<s>
Concurrent	B-Architecture
computing	I-Architecture
is	O
a	O
form	O
of	O
computing	O
in	O
which	O
several	O
computations	O
are	O
executed	O
concurrently	B-Operating_System
—	O
during	O
overlapping	O
time	O
periods	O
—	O
instead	O
of	O
sequentially	O
—	O
with	O
one	O
completing	O
before	O
the	O
next	O
starts	O
.	O
</s>
<s>
This	O
is	O
a	O
property	O
of	O
a	O
system	O
—	O
whether	O
a	O
program	B-Application
,	O
computer	O
,	O
or	O
a	O
network	B-Architecture
—	O
where	O
there	O
is	O
a	O
separate	O
execution	O
point	O
or	O
"	O
thread	B-Operating_System
of	O
control	O
"	O
for	O
each	O
process	B-Operating_System
.	O
</s>
<s>
A	O
concurrent	B-Operating_System
system	I-Operating_System
is	O
one	O
where	O
a	O
computation	O
can	O
advance	O
without	O
waiting	O
for	O
all	O
other	O
computations	O
to	O
complete	O
.	O
</s>
<s>
Concurrent	B-Architecture
computing	I-Architecture
is	O
a	O
form	O
of	O
modular	B-Architecture
programming	I-Architecture
.	O
</s>
<s>
In	O
its	O
paradigm	O
an	O
overall	O
computation	O
is	O
factored	B-Algorithm
into	O
subcomputations	O
that	O
may	O
be	O
executed	O
concurrently	B-Operating_System
.	O
</s>
<s>
Pioneers	O
in	O
the	O
field	O
of	O
concurrent	B-Architecture
computing	I-Architecture
include	O
Edsger	O
Dijkstra	O
,	O
Per	O
Brinch	O
Hansen	O
,	O
and	O
C.A.R.	O
</s>
<s>
The	O
concept	O
of	O
concurrent	B-Architecture
computing	I-Architecture
is	O
frequently	O
confused	O
with	O
the	O
related	O
but	O
distinct	O
concept	O
of	O
parallel	B-Operating_System
computing	I-Operating_System
,	O
although	O
both	O
can	O
be	O
described	O
as	O
"	O
multiple	O
processes	O
executing	O
during	O
the	O
same	O
period	O
of	O
time	O
"	O
.	O
</s>
<s>
In	O
parallel	B-Operating_System
computing	I-Operating_System
,	O
execution	O
occurs	O
at	O
the	O
same	O
physical	O
instant	O
:	O
for	O
example	O
,	O
on	O
separate	O
processors	B-General_Concept
of	O
a	O
multi-processor	B-Operating_System
machine	O
,	O
with	O
the	O
goal	O
of	O
speeding	O
up	O
computations	O
—	O
parallel	B-Operating_System
computing	I-Operating_System
is	O
impossible	O
on	O
a	O
(	O
one-core	B-Architecture
)	O
single	O
processor	O
,	O
as	O
only	O
one	O
computation	O
can	O
occur	O
at	O
any	O
instant	O
(	O
during	O
any	O
single	O
clock	O
cycle	O
)	O
.	O
</s>
<s>
By	O
contrast	O
,	O
concurrent	B-Architecture
computing	I-Architecture
consists	O
of	O
process	B-Operating_System
lifetimes	O
overlapping	O
,	O
but	O
execution	O
need	O
not	O
happen	O
at	O
the	O
same	O
instant	O
.	O
</s>
<s>
The	O
goal	O
here	O
is	O
to	O
model	O
processes	O
in	O
the	O
outside	O
world	O
that	O
happen	O
concurrently	B-Operating_System
,	O
such	O
as	O
multiple	O
clients	O
accessing	O
a	O
server	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
Structuring	O
software	O
systems	O
as	O
composed	O
of	O
multiple	O
concurrent	B-Operating_System
,	O
communicating	O
parts	O
can	O
be	O
useful	O
for	O
tackling	O
complexity	O
,	O
regardless	O
of	O
whether	O
the	O
parts	O
can	O
be	O
