<s>
Coarray	B-Language
Fortran	I-Language
(	O
CAF	O
)	O
,	O
formerly	O
known	O
as	O
F--	O
,	O
started	O
as	O
an	O
extension	O
of	O
Fortran	B-Application
95/2003	O
for	O
parallel	B-Operating_System
processing	I-Operating_System
created	O
by	O
Robert	O
Numrich	O
and	O
John	O
Reid	O
in	O
the	O
1990s	O
.	O
</s>
<s>
The	O
Fortran	B-Application
2008	O
standard	O
(	O
ISO/IEC	O
1539-1:2010	O
)	O
now	O
includes	O
coarrays	O
(	O
spelled	O
without	O
hyphen	O
)	O
,	O
as	O
decided	O
at	O
the	O
May	O
2005	O
meeting	O
of	O
the	O
ISO	O
Fortran	B-Application
Committee	O
;	O
the	O
syntax	O
in	O
the	O
Fortran	B-Application
2008	O
standard	O
is	O
slightly	O
different	O
from	O
the	O
original	O
CAF	O
proposal	O
.	O
</s>
<s>
A	O
CAF	O
program	B-Application
is	O
interpreted	O
as	O
if	O
it	O
were	O
replicated	O
a	O
number	O
of	O
times	O
and	O
all	O
copies	O
were	O
executed	O
asynchronously	O
.	O
</s>
<s>
The	O
array	B-Data_Structure
syntax	O
of	O
Fortran	B-Application
is	O
extended	O
with	O
additional	O
trailing	O
subscripts	O
in	O
square	O
brackets	O
to	O
provide	O
a	O
concise	O
representation	O
of	O
references	O
to	O
data	O
that	O
is	O
spread	O
across	O
images	O
.	O
</s>
<s>
The	O
CAF	O
extension	O
was	O
implemented	O
in	O
some	O
Fortran	B-Application
compilers	B-Language
such	O
as	O
those	O
from	O
Cray	O
(	O
since	O
release	O
3.1	O
)	O
.	O
</s>
<s>
Since	O
the	O
inclusion	O
of	O
coarrays	O
in	O
the	O
Fortran	B-Application
2008	O
standard	O
,	O
the	O
number	O
of	O
implementations	O
is	O
growing	O
.	O
</s>
<s>
The	O
first	O
open-source	B-Application
compiler	B-Language
which	O
implemented	O
coarrays	O
as	O
specified	O
in	O
the	O
Fortran	B-Application
2008	O
standard	O
for	O
Linux	B-Operating_System
architectures	I-Operating_System
is	O
G95	B-Application
.	O
</s>
<s>
Currently	O
,	O
GNU	B-Application
Fortran	I-Application
provides	O
wide	O
coverage	O
of	O
Fortran	B-Application
's	O
coarray	O
features	O
in	O
single	O
-	O
and	O
multi-image	O
configuration	O
(	O
the	O
latter	O
based	O
on	O
the	O
OpenCoarrays	O
library	O
)	O
.	O
</s>
<s>
Another	O
implementation	O
of	O
coarrays	O
and	O
related	O
parallel	B-Operating_System
extensions	O
from	O
Fortran	B-Application
2008	O
is	O
available	O
in	O
the	O
OpenUH	O
compiler	B-Language
(	O
a	O
branch	O
of	O
Open64	B-Application
)	O
developed	O
at	O
the	O
University	O
of	O
Houston	O
.	O
</s>
<s>
CAF	O
is	O
often	O
implemented	O
on	O
top	O
of	O
a	O
Message	B-Application
Passing	I-Application
Interface	I-Application
(	O
MPI	O
)	O
library	O
for	O
portability	O
.	O
</s>
<s>
Some	O
implementations	O
,	O
such	O
as	O
the	O
ones	O
available	O
in	O
the	O
GNU	B-Application
Fortran	I-Application
and	O
OpenUH	O
compilers	B-Language
,	O
may	O
run	O
on	O
top	O
of	O
other	O
low-level	O
layers	O
(	O
for	O
example	O
,	O
GASNet	O
)	O
designed	O
for	O
supporting	O
partitioned	B-Application
global	I-Application
address	I-Application
space	I-Application
languages	O
.	O
</s>
<s>
The	O
program	B-Application
above	O
scales	O
poorly	O
because	O
the	O
loop	O
that	O
distributes	O
information	O
executes	O
sequentially	O
.	O
</s>
<s>
Writing	O
scalable	O
programs	O
often	O
requires	O
a	O
sophisticated	O
understanding	O
of	O
parallel	B-Operating_System
algorithms	O
,	O
a	O
detailed	O
knowledge	O
of	O
the	O
underlying	O
network	O
characteristics	O
,	O
and	O
special	O
tuning	O
for	O
