<s>
Virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
(	O
also	O
referred	O
to	O
as	O
RAM	B-General_Concept
compression	I-General_Concept
and	O
memory	B-General_Concept
compression	I-General_Concept
)	O
is	O
a	O
memory	B-General_Concept
management	I-General_Concept
technique	O
that	O
utilizes	O
data	B-General_Concept
compression	I-General_Concept
to	O
reduce	O
the	O
size	O
or	O
number	O
of	O
paging	B-Architecture
requests	O
to	O
and	O
from	O
the	O
auxiliary	O
storage	O
.	O
</s>
<s>
In	O
a	O
virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
system	O
,	O
pages	O
to	O
be	O
paged	O
out	O
of	O
virtual	B-Architecture
memory	I-Architecture
are	O
compressed	O
and	O
stored	O
in	O
physical	O
memory	O
,	O
which	O
is	O
usually	O
random-access	B-Architecture
memory	I-Architecture
(	O
RAM	B-Architecture
)	O
,	O
or	O
sent	O
as	O
compressed	O
to	O
auxiliary	O
storage	O
such	O
as	O
a	O
hard	B-Device
disk	I-Device
drive	I-Device
(	O
HDD	O
)	O
or	O
solid-state	B-Device
drive	I-Device
(	O
SSD	B-Device
)	O
.	O
</s>
<s>
In	O
both	O
cases	O
the	O
virtual	B-Architecture
memory	I-Architecture
range	O
,	O
whose	O
contents	O
has	O
been	O
compressed	O
,	O
is	O
marked	O
inaccessible	O
so	O
that	O
attempts	O
to	O
access	O
compressed	O
pages	O
can	O
trigger	O
page	B-General_Concept
faults	I-General_Concept
and	O
reversal	O
of	O
the	O
process	O
(	O
retrieval	O
from	O
auxiliary	O
storage	O
and	O
decompression	O
)	O
.	O
</s>
<s>
The	O
footprint	O
of	O
the	O
data	O
being	O
paged	O
is	O
reduced	O
by	O
the	O
compression	O
process	O
;	O
in	O
the	O
first	O
instance	O
,	O
the	O
freed	O
RAM	B-Architecture
is	O
returned	O
to	O
the	O
available	O
physical	O
memory	O
pool	O
,	O
while	O
the	O
compressed	O
portion	O
is	O
kept	O
in	O
RAM	B-Architecture
.	O
</s>
<s>
In	O
the	O
second	O
instance	O
,	O
the	O
compressed	B-General_Concept
data	I-General_Concept
is	O
sent	O
to	O
auxiliary	O
storage	O
but	O
the	O
resulting	O
I/O	O
operation	O
is	O
smaller	O
and	O
therefore	O
takes	O
less	O
time	O
.	O
</s>
<s>
In	O
some	O
implementations	O
,	O
including	O
zswap	B-Application
,	O
zram	B-Application
and	O
Helix	O
Software	O
Company	O
’s	O
Hurricane	O
,	O
the	O
entire	O
process	O
is	O
implemented	O
in	O
software	O
.	O
</s>
<s>
In	O
other	O
systems	O
,	O
such	O
as	O
IBM	O
's	O
MXT	O
,	O
the	O
compression	O
process	O
occurs	O
in	O
a	O
dedicated	O
processor	O
that	O
handles	O
transfers	O
between	O
a	O
local	O
cache	B-General_Concept
and	O
RAM	B-Architecture
.	O
</s>
<s>
Virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
is	O
distinct	O
from	O
garbage	B-General_Concept
collection	I-General_Concept
(	O
GC	O
)	O
systems	O
,	O
which	O
remove	O
unused	O
memory	O
blocks	O
and	O
in	O
some	O
cases	O
consolidate	O
used	O
memory	O
regions	O
,	O
reducing	O
fragmentation	O
and	O
improving	O
efficiency	O
.	O
</s>
<s>
Virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
is	O
also	O
distinct	O
from	O
context	B-Operating_System
switching	I-Operating_System
systems	O
,	O
such	O
as	O
Connectix	O
's	O
RAM	B-Architecture
Doubler	O
(	O
though	O
it	O