executed	O
in	O
parallel	O
.	O
</s>
<s>
For	O
example	O
,	O
concurrent	B-Operating_System
processes	O
can	O
be	O
executed	O
on	O
one	O
core	O
by	O
interleaving	O
the	O
execution	O
steps	O
of	O
each	O
process	B-Operating_System
via	O
time-sharing	B-General_Concept
slices	O
:	O
only	O
one	O
process	B-Operating_System
runs	O
at	O
a	O
time	O
,	O
and	O
if	O
it	O
does	O
not	O
complete	O
during	O
its	O
time	O
slice	O
,	O
it	O
is	O
paused	O
,	O
another	O
process	B-Operating_System
begins	O
or	O
resumes	O
,	O
and	O
then	O
later	O
the	O
original	O
process	B-Operating_System
is	O
resumed	O
.	O
</s>
<s>
In	O
this	O
way	O
,	O
multiple	O
processes	O
are	O
part-way	O
through	O
execution	O
at	O
a	O
single	O
instant	O
,	O
but	O
only	O
one	O
process	B-Operating_System
is	O
being	O
executed	O
at	O
that	O
instant	O
.	O
</s>
<s>
Concurrent	B-Architecture
computations	I-Architecture
may	O
be	O
executed	O
in	O
parallel	O
,	O
for	O
example	O
,	O
by	O
assigning	O
each	O
process	B-Operating_System
to	O
a	O
separate	O
processor	O
or	O
processor	O
core	O
,	O
or	O
distributing	B-Architecture
a	O
computation	O
across	O
a	O
network	B-Architecture
.	O
</s>
<s>
In	O
general	O
,	O
however	O
,	O
the	O
languages	O
,	O
tools	O
,	O
and	O
techniques	O
for	O
parallel	B-Operating_System
programming	I-Operating_System
might	O
not	O
be	O
suitable	O
for	O
concurrent	B-Architecture
programming	I-Architecture
,	O
and	O
vice	O
versa	O
.	O
</s>
<s>
The	O
exact	O
timing	O
of	O
when	O
tasks	O
in	O
a	O
concurrent	B-Operating_System
system	I-Operating_System
are	O
executed	O
depends	O
on	O
the	O
scheduling	O
,	O
and	O
tasks	O
need	O
not	O
always	O
be	O
executed	O
concurrently	B-Operating_System
.	O
</s>
<s>
The	O
word	O
"	O
sequential	O
"	O
is	O
used	O
as	O
an	O
antonym	O
for	O
both	O
"	O
concurrent	B-Operating_System
"	O
and	O
"	O
parallel	O
"	O
;	O
when	O
these	O
are	O
explicitly	O
distinguished	O
,	O
concurrent/sequential	O
and	O
parallel/serial	O
are	O
used	O
as	O
opposing	O
pairs	O
.	O
</s>
<s>
A	O
schedule	O
in	O
which	O
tasks	O
execute	O
one	O
at	O
a	O
time	O
(	O
serially	O
,	O
no	O
parallelism	B-Operating_System
)	O
,	O
without	O
interleaving	O
(	O
sequentially	O
,	O
no	O
concurrency	B-Operating_System
:	O
no	O
task	O
begins	O
until	O
the	O
prior	O
task	O
ends	O
)	O
is	O
called	O
a	O
serial	O
schedule	O
.	O
</s>
<s>
A	O
set	O
of	O
tasks	O
that	O
can	O
be	O
scheduled	O
serially	O
is	O
serializable	B-Operating_System
,	O
which	O
simplifies	O
concurrency	B-Operating_System
control	I-Operating_System
.	O
</s>
<s>
The	O
main	O
challenge	O
in	O
designing	O
concurrent	B-Architecture
programs	I-Architecture
is	O
concurrency	B-Operating_System
control	I-Operating_System
:	O
ensuring	O
the	O
correct	O
sequencing	O
of	O
the	O
interactions	O
or	O
communications	O
between	O
different	O
computational	O
executions	O
,	O
and	O
coordinating	O
access	O
to	O
resources	O
that	O
are	O
shared	O
among	O
executions	O
.	O
</s>
<s>
Potential	O
problems	O
include	O