application	O
characteristics	O
such	O
as	O
the	O
size	O
of	O
data	O
transfers	O
.	O
</s>
<s>
For	O
most	O
application	O
developers	O
,	O
letting	O
the	O
compiler	B-Language
or	O
runtime	O
library	O
decide	O
the	O
best	O
algorithm	O
proves	O
more	O
robust	O
and	O
high-performing	O
.	O
</s>
<s>
Fortran	B-Application
2018	O
will	O
offer	O
collective	O
communication	O
subroutines	O
that	O
empower	O
compiler	B-Language
and	O
runtime	O
library	O
teams	O
to	O
encapsulate	O
efficient	O
parallel	B-Operating_System
algorithms	O
for	O
collective	O
communication	O
and	O
distributed	O
computation	O
in	O
a	O
set	O
of	O
collective	O
subroutines	O
.	O
</s>
<s>
These	O
subroutines	O
and	O
other	O
new	O
parallel	B-Operating_System
programming	O
features	O
are	O
summarized	O
in	O
a	O
technical	O
specification	O
that	O
the	O
Fortran	B-Application
standards	O
committee	O
has	O
voted	O
to	O
incorporate	O
into	O
Fortran	B-Application
2018	O
.	O
</s>
<s>
In	O
2011	O
,	O
Rice	O
University	O
pursued	O
an	O
alternate	O
vision	O
of	O
coarray	O
extensions	O
for	O
the	O
Fortran	B-Application
language	I-Application
.	O
</s>
<s>
Their	O
perspective	O
is	O
that	O
the	O
Fortran	B-Application
2008	O
standard	O
committee	O
's	O
design	O
choices	O
were	O
shaped	O
more	O
by	O
the	O
desire	O
to	O
introduce	O
as	O
few	O
modifications	O
to	O
the	O
language	O
as	O
possible	O
than	O
to	O
assemble	O
the	O
best	O
set	O
of	O
extensions	O
to	O
support	O
parallel	B-Operating_System
programming	I-Operating_System
.	O
</s>
<s>
In	O
their	O
view	O
,	O
both	O
Numrich	O
and	O
Reid	O
's	O
original	O
design	O
and	O
the	O
coarray	O
extensions	O
proposed	O
for	O
Fortran	B-Application
2008	O
suffer	O
from	O
the	O
following	O
shortcomings	O
:	O
</s>
<s>
There	O
is	O
no	O
support	O
for	O
processor	B-General_Concept
subsets	O
;	O
for	O
instance	O
,	O
coarrays	O
must	O
be	O
allocated	O
over	O
all	O
images	O
.	O
</s>
<s>
Reliance	O
on	O
named	O
critical	O
sections	O
for	O
mutual	B-Operating_System
exclusion	I-Operating_System
hinders	O
scalable	O
parallelism	B-Operating_System
by	O
associating	O
mutual	B-Operating_System
exclusion	I-Operating_System
with	O
code	O
regions	O
rather	O
than	O
data	O
objects	O
.	O
</s>
<s>
Fortran	B-Application
2008	O
's	O
sync	O
images	O
statement	O
does	O
not	O
provide	O
a	O
safe	O
synchronization	O
space	O
.	O
</s>
<s>
To	O
address	O
these	O
shortcomings	O
,	O
the	O
Rice	O
University	O
group	O
is	O
developing	O
a	O
clean-slate	O
redesign	O
of	O
the	O
Coarray	B-Language
Fortran	I-Language
programming	O
model	O
.	O
</s>
<s>
Rice	O
's	O
new	O
design	O
for	O
Coarray	B-Language
Fortran	I-Language
,	O
which	O
they	O
call	O
Coarray	B-Language
Fortran	I-Language
2.0	O
,	O
is	O
an	O
expressive	O
set	O
of	O
coarray-based	O
extensions	O
to	O
Fortran	B-Application
designed	O
to	O
provide	O
a	O
productive	O
parallel	B-Operating_System
programming	I-Operating_System
model	O
.	O
</s>
<s>
Compared	O
to	O
Fortran	B-Application
2008	O
,	O
Rice	O
's	O
new	O
coarray-based	O
language	O
extensions	O
include	O
some	O
additional	O
features	O
:	O
</s>
<s>
enhanced	O
support	O
for	O
synchronization	O
for	O
fine-grain	O
control	O
over	O
program	B-Application
execution	O
.	O
</s>
<s>
safe	O
and	O
scalable	O
support	O
for	O
mutual	B-Operating_System
exclusion	I-Operating_System
,	O
including	O
locks	O
and	O
lock	O
sets	O
,	O
</s>