also	O
did	O
online	O
compression	O
)	O
and	O
Apple	O
OS	O
7.1	O
,	O
in	O
which	O
inactive	O
processes	B-Operating_System
are	O
suspended	O
and	O
then	O
compressed	O
as	O
a	O
whole	O
.	O
</s>
<s>
By	O
reducing	O
the	O
I/O	O
activity	O
caused	O
by	O
paging	B-Architecture
requests	O
,	O
virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
can	O
produce	O
overall	O
performance	O
improvements	O
.	O
</s>
<s>
In	O
some	O
situations	O
,	O
such	O
as	O
in	O
embedded	B-Architecture
devices	I-Architecture
,	O
auxiliary	O
storage	O
is	O
limited	O
or	O
non-existent	O
.	O
</s>
<s>
In	O
these	O
cases	O
,	O
virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
can	O
allow	O
a	O
virtual	B-Architecture
memory	I-Architecture
system	O
to	O
operate	O
,	O
where	O
otherwise	O
virtual	B-Architecture
memory	I-Architecture
would	O
have	O
to	O
be	O
disabled	O
.	O
</s>
<s>
This	O
allows	O
the	O
system	O
to	O
run	O
certain	O
software	O
which	O
would	O
otherwise	O
be	O
unable	O
to	O
operate	O
in	O
an	O
environment	O
with	O
no	O
virtual	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
Flash	B-Device
memory	I-Device
has	O
certain	O
endurance	O
limitations	O
on	O
the	O
maximum	O
number	O
of	O
erase	O
cycles	O
it	O
can	O
undergo	O
,	O
which	O
can	O
be	O
as	O
low	O
as	O
100	O
erase	O
cycles	O
.	O
</s>
<s>
In	O
systems	O
where	O
Flash	B-Device
Memory	I-Device
is	O
used	O
as	O
the	O
only	O
auxiliary	O
storage	O
system	O
,	O
implementing	O
virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
can	O
reduce	O
the	O
total	O
quantity	O
of	O
data	O
being	O
written	O
to	O
auxiliary	O
storage	O
,	O
improving	O
system	O
reliability	O
.	O
</s>
<s>
Various	O
studies	O
show	O
typical	O
data	B-General_Concept
compression	I-General_Concept
ratios	I-General_Concept
ranging	O
from	O
2:1	O
to	O
2.5:1	O
for	O
program	O
data	O
,	O
similar	O
to	O
typically	O
achievable	O
compression	O
ratios	O
with	O
disk	B-General_Concept
compression	I-General_Concept
.	O
</s>
<s>
In	O
order	O
for	O
virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
to	O
provide	O
measurable	O
performance	O
improvements	O
,	O
the	O
throughput	O
of	O
the	O
virtual	B-Architecture
memory	I-Architecture
system	O
must	O
be	O
improved	O
when	O
compared	O
to	O
the	O
uncompressed	B-General_Concept
equivalent	O
.	O
</s>
<s>
However	O
,	O
in	O
I/O	B-General_Concept
-bound	I-General_Concept
systems	O
or	O
applications	O
with	O
highly	O
compressible	O
data	O
sets	O
,	O
the	O
gains	O
can	O
be	O
substantial	O
.	O
</s>
<s>
The	O
physical	O
memory	O
used	O
by	O
a	O
compression	O
system	O
reduces	O
the	O
amount	O
of	O
physical	O
memory	O
available	O
to	O
processes	B-Operating_System
that	O
a	O
system	O
runs	O
,	O
which	O
may	O
result	O
in	O
increased	O
paging	B-Architecture
activity	O
and	O
reduced	O
overall	O
effectiveness	O
of	O
virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
.	O
</s>
<s>
This	O
relationship	O
between	O
the	O
paging	B-Architecture
activity	O
and	O
available	O
physical	O
memory	O
is	O
roughly	O
exponential	O
,	O
meaning	O
that	O
reducing	O
the	O
amount	O
