<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
distributed	B-Operating_System
shared	I-Operating_System
memory	I-Operating_System
(	O
DSM	O
)	O
is	O
a	O
form	O
of	O
memory	B-General_Concept
architecture	I-General_Concept
where	O
physically	O
separated	O
memories	O
can	O
be	O
addressed	O
as	O
a	O
single	O
shared	O
address	B-General_Concept
space	I-General_Concept
.	O
</s>
<s>
The	O
term	O
"	O
shared	O
"	O
does	O
not	O
mean	O
that	O
there	O
is	O
a	O
single	O
centralized	O
memory	B-Architecture
,	O
but	O
that	O
the	O
address	B-General_Concept
space	I-General_Concept
is	O
shared	O
—	O
i.e.	O
,	O
the	O
same	O
physical	O
address	O
on	O
two	O
processors	O
refers	O
to	O
the	O
same	O
location	O
in	O
memory	B-Architecture
.	O
</s>
<s>
Distributed	B-Operating_System
global	I-Operating_System
address	I-Operating_System
space	I-Operating_System
(	O
DGAS	B-Operating_System
)	O
,	O
is	O
a	O
similar	O
term	O
for	O
a	O
wide	O
class	O
of	O
software	O
and	O
hardware	O
implementations	O
,	O
in	O
which	O
each	O
node	B-Protocol
of	O
a	O
cluster	B-Architecture
has	O
access	O
to	O
shared	B-Operating_System
memory	I-Operating_System
in	O
addition	O
to	O
each	O
node	B-Protocol
's	O
private	O
(	O
i.e.	O
,	O
not	O
shared	O
)	O
memory	B-Architecture
.	O
</s>
<s>
A	O
distributed-memory	O
system	O
,	O
often	O
called	O
a	O
multicomputer	B-Operating_System
,	O
consists	O
of	O
multiple	O
independent	O
processing	O
nodes	O
with	O
local	O
memory	B-Architecture
modules	O
which	O
is	O
connected	O
by	O
a	O
general	O
interconnection	O
network	O
.	O
</s>
<s>
Software	O
DSM	O
systems	O
can	O
be	O
implemented	O
in	O
an	O
operating	B-General_Concept
system	I-General_Concept
,	O
or	O
as	O
a	O
programming	O
library	O
and	O
can	O
be	O
thought	O
of	O
as	O
extensions	O
of	O
the	O
underlying	O
virtual	B-Architecture
memory	I-Architecture
architecture	O
.	O
</s>
<s>
When	O
implemented	O
in	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
such	O
systems	O
are	O
transparent	O
to	O
the	O
developer	O
;	O
which	O
means	O
that	O
the	O
underlying	O
distributed	B-Operating_System
memory	I-Operating_System
is	O
completely	O
hidden	O
from	O
the	O
users	O
.	O
</s>
<s>
A	O
DSM	O
system	O
implements	O
the	O
shared-memory	B-Operating_System
model	O
on	O
a	O
physically	O
distributed	B-Operating_System
memory	I-Operating_System
system	O
.	O
</s>
<s>
Hardware	O
examples	O
include	O
cache	B-General_Concept
coherence	I-General_Concept
circuits	O
and	O
network	B-Protocol
interface	I-Protocol
controllers	I-Protocol
.	O
</s>
<s>
Software	O
DSM	O
systems	O
also	O
have	O
the	O
flexibility	O
to	O
organize	O
the	O
shared	B-Operating_System
memory	I-Operating_System
region	O
in	O
different	O
ways	O
.	O
</s>
<s>
The	O
page	B-General_Concept
based	O
approach	O
organizes	O
shared	B-Operating_System
memory	I-Operating_System
into	O
pages	O
of	O
fixed	O
size	O
.	O
</s>
<s>
In	O
contrast	O
,	O
the	O
object	O
based	O
approach	O
organizes	O
the	O
shared	B-Operating_System
memory	I-Operating_System
region	O
as	O
an	O
abstract	O
space	O
for	O
storing	O
shareable	O
objects	O
of	O
variable	O
sizes	O
.	O
</s>
<s>
Another	O
commonly	O
seen	O
implementation	O
uses	O
a	O
tuple	B-Operating_System
space	I-Operating_System
,	O
in	O
which	O
the	O
unit	O
of	O
sharing	O
is	O
a	O
tuple	B-Application
.	O
</s>
<s>
Shared	B-Operating_System
memory	I-Operating_System
architecture	I-Operating_System
may	O
involve	O
separating	O
memory	B-Architecture
into	O
shared	O
parts	O
distributed	O
amongst	O
nodes	O
and	O
main	O
memory	B-Architecture
;	O
or	O
distributing	O
all	O
memory	B-Architecture
between	O
nodes	O
.	O
</s>
<s>
A	O
coherence	B-General_Concept
protocol	I-General_Concept
,	O
chosen	O
in	O
accordance	O
with	O
a	O
consistency	B-General_Concept
model	I-General_Concept
,	O
maintains	O
memory	B-Operating_System
coherence	I-Operating_System
.	O
</s>
<s>
Memory	B-Operating_System
