<s>
In	O
computer	B-General_Concept
science	I-General_Concept
,	O
memory	B-General_Concept
virtualization	I-General_Concept
decouples	O
volatile	O
random	B-Architecture
access	I-Architecture
memory	I-Architecture
(	O
RAM	B-Architecture
)	O
resources	O
from	O
individual	O
systems	O
in	O
the	O
data	O
centre	O
,	O
and	O
then	O
aggregates	O
those	O
resources	O
into	O
a	O
virtualized	O
memory	O
pool	O
available	O
to	O
any	O
computer	O
in	O
the	O
cluster	O
.	O
</s>
<s>
The	O
distributed	O
memory	O
pool	O
can	O
then	O
be	O
utilized	O
as	O
a	O
high-speed	O
cache	O
,	O
a	O
messaging	O
layer	O
,	O
or	O
a	O
large	O
,	O
shared	B-Operating_System
memory	I-Operating_System
resource	O
for	O
a	O
CPU	O
or	O
a	O
GPU	O
application	O
.	O
</s>
<s>
Memory	B-General_Concept
virtualization	I-General_Concept
allows	O
networked	O
,	O
and	O
therefore	O
distributed	O
,	O
servers	O
to	O
share	O
a	O
pool	O
of	O
memory	O
to	O
overcome	O
physical	O
memory	O
limitations	O
,	O
a	O
common	O
bottleneck	O
in	O
software	O
performance	O
.	O
</s>
<s>
With	O
this	O
capability	O
integrated	O
into	O
the	O
network	O
,	O
applications	O
can	O
take	O
advantage	O
of	O
a	O
very	O
large	O
amount	O
of	O
memory	O
to	O
improve	O
overall	O
performance	O
,	O
system	O
utilization	O
,	O
increase	O
memory	B-General_Concept
usage	I-General_Concept
efficiency	O
,	O
and	O
enable	O
new	O
use	O
cases	O
.	O
</s>
<s>
Management	O
software	O
and	O
the	O
technologies	O
of	O
memory	B-General_Concept
overcommitment	I-General_Concept
manage	O
shared	B-Operating_System
memory	I-Operating_System
,	O
data	O
insertion	O
,	O
eviction	O
and	O
provisioning	O
policies	O
,	O
data	O
assignment	O
to	O
contributing	O
nodes	O
,	O
and	O
handles	O
requests	O
from	O
client	O
nodes	O
.	O
</s>
<s>
At	O
the	O
application	O
level	O
,	O
the	O
pool	O
is	O
accessed	O
through	O
an	O
API	O
or	O
as	O
a	O
networked	O
file	O
system	O
to	O
create	O
a	O
high-speed	O
shared	B-Operating_System
memory	I-Operating_System
cache	O
.	O
</s>
<s>
At	O
the	O
operating	O
system	O
level	O
,	O
a	O
page	B-General_Concept
cache	I-General_Concept
can	O
utilize	O
the	O
pool	O
as	O
a	O
very	O
large	O
memory	O
resource	O
that	O
is	O
much	O
faster	O
than	O
local	O
or	O
networked	O
storage	O
.	O
</s>
<s>
Memory	B-General_Concept
virtualization	I-General_Concept
implementations	O
are	O
distinguished	O
from	O
shared	B-Operating_System
memory	I-Operating_System
systems	O
.	O
</s>
<s>
Shared	B-Operating_System
memory	I-Operating_System
systems	O
do	O
not	O
permit	O
abstraction	O
of	O
memory	O
resources	O
,	O
thus	O
requiring	O
implementation	O
with	O
a	O
single	O
operating	O
system	O
instance	O
(	O
i.e.	O
</s>
<s>
Memory	B-General_Concept
virtualization	I-General_Concept
is	O
also	O
different	O
from	O
storage	O
based	O
on	O
flash	O
memory	O
such	O
as	O
solid-state	B-Device
drives	I-Device
(	O
SSDs	B-Device
)	O
-	O