of	O
physical	O
memory	O
available	O
to	O
system	O
processes	B-Operating_System
results	O
in	O
an	O
exponential	O
increase	O
of	O
paging	B-Architecture
activity	O
.	O
</s>
<s>
In	O
circumstances	O
where	O
the	O
amount	O
of	O
free	O
physical	O
memory	O
is	O
low	O
and	O
paging	B-Architecture
is	O
fairly	O
prevalent	O
,	O
any	O
performance	O
gains	O
provided	O
by	O
the	O
compression	O
system	O
(	O
compared	O
to	O
paging	B-Architecture
directly	O
to	O
and	O
from	O
auxiliary	O
storage	O
)	O
may	O
be	O
offset	O
by	O
an	O
increased	O
page	B-General_Concept
fault	I-General_Concept
rate	O
that	O
leads	O
to	O
thrashing	B-General_Concept
and	O
degraded	O
system	O
performance	O
.	O
</s>
<s>
In	O
an	O
opposite	O
state	O
,	O
where	O
enough	O
physical	O
memory	O
is	O
available	O
and	O
paging	B-Architecture
activity	O
is	O
low	O
,	O
compression	O
may	O
not	O
impact	O
performance	O
enough	O
to	O
be	O
noticeable	O
.	O
</s>
<s>
The	O
middle	O
ground	O
between	O
these	O
two	O
circumstanceslow	O
RAM	B-Architecture
with	O
high	O
paging	B-Architecture
activity	O
,	O
and	O
plenty	O
of	O
RAM	B-Architecture
with	O
low	O
paging	B-Architecture
activityis	O
where	O
virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
may	O
be	O
most	O
useful	O
.	O
</s>
<s>
However	O
,	O
the	O
more	O
compressible	O
the	O
program	O
data	O
is	O
,	O
the	O
more	O
pronounced	O
are	O
the	O
performance	O
improvements	O
as	O
less	O
physical	O
memory	O
is	O
needed	O
to	O
hold	O
the	O
compressed	B-General_Concept
data	I-General_Concept
.	O
</s>
<s>
For	O
example	O
,	O
in	O
order	O
to	O
maximize	O
the	O
use	O
of	O
a	O
compressed	O
pages	O
cache	B-General_Concept
,	O
Helix	O
Software	O
Company	O
's	O
Hurricane2.0	O
provides	O
a	O
user-configurable	O
compression	O
rejection	O
threshold	O
.	O
</s>
<s>
By	O
compressing	O
the	O
first	O
256	O
to	O
512	O
bytes	O
of	O
a	O
4KiB	O
page	O
,	O
this	O
virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
system	O
determines	O
whether	O
the	O
configured	O
compression	O
level	O
threshold	O
can	O
be	O
achieved	O
for	O
a	O
particular	O
page	O
;	O
if	O
achievable	O
,	O
the	O
rest	O
of	O
the	O
page	O
would	O
be	O
compressed	O
and	O
retained	O
in	O
a	O
compressed	O
cache	B-General_Concept
,	O
and	O
otherwise	O
the	O
page	O
would	O
be	O
sent	O
to	O
auxiliary	O
storage	O
through	O
the	O
normal	O
paging	B-Architecture
system	O
.	O
</s>
<s>
In	O
hardware	O
implementations	O
the	O
technology	O
also	O
relies	O
on	O
price	O
differentials	O
between	O
the	O
various	O
components	O
of	O
the	O
system	O
,	O
for	O
example	O
,	O
the	O
difference	O
between	O
the	O
cost	O
of	O
RAM	B-Architecture
and	O
the	O
cost	O
of	O
a	O
processor	O
dedicated	O
to	O
compression	O
.	O
</s>
<s>
In	O
a	O
typical	O
virtual	B-Architecture
memory	I-Architecture
implementation	O
,	O
paging	B-Architecture
happens	O
on	O
a	O
least	B-General_Concept
recently	I-General_Concept
used	I-General_Concept
basis	O
,	O
potentially	O
causing	O
the	O
compression	B-General_Concept
algorithm	I-General_Concept
to	O
use	O
up	O
CPU	O
cycles	O
dealing	O
with	O
the	O
