_System
starvation	I-Operating_System
.	O
</s>
<s>
Design	O
of	O
concurrent	B-Operating_System
systems	I-Operating_System
often	O
entails	O
finding	O
reliable	O
techniques	O
for	O
coordinating	O
their	O
execution	O
,	O
data	O
exchange	O
,	O
memory	B-General_Concept
allocation	I-General_Concept
,	O
and	O
execution	O
scheduling	O
to	O
minimize	O
response	O
time	O
and	O
maximise	O
throughput	O
.	O
</s>
<s>
Concurrency	B-Architecture
theory	O
has	O
been	O
an	O
active	O
field	O
of	O
research	O
in	O
theoretical	O
computer	B-General_Concept
science	I-General_Concept
.	O
</s>
<s>
One	O
of	O
the	O
first	O
proposals	O
was	O
Carl	O
Adam	O
Petri	O
's	O
seminal	O
work	O
on	O
Petri	B-Operating_System
nets	I-Operating_System
in	O
the	O
early	O
1960s	O
.	O
</s>
<s>
In	O
the	O
years	O
since	O
,	O
a	O
wide	O
variety	O
of	O
formalisms	O
have	O
been	O
developed	O
for	O
modeling	O
and	O
reasoning	O
about	O
concurrency	B-Architecture
.	O
</s>
<s>
A	O
number	O
of	O
formalisms	O
for	O
modeling	O
and	O
understanding	O
concurrent	B-Operating_System
systems	I-Operating_System
have	O
been	O
developed	O
,	O
including	O
:	O
</s>
<s>
Some	O
of	O
these	O
models	O
of	O
concurrency	B-Architecture
are	O
primarily	O
intended	O
to	O
support	O
reasoning	O
and	O
specification	O
,	O
while	O
others	O
can	O
be	O
used	O
through	O
the	O
entire	O
development	O
cycle	O
,	O
including	O
design	O
,	O
implementation	O
,	O
proof	O
,	O
testing	O
and	O
simulation	O
of	O
concurrent	B-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
Some	O
of	O
these	O
are	O
based	O
on	O
message	B-Architecture
passing	I-Architecture
,	O
while	O
others	O
have	O
different	O
mechanisms	O
for	O
concurrency	B-Architecture
.	O
</s>
<s>
The	O
proliferation	O
of	O
different	O
models	O
of	O
concurrency	B-Architecture
has	O
motivated	O
some	O
researchers	O
to	O
develop	O
ways	O
to	O
unify	O
these	O
different	O
theoretical	O
models	O
.	O
</s>
<s>
For	O
example	O
,	O
Lee	O
and	O
Sangiovanni-Vincentelli	O
have	O
demonstrated	O
that	O
a	O
so-called	O
"	O
tagged-signal	O
"	O
model	O
can	O
be	O
used	O
to	O
provide	O
a	O
common	O
framework	O
for	O
defining	O
the	O
denotational	B-Application
semantics	I-Application
of	O
a	O
variety	O
of	O
different	O
models	O
of	O
concurrency	B-Architecture
,	O
while	O
Nielsen	O
,	O
Sassone	O
,	O
and	O
Winskel	O
have	O
demonstrated	O
that	O
category	O
theory	O
can	O
be	O
used	O
to	O
provide	O
a	O
similar	O
unified	O
understanding	O
of	O
different	O
models	O
.	O
</s>
<s>
The	O
Concurrency	B-Architecture
Representation	O
Theorem	O
in	O
the	O
actor	B-Application
model	I-Application
provides	O
a	O
fairly	O
general	O
way	O
to	O
represent	O
concurrent	B-Operating_System
systems	I-Operating_System
that	O
are	O
closed	O
in	O
the	O
sense	O
that	O
they	O
do	O
not	O
receive	O
communications	O
from	O
outside	O
.	O
</s>
<s>
(	O
Other	O
concurrency	B-Architecture
systems	O
,	O
e.g.	O
,	O
process	O
calculi	O
can	O
be	O
modeled	O
in	O
the	O
actor	B-Application
model	I-Application
using	O
a	O
two-phase	B-General_Concept
commit	I-General_Concept
protocol	I-General_Concept
.	O
)	O
</s>
<s>
Various	O
types	O
of	O
temporal	O
logic	O
can	O
be	O
used	O
to	O
help	O
reason	O
about	O
concurrent	B-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
Some	O
of	O
these	O
logics	O
,	O
such	O
as	O
linear	O
temporal	O
logic	O
and	O
computation	O
tree	O
logic	O
,	O
allow	O
assertions	O
to	O
be	O
made	O
about	O
the	O
sequences	O
of	O
states	O
that	O
a	O
concurrent	B-Operating_System
system	I-Operating_System