coherence	I-Operating_System
is	O
necessary	O
such	O
that	O
the	O
system	O
which	O
organizes	O
the	O
DSM	O
is	O
able	O
to	O
track	O
and	O
maintain	O
the	O
state	O
of	O
data	O
blocks	O
in	O
nodes	O
across	O
the	O
memories	O
comprising	O
the	O
system	O
.	O
</s>
<s>
As	O
blocks	O
come	O
into	O
the	O
directory	O
organization	O
,	O
they	O
will	O
transition	O
from	O
U	O
to	O
EM	O
(	O
ownership	O
state	O
)	O
in	O
the	O
initial	O
node	B-Protocol
.	O
</s>
<s>
There	O
are	O
two	O
primary	O
methods	O
for	O
allowing	O
the	O
system	O
to	O
track	O
where	O
blocks	O
are	O
cached	O
and	O
in	O
what	O
condition	O
across	O
each	O
node	B-Protocol
.	O
</s>
<s>
Home-centric	O
request-response	O
uses	O
the	O
home	O
to	O
service	O
requests	O
and	O
drive	O
states	O
,	O
whereas	O
requester-centric	O
allows	O
each	O
node	B-Protocol
to	O
drive	O
and	O
manage	O
its	O
own	O
requests	O
through	O
the	O
home	O
.	O
</s>
<s>
In	O
a	O
home-centric	O
system	O
,	O
the	O
DSM	O
will	O
avoid	O
having	O
to	O
handle	O
request-response	O
races	O
between	O
nodes	O
by	O
allowing	O
only	O
one	O
transaction	O
to	O
occur	O
at	O
a	O
time	O
until	O
the	O
home	O
node	B-Protocol
has	O
decided	O
that	O
the	O
transaction	O
is	O
finished	O
—	O
usually	O
when	O
the	O
home	O
has	O
received	O
every	O
responding	O
processor	O
's	O
response	O
to	O
the	O
request	O
.	O
</s>
<s>
An	O
example	O
of	O
this	O
is	O
Intel	O
's	O
QPI	B-Architecture
home-source	O
mode	O
.	O
</s>
<s>
The	O
advantages	O
of	O
this	O
approach	O
are	O
that	O
it	O
's	O
simple	O
to	O
implement	O
but	O
its	O
request-response	O
strategy	O
is	O
slow	O
and	O
buffered	O
due	O
to	O
the	O
home	O
node	B-Protocol
's	O
limitations	O
.	O
</s>
<s>
For	O
example	O
:	O
when	O
one	O
node	B-Protocol
is	O
processing	O
a	O
block	O
,	O
if	O
it	O
receives	O
a	O
request	O
for	O
that	O
block	O
from	O
another	O
node	B-Protocol
it	O
will	O
send	O
a	O
NAck	O
(	O
Negative	O
Acknowledgement	O
)	O
to	O
tell	O
the	O
initiator	O
that	O
the	O
processing	O
node	B-Protocol
ca	O
n't	O
fulfill	O
that	O
request	O
right	O
away	O
.	O
</s>
<s>
An	O
example	O
of	O
this	O
is	O
Intel	O
's	O
QPI	B-Architecture
snoop-source	O
mode	O
.	O
</s>
<s>
The	O
DSM	O
must	O
follow	O
certain	O
rules	O
to	O
maintain	O
consistency	O
over	O
how	O
read	O
and	O
write	O
order	O
is	O
viewed	O
among	O
nodes	O
,	O
called	O
the	O
system	O
's	O
consistency	B-General_Concept
model	I-General_Concept
.	O
</s>
<s>
Suppose	O
we	O
have	O
n	O
processes	O
and	O
Mi	O
memory	B-Architecture
operations	O
for	O
each	O
process	O
i	O
,	O
and	O
that	O
all	O
the	O
operations	O
are	O
executed	O
sequentially	O
.	O
</s>
<s>
Memory	B-Operating_System
coherence	I-Operating_System
for	O
DSM	O
defines	O
which	O
interleavings	O
are	O
permitted	O
.	O
</s>
<s>
In	O
Read	O
replication	O
multiple	O
nodes	O
can	O
read	O
at	O
the	O
same	O
time	O
but	O
only	O
one	O
node	B-Protocol
can	O
write	O
.	O
</s>
<s>
Replication	O
of	O
shared	B-Operating_System
data	I-Operating_System
in	O
general	O
tends	O
to	O
:	O
</s>
<s>
Release	O
consistency	O
:	O
when	O
a	O
process	O
exits	O
a	O
critical	B-Operating_System
section	I-Operating_System
,	O
new	O
values	O
of	O
the	O
variables	O
are	O
propagated	O
to	O
all	O
sites	O
.	O
</s>
<s>
Entry	O
consistency	O
:	O
when	O
a	O
process	O
enters	O
a	O
critical	B-Operating_System
section	I-Operating_System
,	O
it	O
will	O
automatically	O
update	O
the	O
values	O
of	O
the	O
shared	O
variables	O
.	O
</s>
<s>
View-based	O
Consistency	O
:	O
it	O
is	O
a	O
variant	O
of	O
Entry	O
Consistency	O
,	O
except	O
the	O
shared	O
variables	O
of	O
a	O
critical	B-Operating_System
section	I-Operating_System
are	O
automatically	O
detected	O
by	O
the	O
system	O
.	O
</s>
<s>
An	O
implementation	O
of	O
view-based	O
consistency	O
is	O
which	O
has	O
comparable	O
performance	O
to	O
MPI	B-Application
on	O
cluster	B-Architecture
computers	I-Architecture
.	O
</s>