lowest	O
priority	O
data	O
.	O
</s>
<s>
In	O
this	O
case	O
the	O
bar	O
for	O
compression	O
is	O
higher	O
,	O
since	O
the	O
I/O	O
cycle	O
it	O
is	O
attempting	O
to	O
eliminate	O
is	O
much	O
shorter	O
,	O
particularly	O
on	O
flash	B-Device
memory	I-Device
devices	O
.	O
</s>
<s>
Virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
has	O
gone	O
in	O
and	O
out	O
of	O
favor	O
as	O
a	O
technology	O
.	O
</s>
<s>
The	O
price	O
and	O
speed	O
of	O
RAM	B-Architecture
and	O
external	O
storage	O
have	O
plummeted	O
due	O
to	O
Moore	O
's	O
Law	O
and	O
improved	O
RAM	B-Architecture
interfaces	O
such	O
as	O
DDR3	O
,	O
thus	O
reducing	O
the	O
need	O
for	O
virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
,	O
while	O
multi-core	O
processors	O
,	O
server	O
farms	O
,	O
and	O
mobile	O
technology	O
together	O
with	O
the	O
advent	O
of	O
flash	O
based	O
systems	O
make	O
virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
more	O
attractive	O
.	O
</s>
<s>
Acorn	O
Computers	O
 '	O
Unix	O
variant	O
,	O
RISC	B-Operating_System
iX	I-Operating_System
,	O
was	O
supplied	O
as	O
the	O
primary	O
operating	O
system	O
for	O
its	O
R140	O
workstation	O
released	O
in	O
1989	O
.	O
</s>
<s>
RISC	B-Operating_System
iX	I-Operating_System
provided	O
support	O
for	O
demand	O
paging	B-Architecture
of	O
compressed	O
executable	O
files	O
.	O
</s>
<s>
However	O
,	O
the	O
principal	O
motivation	O
for	O
providing	O
compressed	O
executables	O
was	O
to	O
be	O
able	O
to	O
accommodate	O
a	O
complete	O
Unix	O
system	O
in	O
a	O
hard	B-Device
disk	I-Device
of	O
relatively	O
modest	O
size	O
.	O
</s>
<s>
Compressed	B-General_Concept
data	I-General_Concept
was	O
not	O
paged	O
out	O
to	O
disk	O
under	O
this	O
scheme	O
.	O
</s>
<s>
Paul	O
R	O
.	O
Wilson	O
proposed	O
compressed	O
caching	B-General_Concept
of	O
virtual	B-Architecture
memory	I-Architecture
pages	O
in	O
1990	O
,	O
in	O
a	O
paper	O
circulated	O
at	O
the	O
ACM	O
OOPSLA/ECOOP	O
'	O
90	O
Workshop	O
on	O
Garbage	B-General_Concept
Collection	I-General_Concept
(	O
"	O
Some	O
Issues	O
and	O
Strategies	O
in	O
Heap	B-General_Concept
Management	I-General_Concept
and	O
Memory	O
Hierarchies	O
"	O
)	O
,	O
and	O
appearing	O
in	O
ACM	O
SIGPLAN	O
Notices	O
in	O
January	O
1991	O
.	O
</s>
<s>
Helix	O
Software	O
Company	O
pioneered	O
virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
in	O
1992	O
,	O
filing	O
a	O
patent	O
application	O
for	O
the	O
process	O
in	O
October	O
of	O
that	O
year	O
.	O
</s>
<s>
In	O
1994	O
and	O
1995	O
,	O
Helix	O
refined	O
the	O
process	O
using	O
test-compression	O
and	O
secondary	O
memory	B-General_Concept
caches	I-General_Concept
on	O
video	O
cards	O
and	O
other	O
devices	O
.	O
</s>
<s>
However	O
,	O
Helix	O
did	O
not	O
release	O
a	O
product	O
incorporating	O
virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
until	O
July	O
1996	O
and	O
the	O
release	O
of	O
Hurricane2.0	O
,	O
which	O
used	O
the	O
Stac	O
Electronics	O
Lempel	B-Algorithm
–	I-Algorithm
Ziv	I-Algorithm
–	I-Algorithm
Stac	I-Algorithm
compression	B-General_Concept
algorithm	I-General_Concept
and	O
also	O