race	B-Operating_System
conditions	I-Operating_System
,	O
deadlocks	B-Operating_System
,	O
and	O
resource	B-Operating_System
starvation	I-Operating_System
.	O
</s>
<s>
Suppose	O
balance	O
=	O
500	O
,	O
and	O
two	O
concurrent	B-Operating_System
threads	B-Operating_System
make	O
the	O
calls	O
withdraw(300 )	O
and	O
withdraw(350 )	O
.	O
</s>
<s>
These	O
sorts	O
of	O
problems	O
with	O
shared	O
resources	O
benefit	O
from	O
the	O
use	O
of	O
concurrency	B-Operating_System
control	I-Operating_System
,	O
or	O
non-blocking	B-Operating_System
algorithms	I-Operating_System
.	O
</s>
<s>
The	O
advantages	O
of	O
concurrent	B-Architecture
computing	I-Architecture
include	O
:	O
</s>
<s>
Concurrent	B-Architecture
programming	I-Architecture
allows	O
the	O
time	O
that	O
would	O
be	O
spent	O
waiting	O
to	O
be	O
used	O
for	O
another	O
task	O
.	O
</s>
<s>
More	O
appropriate	O
program	B-Application
structure	O
—	O
some	O
problems	O
and	O
problem	O
domains	O
are	O
well-suited	O
to	O
representation	O
as	O
concurrent	B-Operating_System
tasks	O
or	O
processes	O
.	O
</s>
<s>
Introduced	O
in	O
1962	O
,	O
Petri	B-Operating_System
nets	I-Operating_System
were	O
an	O
early	O
attempt	O
to	O
codify	O
the	O
rules	O
of	O
concurrent	B-Operating_System
execution	O
.	O
</s>
<s>
Dataflow	O
theory	O
later	O
built	O
upon	O
these	O
,	O
and	O
Dataflow	B-General_Concept
architectures	I-General_Concept
were	O
created	O
to	O
physically	O
implement	O
the	O
ideas	O
of	O
dataflow	O
theory	O
.	O
</s>
<s>
Beginning	O
in	O
the	O
late	O
1970s	O
,	O
process	B-Operating_System
calculi	O
such	O
as	O
Calculus	O
of	O
Communicating	O
Systems	O
(	O
CCS	O
)	O
and	O
Communicating	O
Sequential	O
Processes	O
(	O
CSP	O
)	O
were	O
developed	O
to	O
permit	O
algebraic	O
reasoning	O
about	O
systems	O
composed	O
of	O
interacting	O
components	O
.	O
</s>
<s>
Input/output	B-Operating_System
automata	I-Operating_System
were	O
introduced	O
in	O
1987	O
.	O
</s>
<s>
Logics	O
such	O
as	O
Lamport	O
's	O
TLA+	O
,	O
and	O
mathematical	O
models	O
such	O
as	O
traces	O
and	O
Actor	O
event	O
diagrams	O
,	O
have	O
also	O
been	O
developed	O
to	O
describe	O
the	O
behavior	O
of	O
concurrent	B-Operating_System
systems	I-Operating_System
.	O
</s>
<s>
Software	B-Operating_System
transactional	I-Operating_System
memory	I-Operating_System
borrows	O
from	O
database	B-Application
theory	I-Application
the	O
concept	O
of	O
atomic	B-Operating_System
transactions	I-Operating_System
and	O
applies	O
them	O
to	O
memory	O
accesses	O
.	O
</s>
<s>
Concurrent	B-Architecture
programming	I-Architecture
languages	O
and	O
multiprocessor	B-Operating_System
programs	O
must	O
have	O
a	O
consistency	B-General_Concept
model	I-General_Concept
(	O
also	O
known	O
as	O
a	O
memory	O
model	O
)	O
.	O
</s>
<s>
The	O
consistency	B-General_Concept
model	I-General_Concept
defines	O
rules	O
for	O
how	O
operations	O
on	O
computer	B-General_Concept
memory	I-General_Concept
occur	O
and	O
how	O
results	O
are	O
produced	O
.	O
</s>
