<s>
In	O
computing	O
,	O
a	O
parallel	B-Application
programming	I-Application
model	I-Application
is	O
an	O
abstraction	B-Application
of	O
parallel	B-Operating_System
computer	I-Operating_System
architecture	O
,	O
with	O
which	O
it	O
is	O
convenient	O
to	O
express	O
algorithms	O
and	O
their	O
composition	O
in	O
programs	B-Application
.	O
</s>
<s>
The	O
value	O
of	O
a	O
programming	O
model	O
can	O
be	O
judged	O
on	O
its	O
generality	O
:	O
how	O
well	O
a	O
range	O
of	O
different	O
problems	O
can	O
be	O
expressed	O
for	O
a	O
variety	O
of	O
different	O
architectures	O
,	O
and	O
its	O
performance	O
:	O
how	O
efficiently	O
the	O
compiled	O
programs	B-Application
can	O
execute	O
.	O
</s>
<s>
The	O
implementation	O
of	O
a	O
parallel	B-Application
programming	I-Application
model	I-Application
can	O
take	O
the	O
form	O
of	O
a	O
library	B-Library
invoked	O
from	O
a	O
sequential	B-Architecture
language	I-Architecture
,	O
as	O
an	O
extension	O
to	O
an	O
existing	O
language	O
,	O
or	O
as	O
an	O
entirely	O
new	O
language	O
.	O
</s>
<s>
Consensus	O
around	O
a	O
particular	O
programming	O
model	O
is	O
important	O
because	O
it	O
leads	O
to	O
different	O
parallel	B-Operating_System
computers	I-Operating_System
being	O
built	O
with	O
support	O
for	O
the	O
model	O
,	O
thereby	O
facilitating	O
portability	B-Architecture
of	O
software	O
.	O
</s>
<s>
In	O
this	O
sense	O
,	O
programming	O
models	O
are	O
referred	O
to	O
as	O
bridging	B-Architecture
between	O
hardware	O
and	O
software	O
.	O
</s>
<s>
Classifications	O
of	O
parallel	B-Application
programming	I-Application
models	I-Application
can	O
be	O
divided	O
broadly	O
into	O
two	O
areas	O
:	O
process	O
interaction	O
and	O
problem	O
decomposition	O
.	O
</s>
<s>
In	O
a	O
shared-memory	O
model	O
,	O
parallel	O
processes	O
share	O
a	O
global	B-Application
address	I-Application
space	I-Application
that	O
they	O
read	O
and	O
write	O
to	O
asynchronously	O
.	O
</s>
<s>
Asynchronous	O
concurrent	B-Operating_System
access	O
can	O
lead	O
to	O
race	B-Operating_System
conditions	I-Operating_System
,	O
and	O
mechanisms	O
such	O
as	O
locks	B-Operating_System
,	O
semaphores	B-Operating_System
and	O
monitors	O
can	O
be	O
used	O
to	O
avoid	O
these	O
.	O
</s>
<s>
Conventional	O
multi-core	B-Architecture
processors	I-Architecture
directly	O
support	O
shared	O
memory	O
,	O
which	O
many	O
parallel	B-Operating_System
programming	I-Operating_System
languages	O
and	O
libraries	O
,	O
such	O
as	O
Cilk	B-Language
,	O
OpenMP	B-Application
and	O
Threading	B-Application
Building	I-Application
Blocks	I-Application
,	O
are	O
designed	O
to	O
exploit	O
.	O
</s>
<s>
The	O
Communicating	O
sequential	O
processes	O
(	O
CSP	O
)	O
formalisation	O
of	O
message	O
passing	O
uses	O
synchronous	O
communication	O
channels	O
to	O
connect	O
processes	O
,	O
and	O
led	O
to	O
important	O
languages	O
such	O
as	O
Occam	B-Language
,	O
Limbo	B-Application
and	O
Go	B-Application
.	O
</s>
<s>
In	O
contrast	O
,	O
the	O
actor	B-Application
model	I-Application
uses	O
asynchronous	O
message	O
passing	O
and	O
has	O
been	O
employed	O
in	O
the	O
design	O
of	O
languages	O
such	O
as	O
D	B-Application
,	O
Scala	B-Application
and	O
SALSA	O
.	O
</s>
<s>
Partitioned	B-Application
Global	I-Application
Address	I-Application
Space	I-Application
(	O
PGAS	B-Application
)	O
models	O
provide	O
a	O
middle	O
ground	O
between	O
shared	O
memory	O
and	O
message	O
passing	O
.	O
</s>
<s>
PGAS	B-Application
provides	O
a	O
global	O
memory	O
address	O
space	O
abstraction	B-Application
that	O
is	O
logically	O
partitioned	O
,	O
where	O
a	O
portion	O
is	O
local	O
to	O
each	O
process	O
.	O
</s>
<s>
reads	O
and	O
writes	O
)	O
on	O
the	O
global	B-Application
address	I-Application
space	I-Application
,	O
in	O
a	O
manner	O
reminiscent	O
of	O
shared	O
memory	O
models	O
.	O
</s>
<s>
However	O
by	O
semantically	O
partitioning	O
the	O
global	B-Application