used	O
off-screen	O
video	O
RAM	B-Architecture
as	O
a	O
compression	O
buffer	O
to	O
gain	O
performance	O
benefits	O
.	O
</s>
<s>
In	O
1995	O
,	O
RAM	B-Architecture
cost	O
nearly	O
$50	O
per	O
megabyte	O
,	O
and	O
Microsoft	O
's	O
Windows	O
95	O
listed	O
a	O
minimum	O
requirement	O
of	O
4MB	O
of	O
RAM	B-Architecture
.	O
</s>
<s>
Due	O
to	O
the	O
high	O
RAM	B-Architecture
requirement	O
,	O
several	O
programs	O
were	O
released	O
which	O
claimed	O
to	O
use	O
compression	O
technology	O
to	O
gain	O
“	O
memory	O
”	O
.	O
</s>
<s>
Most	O
notorious	O
was	O
the	O
SoftRAM	B-Application
program	O
from	O
Syncronys	B-Application
Softcorp	O
.	O
</s>
<s>
SoftRAM	B-Application
was	O
revealed	O
to	O
be	O
“	O
placebo	O
software	O
”	O
which	O
did	O
not	O
include	O
any	O
compression	O
technology	O
at	O
all	O
.	O
</s>
<s>
Other	O
products	O
,	O
including	O
Hurricane	O
and	O
MagnaRAM	B-Application
,	O
included	O
virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
,	O
but	O
implemented	O
only	O
run-length	B-Algorithm
encoding	I-Algorithm
,	O
with	O
poor	O
results	O
,	O
giving	O
the	O
technology	O
a	O
negative	O
reputation	O
.	O
</s>
<s>
In	O
its	O
8	O
April	O
1997	O
issue	O
,	O
PC	O
Magazine	O
published	O
a	O
comprehensive	O
test	O
of	O
the	O
performance	O
enhancement	O
claims	O
of	O
several	O
software	O
virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
tools	O
.	O
</s>
<s>
However	O
the	O
tests	O
were	O
run	O
on	O
Intel	B-General_Concept
Pentium	I-General_Concept
systems	O
which	O
had	O
a	O
single	O
core	O
and	O
were	O
single	O
threaded	O
,	O
and	O
thus	O
compression	O
directly	O
impacted	O
all	O
system	O
activity	O
.	O
</s>
<s>
MXT	O
was	O
a	O
stand-alone	O
chip	O
which	O
acted	O
as	O
a	O
CPU	B-General_Concept
cache	I-General_Concept
between	O
the	O
CPU	O
and	O
memory	O
controller	O
.	O
</s>
<s>
In	O
2010	O
,	O
IBM	O
released	O
Active	O
Memory	O
Expansion	O
(	O
AME	O
)	O
for	O
AIX	B-Application
6.1	O
which	O
implements	O
virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
.	O
</s>
<s>
In	O
2012	O
,	O
some	O
versions	O
of	O
the	O
POWER7+	O
chip	O
included	O
AME	O
hardware	O
accelerators	O
using	O
the	O
842	B-Algorithm
compression	I-Algorithm
algorithm	I-Algorithm
for	O
data	B-General_Concept
compression	I-General_Concept
support	O
,	O
used	O
on	O
AIX	B-Application
,	O
for	O
virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
.	O
</s>
<s>
In	O
December	O
2012	O
,	O
the	O
zswap	B-Application
project	O
was	O
announced	O
;	O
it	O
was	O
merged	O
into	O
the	O
Linux	B-Application
kernel	O
mainline	O
in	O
September	O
2013	O
.	O
</s>
<s>
In	O
June	O
2013	O
,	O
Apple	O
announced	O
that	O
it	O
would	O
include	O
virtual	B-General_Concept
memory	I-General_Concept
compression	I-General_Concept
in	O
OS	O
X	O
Mavericks	O
,	O
using	O
the	O
Wilson-Kaplan	O
WKdm	O
algorithm	O
.	O
</s>
<s>
A	O
10	O
August	O
2015	O
"	O
Windows	B-Application
Insider	I-Application
Preview	O
"	O
update	O
for	O
Windows	O
10	O
(	O
build	O
10525	O
)	O
added	O
support	O
for	O
RAM	B-General_Concept
compression	I-General_Concept
.	O
</s>