<s>
One	O
of	O
the	O
first	O
consistency	B-General_Concept
models	I-General_Concept
was	O
Leslie	O
Lamport	O
's	O
sequential	B-General_Concept
consistency	I-General_Concept
model	O
.	O
</s>
<s>
Sequential	B-General_Concept
consistency	I-General_Concept
is	O
the	O
property	O
of	O
a	O
program	B-Application
that	O
its	O
execution	O
produces	O
the	O
same	O
results	O
as	O
a	O
sequential	O
program	B-Application
.	O
</s>
<s>
Specifically	O
,	O
a	O
program	B-Application
is	O
sequentially	B-General_Concept
consistent	I-General_Concept
if	O
"	O
the	O
results	O
of	O
any	O
execution	O
is	O
the	O
same	O
as	O
if	O
the	O
operations	O
of	O
all	O
the	O
processors	B-General_Concept
were	O
executed	O
in	O
some	O
sequential	O
order	O
,	O
and	O
the	O
operations	O
of	O
each	O
individual	O
processor	O
appear	O
in	O
this	O
sequence	O
in	O
the	O
order	O
specified	O
by	O
its	O
program	B-Application
"	O
.	O
</s>
<s>
A	O
number	O
of	O
different	O
methods	O
can	O
be	O
used	O
to	O
implement	O
concurrent	B-Architecture
programs	I-Architecture
,	O
such	O
as	O
implementing	O
each	O
computational	O
execution	O
as	O
an	O
operating	B-Operating_System
system	I-Operating_System
process	I-Operating_System
,	O
or	O
implementing	O
the	O
computational	O
processes	O
as	O
a	O
set	O
of	O
threads	B-Operating_System
within	O
a	O
single	O
operating	B-Operating_System
system	I-Operating_System
process	I-Operating_System
.	O
</s>
<s>
In	O
some	O
concurrent	B-Architecture
computing	I-Architecture
systems	O
,	O
communication	O
between	O
the	O
concurrent	B-Operating_System
components	O
is	O
hidden	O
from	O
the	O
programmer	O
(	O
e.g.	O
,	O
by	O
using	O
futures	B-Operating_System
)	O
,	O
while	O
in	O
others	O
it	O
must	O
be	O
handled	O
explicitly	O
.	O
</s>
<s>
Shared	B-Operating_System
memory	I-Operating_System
communication	O
Concurrent	B-Operating_System
components	O
communicate	O
by	O
altering	O
the	O
contents	O
of	O
shared	B-Operating_System
memory	I-Operating_System
locations	O
(	O
exemplified	O
by	O
Java	B-Language
and	O
C#	B-Application
)	O
.	O
</s>
<s>
This	O
style	O
of	O
concurrent	B-Architecture
programming	I-Architecture
usually	O
needs	O
the	O
use	O
of	O
some	O
form	O
of	O
locking	O
(	O
e.g.	O
,	O
mutexes	B-Operating_System
,	O
semaphores	B-Operating_System
,	O
or	O
monitors	O
)	O
to	O
coordinate	O
between	O
threads	B-Operating_System
.	O
</s>
<s>
A	O
program	B-Application
that	O
properly	O
implements	O
any	O
of	O
these	O
is	O
said	O
to	O
be	O
thread-safe	B-Operating_System
.	O
</s>
<s>
Message	B-Architecture
passing	I-Architecture
communication	O
Concurrent	B-Operating_System
components	O
communicate	O
by	O
exchanging	B-Architecture
messages	I-Architecture
(	O
exemplified	O
by	O
MPI	B-Library
,	O
Go	B-Application
,	O
Scala	B-Application
,	O
Erlang	B-Operating_System
and	O
occam	B-Language
)	O
.	O
</s>
<s>
Asynchronous	O
message	B-Architecture
passing	I-Architecture
may	O
be	O
reliable	O
or	O
unreliable	O
(	O
sometimes	O
referred	O
to	O
