<s>
In	O
IBM	B-Device
System/390	I-Device
and	O
IBM	B-Device
Z	I-Device
mainframe	B-Architecture
computers	I-Architecture
,	O
a	O
Coupling	B-Device
Facility	I-Device
or	O
CF	O
is	O
a	O
piece	O
of	O
computer	B-Architecture
hardware	I-Architecture
or	O
virtual	B-Architecture
machine	I-Architecture
that	O
coordinates	O
multiple	O
processors	O
.	O
</s>
<s>
A	O
Parallel	B-Operating_System
Sysplex	I-Operating_System
relies	O
on	O
one	O
or	O
more	O
Coupling	B-Device
Facilities	I-Device
(	O
CFs	O
)	O
.	O
</s>
<s>
A	O
coupling	B-Device
facility	I-Device
is	O
a	O
mainframe	B-Architecture
processor	O
(	O
runs	O
in	O
an	O
own	O
LPAR	B-Device
,	O
with	O
dedicated	O
physical	O
CP	O
,	O
defined	O
through	O
Hardware	B-Device
Management	I-Device
Console	I-Device
(	O
HMC	O
)	O
)	O
,	O
with	O
memory	O
and	O
special	O
channels	O
(	O
CF	O
Links	O
)	O
,	O
and	O
a	O
specialized	O
operating	O
system	O
called	O
Coupling	B-Device
Facility	I-Device
Control	O
Code	O
(	O
CFCC	O
)	O
.	O
</s>
<s>
The	O
information	O
in	O
the	O
CF	O
resides	O
entirely	O
in	O
memory	O
as	O
CFCC	O
is	O
not	O
a	O
virtual	B-Architecture
memory	I-Architecture
operating	O
system	O
.	O
</s>
<s>
When	O
originally	O
introduced	O
,	O
the	O
CFCC	O
executed	O
in	O
a	O
separate	O
9674	O
mainframe	B-Architecture
unit	O
that	O
was	O
essentially	O
a	O
processor	O
without	O
I/O	O
facilities	O
other	O
than	O
the	O
CF	O
links	O
.	O
</s>
<s>
Later	O
IBM	O
enabled	O
the	O
use	O
of	O
an	O
Internal	O
Coupling	B-Device
Facility	I-Device
where	O
the	O
CFCC	O
runs	O
in	O
a	O
logical	B-Device
partition	I-Device
(	O
LPAR	B-Device
)	O
defined	O
in	O
standard	O
processor	O
complex	O
and	O
communicates	O
over	O
internal	O
links	O
within	O
that	O
processor	O
complex	O
hardware	B-Architecture
.	O
</s>
<s>
Recovery	O
support	O
in	O
the	O
z/OS	B-Application
operating	O
system	O
allows	O
structures	O
to	O
be	O
rebuilt	O
in	O
the	O
alternate	O
CF	O
in	O
the	O
event	O
of	O
a	O
failure	O
.	O
</s>
<s>
Using	O
the	O
CF	O
links	O
,	O
data	O
can	O
be	O
directly	O
exchanged	O
between	O
the	O
CF	O
memory	O
and	O
the	O
memory	O
of	O
the	O
attached	O
systems	O
,	O
using	O
a	O
direct	B-General_Concept
memory	I-General_Concept
access	I-General_Concept
like	O
mechanism	O
,	O
without	O
interrupting	O
a	O
running	O
program	O
.	O
</s>
<s>
The	O
System	B-Device
z	I-Device
Architecture	O
includes	O
18	O
special	O
machine	O
instructions	O
and	O
additional	O
hardware	B-Architecture
features	O
supporting	O
CF	O
operation	O
.	O
</s>
<s>
It	O
is	O
said	O
to	O
be	O
connected	O
to	O
by	O
specific	O
CF-exploiting	O
applications	O
on	O
the	O
coupled	O
z/OS	B-Application
systems	O
.	O
</s>
<s>
A	O
typical	O
Parallel	B-Operating_System
Sysplex	I-Operating_System
contains	O
several	O
structures	O
of	O
each	O
type	O
.	O
</s>
<s>
For	O
example	O
,	O
each	O
Db2	B-Application
Data	O
Sharing	O
Group	O
uses	O
one	O
Lock	O
structure	O
,	O
one	O
List	O
structure	O
and	O
several	O
cache	O
structures	O
(	O
one	O
for	O
each	O
Group	O
Buffer	O
Pool	O
(	O
GBP	O
)	O
)	O
.	O
</s>
<s>
When	O
a	O
z/OS	B-Application
system	O
issues	O
a	O
request	O
it	O
waits	O
for	O
the	O
request	O
to	O
complete	O
,	O
actively	O
spinning	O
on	O
one	O
of	O
its	O
own	O
processors	O
.	O
</s>
<s>
When	O
a	O
z/OS	B-Application
system	O
issues	O
a	O
request	O
it	O
does	O
n't	O
wait	O
for	O
the	O
request	O
to	O
complete	O
.	O
</s>
<s>
Exploiting	O
z/OS	B-Application
applications	O
explicitly	O
issue	O
CF	O
requests	O
as	O
synch	O
or	O
asynch	O
.	O
</s>
<s>
In	O
z/OS	B-Application
Release	O
2	O
,	O
the	O
Dynamic	O
Request	O
Conversion	O
heuristic	O
algorithm	O
was	O
introduced	O
.	O
</s>
<s>
The	O
greater	O
the	O
distance	O
between	O
the	O
coupled	O
z/OS	B-Application
system	O
and	O
the	O
CF	O
the	O
greater	O
the	O
likelihood	O
requests	O
will	O
be	O
converted	O
to	O
Async	O
from	O
Sync	O
.	O
</s>
<s>
CFs	O
are	O
unique	O
to	O
S/390	B-Device
,	O
zSeries	O
and	O
System	B-Device
z	I-Device
mainframes	B-Architecture
.	O
</s>
<s>
They	O
are	O
key	O
to	O
Parallel	B-Operating_System
Sysplex	I-Operating_System
technology	O
.	O
</s>
<s>
In	O
most	O
cases	O
the	O
new	O
function	O
or	O
performance	O
improvement	O
requires	O
a	O
corequisite	O
release	O
of	O
z/OS	B-Application
and	O
perhaps	O
new	O
function	O
in	O
some	O
subsystem	O
(	O
such	O
as	O
Db2	B-Application
)	O
.	O
</s>
<s>
One	O
such	O
example	O
is	O
Coupling	B-Device
Facility	I-Device
Structure	O
Duplexing	O
.	O
</s>