can	O
pass	O
through	O
.	O
</s>
<s>
Others	O
,	O
such	O
as	O
action	O
computational	O
tree	O
logic	O
,	O
Hennessy	B-Application
–	I-Application
Milner	I-Application
logic	I-Application
,	O
and	O
Lamport	O
's	O
temporal	O
logic	O
of	O
actions	O
,	O
build	O
their	O
assertions	O
from	O
sequences	O
of	O
actions	O
(	O
changes	O
in	O
state	O
)	O
.	O
</s>
<s>
The	O
principal	O
application	O
of	O
these	O
logics	O
is	O
in	O
writing	O
specifications	O
for	O
concurrent	B-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
Concurrent	B-Architecture
programming	I-Architecture
encompasses	O
programming	O
languages	O
and	O
algorithms	O
used	O
to	O
implement	O
concurrent	B-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
Concurrent	B-Architecture
programming	I-Architecture
is	O
usually	O
considered	O
to	O
be	O
more	O
general	O
than	O
parallel	B-Operating_System
programming	I-Operating_System
because	O
it	O
can	O
involve	O
arbitrary	O
and	O
dynamic	O
patterns	O
of	O
communication	O
and	O
interaction	O
,	O
whereas	O
parallel	B-Operating_System
systems	O
generally	O
have	O
a	O
predefined	O
and	O
well-structured	O
communications	O
pattern	O
.	O
</s>
<s>
The	O
base	O
goals	O
of	O
concurrent	B-Architecture
programming	I-Architecture
include	O
correctness	O
,	O
performance	O
and	O
robustness	O
.	O
</s>
<s>
Concurrent	B-Operating_System
systems	I-Operating_System
such	O
as	O
Operating	B-General_Concept
systems	I-General_Concept
and	O
Database	B-Application
management	I-Application
systems	I-Application
are	O
generally	O
designed	O
to	O
operate	O
indefinitely	O
,	O
including	O
automatic	O
recovery	O
from	O
failure	O
,	O
and	O
not	O
terminate	O
unexpectedly	O
(	O
see	O
Concurrency	B-Operating_System
control	I-Operating_System
)	O
.	O
</s>
<s>
Some	O
concurrent	B-Operating_System
systems	I-Operating_System
implement	O
a	O
form	O
of	O
transparent	O
concurrency	B-Architecture
,	O
in	O
which	O
concurrent	B-Operating_System
computational	O
entities	O
may	O
compete	O
for	O
and	O
share	O
a	O
single	O
resource	O
,	O
but	O
the	O
complexities	O
of	O
this	O
competition	O
and	O
sharing	O
are	O
shielded	O
from	O
the	O
programmer	O
.	O
</s>
<s>
Because	O
they	O
use	O
shared	O
resources	B-General_Concept
,	O
concurrent	B-Operating_System
systems	I-Operating_System
in	O
general	O
require	O
the	O
inclusion	O
of	O
some	O
kind	O
of	O
arbiter	O
somewhere	O
in	O
their	O
implementation	O
(	O
often	O
in	O
the	O
underlying	O
hardware	O
)	O
,	O
to	O
control	O
access	O
to	O
those	O
resources	B-General_Concept
.	O
</s>
<s>
The	O
use	O
of	O
arbiters	O
introduces	O
the	O
possibility	O
of	O
indeterminacy	B-Application
in	I-Application
concurrent	I-Application
computation	I-Application
which	O
has	O
major	O
implications	O
for	O
practice	O
including	O
correctness	O
and	O
performance	O
.	O
</s>
<s>
For	O
example	O
,	O
arbitration	O
introduces	O
unbounded	B-Application
nondeterminism	I-Application
which	O
raises	O
issues	O
with	O
model	B-Application
checking	I-Application
because	O
it	O
causes	O
explosion	O
in	O
the	O
state	O
space	O
and	O
can	O
even	O
cause	O
models	O
to	O
have	O
an	O
infinite	O
number	O
of	O
states	O
.	O
</s>
<s>
Some	O
concurrent	B-Architecture
programming	I-Architecture
models	O
include	O
coprocesses	O
and	O
deterministic	B-Architecture
concurrency	I-Architecture
.	O
</s>
<s>
In	O
these	O
models	O
,	O
threads	B-Operating_System
of	O
control	O
explicitly	O
yield	B-Language
their	O
timeslices	O
,	O
either	O
to	O
the	O
system	O
or	O
to	O
another	O
process	O
.	O
</s>