address	I-Application
space	I-Application
into	O
portions	O
with	O
affinity	O
to	O
a	O
particular	O
processes	O
,	O
they	O
allow	O
programmers	O
to	O
exploit	O
locality	B-General_Concept
of	I-General_Concept
reference	I-General_Concept
and	O
enable	O
efficient	O
implementation	O
on	O
distributed	B-Operating_System
memory	I-Operating_System
parallel	B-Operating_System
computers	I-Operating_System
.	O
</s>
<s>
PGAS	B-Application
is	O
offered	O
by	O
many	O
many	O
parallel	B-Operating_System
programming	I-Operating_System
languages	O
and	O
libraries	O
,	O
such	O
as	O
Fortran	O
2008	O
,	O
Chapel	B-Language
,	O
,	O
and	O
SHMEM	B-Operating_System
.	O
</s>
<s>
Two	O
examples	O
of	O
implicit	O
parallelism	B-Operating_System
are	O
with	O
domain-specific	B-Language
languages	I-Language
where	O
the	O
concurrency	B-Architecture
within	O
high-level	O
operations	O
is	O
prescribed	O
,	O
and	O
with	O
functional	B-Language
programming	I-Language
languages	I-Language
because	O
the	O
absence	O
of	O
side-effects	O
allows	O
non-dependent	O
functions	O
to	O
be	O
executed	O
in	O
parallel	O
.	O
</s>
<s>
However	O
,	O
this	O
kind	O
of	O
parallelism	B-Operating_System
is	O
difficult	O
to	O
manage	O
and	O
functional	B-Language
languages	I-Language
such	O
as	O
Concurrent	B-Language
Haskell	I-Language
and	O
Concurrent	B-Language
ML	I-Language
provide	O
features	O
to	O
manage	O
parallelism	B-Operating_System
explicitly	O
and	O
correctly	O
.	O
</s>
<s>
A	O
parallel	B-Operating_System
program	I-Operating_System
is	O
composed	O
of	O
simultaneously	O
executing	O
processes	O
.	O
</s>
<s>
Task	B-Operating_System
parallelism	I-Operating_System
is	O
a	O
natural	O
way	O
to	O
express	O
message-passing	O
communication	O
.	O
</s>
<s>
In	O
Flynn	B-Operating_System
's	I-Operating_System
taxonomy	I-Operating_System
,	O
task	B-Operating_System
parallelism	I-Operating_System
is	O
usually	O
classified	O
as	O
MIMD/MPMD	O
or	O
MISD	B-Operating_System
.	O
</s>
<s>
In	O
Flynn	B-Operating_System
's	I-Operating_System
taxonomy	I-Operating_System
,	O
data	O
parallelism	B-Operating_System
is	O
usually	O
classified	O
as	O
MIMD/SPMD	O
or	O
SIMD	B-Device
.	O
</s>
<s>
As	O
with	O
implicit	O
process	O
interaction	O
,	O
an	O
implicit	O
model	O
of	O
parallelism	B-Operating_System
reveals	O
nothing	O
to	O
the	O
programmer	O
as	O
the	O
compiler	O
,	O
the	O
runtime	O
or	O
the	O
hardware	O
is	O
responsible	O
.	O
</s>
<s>
For	O
example	O
,	O
in	O
compilers	O
,	O
automatic	O
parallelization	B-Operating_System
is	O
the	O
process	O
of	O
converting	O
sequential	O
code	O
into	O
parallel	B-Operating_System
code	I-Operating_System
,	O
and	O
in	O
computer	O
architecture	O
,	O
superscalar	B-General_Concept
execution	I-General_Concept
is	O
a	O
mechanism	O
whereby	O
instruction-level	B-Operating_System
parallelism	I-Operating_System
is	O
exploited	O
to	O
perform	O
operations	O
in	O
parallel	O
.	O
</s>
<s>
Parallel	B-Application
programming	I-Application
models	I-Application
are	O
closely	O
related	O
to	O
models	O
of	O
computation	O
.	O
</s>
<s>
A	O
model	O
of	O
parallel	B-Operating_System
computation	I-Operating_System
is	O
an	O
abstraction	B-Application
used	O
to	O
analyze	O
the	O
cost	O
of	O
computational	O
processes	O
,	O
but	O
it	O
does	O
not	O
necessarily	O
need	O
to	O
be	O
practical	O
,	O
in	O
that	O
it	O
can	O
be	O
implemented	O
efficiently	O
in	O
hardware	O
and/or	O
software	O
.	O
</s>
<s>
A	O
parallel	B-Operating_System
programming	I-Operating_System
language	O
may	O
be	O
based	O
on	O
one	O
or	O
a	O
combination	O
of	O
programming	O
models	O
.	O
</s>
<s>
For	O
example	O
,	O
High	B-Language
Performance	I-Language
Fortran	I-Language
is	O
based	O
on	O
shared-memory	O
interactions	O
and	O
data-parallel	O
problem	O
decomposition	O
,	O
and	O
Go	B-Application
provides	O
mechanism	O
for	O
shared-memory	O
and	O
message-passing	O
interaction	O
.	O
</s>