SSDs	B-Device
and	O
other	O
similar	O
technologies	O
replace	O
hard-drives	O
(	O
networked	O
or	O
otherwise	O
)	O
,	O
while	O
memory	B-General_Concept
virtualization	I-General_Concept
replaces	O
or	O
complements	O
traditional	O
RAM	B-Architecture
.	O
</s>
<s>
AppFabric	B-Application
Caching	I-Application
Service	I-Application
is	O
a	O
distributed	B-Operating_System
cache	I-Operating_System
platform	O
for	O
in-memory	B-General_Concept
caches	O
spread	O
across	O
multiple	O
systems	O
,	O
developed	O
by	O
Microsoft	O
.	O
</s>
<s>
Memory	B-General_Concept
virtualization	I-General_Concept
technology	O
follows	O
from	O
memory	B-General_Concept
management	I-General_Concept
architectures	O
and	O
virtual	B-Architecture
memory	I-Architecture
techniques	O
.	O
</s>
<s>
Virtual	B-Architecture
memory	I-Architecture
systems	O
abstract	O
between	O
physical	O
RAM	B-Architecture
and	O
virtual	O
addresses	O
,	O
assigning	O
virtual	B-Architecture
memory	I-Architecture
addresses	O
both	O
to	O
physical	O
RAM	B-Architecture
and	O
to	O
disk-based	O
storage	O
,	O
expanding	O
addressable	O
memory	O
,	O
but	O
at	O
the	O
cost	O
of	O
speed	O
.	O
</s>
<s>
NUMA	B-Operating_System
and	O
SMP	B-Operating_System
architectures	O
optimize	O
memory	B-General_Concept
allocation	I-General_Concept
within	O
multi-processor	O
systems	O
.	O
</s>
<s>
While	O
these	O
technologies	O
dynamically	O
manage	O
memory	O
within	O
individual	O
computers	O
,	O
memory	B-General_Concept
virtualization	I-General_Concept
manages	O
the	O
aggregated	O
memory	O
of	O
multiple	O
networked	O
computers	O
as	O
a	O
single	O
memory	O
pool	O
.	O
</s>
<s>
In	O
tandem	O
with	O
memory	B-General_Concept
management	I-General_Concept
innovations	O
,	O
a	O
number	O
of	O
virtualization	O
techniques	O
have	O
arisen	O
to	O
make	O
the	O
best	O
use	O
of	O
available	O
hardware	O
resources	O
.	O
</s>
<s>
Application	B-Operating_System
virtualization	I-Operating_System
was	O
demonstrated	O
in	O
mainframe	O
systems	O
first	O
.	O
</s>
<s>
The	O
next	O
wave	O
was	O
storage	B-Application
virtualization	I-Application
,	O
as	O
servers	O
connected	O
to	O
storage	O
systems	O
such	O
as	O
NAS	B-Application
or	O
SAN	O
in	O
addition	O
to	O
,	O
or	O
instead	O
of	O
,	O
on-board	O
hard	O
disk	O
drives	O
.	O
</s>
<s>
Server	O
virtualization	O
,	O
or	O
Full	B-Architecture
virtualization	I-Architecture
,	O
partitions	O
a	O
single	O
physical	O
server	O
into	O
multiple	O
virtual	B-Architecture
machines	I-Architecture
,	O
consolidating	O
multiple	O
instances	O
of	O
operating	O
systems	O
onto	O
the	O
same	O
machine	O
for	O
the	O
purpose	O
of	O
efficiency	O
and	O
flexibility	O
.	O
</s>
<s>
In	O
the	O
same	O
way	O
,	O
memory	B-General_Concept
virtualization	I-General_Concept
allocates	O
the	O
memory	O
of	O
an	O
entire	O
networked	O
cluster	O
of	O
servers	O
among	O
the	O
computers	O
in	O
that	O
cluster	O
.	O
</s>
