<s>
Memory	B-General_Concept
overcommitment	I-General_Concept
is	O
a	O
concept	O
in	O
computing	O
that	O
covers	O
the	O
assignment	O
of	O
more	O
memory	B-Architecture
to	O
virtual	B-Architecture
computing	I-Architecture
devices	O
(	O
or	O
processes	O
)	O
than	O
the	O
physical	O
machine	O
they	O
are	O
hosted	O
,	O
or	O
running	O
on	O
,	O
actually	O
has	O
.	O
</s>
<s>
This	O
is	O
possible	O
because	O
virtual	B-Architecture
machines	I-Architecture
(	O
or	O
processes	O
)	O
do	O
not	O
necessarily	O
use	O
as	O
much	O
memory	B-Architecture
at	O
any	O
one	O
point	O
as	O
they	O
are	O
assigned	O
,	O
creating	O
a	O
buffer	O
.	O
</s>
<s>
If	O
four	O
virtual	B-Architecture
machines	I-Architecture
each	O
have	O
1GB	O
of	O
memory	B-Architecture
on	O
a	O
physical	O
machine	O
with	O
4GB	O
of	O
memory	B-Architecture
,	O
but	O
those	O
virtual	B-Architecture
machines	I-Architecture
are	O
only	O
using	O
500MB	O
,	O
it	O
is	O
possible	O
to	O
create	O
additional	O
virtual	B-Architecture
machines	I-Architecture
that	O
take	O
advantage	O
of	O
the	O
500MB	O
each	O
existing	O
machine	O
is	O
leaving	O
free	O
.	O
</s>
<s>
Memory	B-Architecture
swapping	I-Architecture
is	O
then	O
used	O
to	O
handle	O
spikes	O
in	O
memory	B-Architecture
usage	O
.	O
</s>
<s>
The	O
disadvantage	O
of	O
this	O
approach	O
is	O
that	O
memory	B-Architecture
swap	O
files	O
are	O
slower	O
to	O
read	O
from	O
than	O
'	O
actual	O
 '	O
memory	B-Architecture
,	O
which	O
can	O
lead	O
to	O
performance	O
drops	O
.	O
</s>
<s>
While	O
memory	B-General_Concept
overcommitment	I-General_Concept
is	O
usually	O
talked	O
about	O
in	O
the	O
context	O
of	O
virtualization	B-General_Concept
,	O
it	O
is	O
actually	O
a	O
generalised	O
concept	O
;	O
Windows	B-Device
NT	I-Device
contained	O
overcommitment	B-General_Concept
features	O
,	O
as	O
do	O
most	O
modern	O
generalised	O
operating	O
systems	O
,	O
including	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
.	O
</s>