as	O
"	O
send	O
and	O
pray	O
"	O
)	O
.	O
</s>
<s>
Message-passing	B-Architecture
concurrency	B-Operating_System
tends	O
to	O
be	O
far	O
easier	O
to	O
reason	O
about	O
than	O
shared-memory	B-Operating_System
concurrency	B-Operating_System
,	O
and	O
is	O
typically	O
considered	O
a	O
more	O
robust	O
form	O
of	O
concurrent	B-Architecture
programming	I-Architecture
.	O
</s>
<s>
A	O
wide	O
variety	O
of	O
mathematical	O
theories	O
to	O
understand	O
and	O
analyze	O
message-passing	B-Architecture
systems	O
are	O
available	O
,	O
including	O
the	O
actor	B-Application
model	I-Application
,	O
and	O
various	O
process	B-Operating_System
calculi	O
.	O
</s>
<s>
Message	B-Architecture
passing	I-Architecture
can	O
be	O
efficiently	O
implemented	O
via	O
symmetric	B-Operating_System
multiprocessing	I-Operating_System
,	O
with	O
or	O
without	O
shared	B-Operating_System
memory	I-Operating_System
cache	B-General_Concept
coherence	I-General_Concept
.	O
</s>
<s>
Shared	B-Operating_System
memory	I-Operating_System
and	O
message	B-Architecture
passing	I-Architecture
concurrency	B-Operating_System
have	O
different	O
performance	O
characteristics	O
.	O
</s>
<s>
Typically	O
(	O
although	O
not	O
always	O
)	O
,	O
the	O
per-process	O
memory	O
overhead	O
and	O
task	O
switching	O
overhead	O
is	O
lower	O
in	O
a	O
message	B-Architecture
passing	I-Architecture
system	O
,	O
but	O
the	O
overhead	O
of	O
message	B-Architecture
passing	I-Architecture
is	O
greater	O
than	O
for	O
a	O
procedure	O
call	O
.	O
</s>
<s>
Concurrent	B-Architecture
computing	I-Architecture
developed	O
out	O
of	O
earlier	O
work	O
on	O
railroads	O
and	O
telegraphy	O
,	O
from	O
the	O
19th	O
and	O
early	O
20th	O
century	O
,	O
and	O
some	O
terms	O
date	O
to	O
this	O
period	O
,	O
such	O
as	O
semaphores	B-Operating_System
.	O
</s>
<s>
These	O
arose	O
to	O
address	O
the	O
question	O
of	O
how	O
to	O
handle	O
multiple	O
trains	O
on	O
the	O
same	O
railroad	O
system	O
(	O
avoiding	O
collisions	O
and	O
maximizing	O
efficiency	O
)	O
and	O
how	O
to	O
handle	O
multiple	O
transmissions	O
over	O
a	O
given	O
set	O
of	O
wires	O
(	O
improving	O
efficiency	O
)	O
,	O
such	O
as	O
via	O
time-division	B-Protocol
multiplexing	I-Protocol
(	O
1870s	O
)	O
.	O
</s>
<s>
The	O
academic	O
study	O
of	O
concurrent	B-Architecture
algorithms	I-Architecture
started	O
in	O
the	O
1960s	O
,	O
with	O
credited	O
with	O
being	O
the	O
first	O
paper	O
in	O
this	O
field	O
,	O
identifying	O
and	O
solving	O
mutual	B-Operating_System
exclusion	I-Operating_System
.	O
</s>
<s>
Concurrency	B-Operating_System
is	O
pervasive	O
in	O
computing	O
,	O
occurring	O
from	O
low-level	O
hardware	O
on	O
a	O
single	O
chip	O
to	O
worldwide	O
networks	O
.	O
</s>
<s>
At	O
the	O
network	B-Architecture
level	O
,	O
networked	O
systems	O
are	O
generally	O
concurrent	B-Operating_System
by	O
their	O
nature	O
,	O
as	O
they	O
consist	O
of	O
separate	O
devices	O
.	O
</s>
<s>
Concurrent	B-Architecture
programming	I-Architecture
languages	O
are	O
programming	O
languages	O
that	O
use	O
language	O
constructs	O
for	O
concurrency	B-Operating_System
.	O
</s>
<s>
These	O
constructs	O
may	O
involve	O
multi-threading	B-Operating_System
,	O
support	O
for	O
distributed	B-Architecture
computing	I-Architecture
,	O
message	B-Architecture
passing	I-Architecture
,	O
shared	O
resources	O
(	O
including	O
shared	B-Operating_System
memory	I-Operating_System
)	O
or	O
futures	B-Operating_System
and	I-Operating_System
promises	I-Operating_System
.	O
</s>
<s>
Such	O
languages	O
are	O
sometimes	O
described	O
as	O
concurrency-oriented	O
languages	O
or	O
concurrency-oriented	O
programming	O
languages	O
(	O
COPL	O
)	O
.	O
</s>
<s>
Today	O
,	O
the	O
most	O
commonly	O
used	O
programming	O
languages	O
that	O
have	O
specific	O
constructs	O
for	O
concurrency	B-Operating_System
are	O
Java	B-Language
and	O
C#	B-Application
.	O
</s>
<s>
Both	O
of	O
these	O
languages	O
fundamentally	O
use	O
a	O
shared-memory	B-Operating_System
concurrency	B-Operating_System
model	O
,	O
with	O
locking	O
provided	O
by	O
monitors	O
(	O
although	O
message-passing	B-Architecture
models	O
can	O
and	O
have	O
been	O
implemented	O
on	O
top	O
of	O
the	O
underlying	O
shared-memory	B-Operating_System
model	O
)	O
.	O
</s>
<s>
Of	O
the	O
languages	O
that	O
use	O
a	O
message-passing	B-Architecture
concurrency	B-Operating_System
model	O
,	O
Erlang	B-Operating_System
is	O
probably	O
the	O
most	O
widely	O
used	O
in	O
industry	O
at	O
present	O
.	O
</s>
<s>
Many	O
concurrent	B-Architecture
programming	I-Architecture
languages	O
have	O
been	O
developed	O
more	O
as	O
research	O
languages	O
(	O
e.g.	O
</s>
<s>
Pict	B-Language
)	O
rather	O
than	O
as	O
languages	O
for	O
production	O
use	O
.	O
</s>
<s>
However	O
,	O
languages	O
such	O
as	O
Erlang	B-Operating_System
,	O
Limbo	B-Application
,	O
and	O
occam	B-Language
have	O
seen	O
industrial	O
use	O
at	O
various	O
times	O
in	O
the	O
last	O
20	O
years	O
.	O
</s>
<s>
A	O
non-exhaustive	O
list	O
of	O
languages	O
which	O
use	O
or	O
provide	O
concurrent	B-Architecture
programming	I-Architecture
facilities	O
:	O
</s>
<s>
Elixir	B-Language
—	O
dynamic	O
and	O
functional	B-Language
meta-programming	O
aware	O
language	O
running	O
on	O
the	O
Erlang	B-Operating_System
VM	O
.	O
</s>
<s>
Julia	B-Application
—	O
"	O
concurrent	B-Architecture
programming	I-Architecture
primitives	O
:	O
Tasks	O
,	O
async-wait	O
,	O
Channels.	O
"	O
</s>
<s>
JavaScript	B-Language
—	O
via	O
web	O
workers	O
,	O
in	O
a	O
browser	O
environment	O
,	O
promises	B-Operating_System
,	O
and	O
callbacks	O
.	O
</s>
<s>
Rust	B-Application
—	O
for	O
system	O
programming	O
,	O
using	O
message-passing	B-Architecture
with	O
move	O
semantics	O
,	O
shared	O
immutable	O
memory	O
,	O
and	O
shared	O
mutable	O
memory	O
.	O
</s>
<s>
Many	O
other	O
languages	O
provide	O
support	O
for	O
concurrency	B-Operating_System
in	O
the	O
form	O
of	O
libraries	O
,	O
at	O
levels	O
roughly	O
comparable	O
with	O
the	O
above	O
list	O
.	O
</s>
