<s>
The	O
garbage-first	B-Language
collector	I-Language
(	O
G1	B-Language
)	O
is	O
a	O
garbage	B-General_Concept
collection	I-General_Concept
algorithm	O
introduced	O
in	O
the	O
Oracle	B-Application
HotSpot	B-Language
Java	B-Language
virtual	I-Language
machine	I-Language
(	O
JVM	B-Language
)	O
6	O
and	O
supported	O
from	O
7	O
Update	O
4	O
.	O
</s>
<s>
It	O
was	O
planned	O
to	O
replace	O
concurrent	B-Language
mark	I-Language
sweep	I-Language
collector	I-Language
(	O
CMS	O
)	O
in	O
JVM	B-Language
7	O
and	O
was	O
made	O
default	O
in	O
Java	B-Device
9	O
.	O
</s>
<s>
Garbage-first	O
(	O
G1	B-Language
)	O
collector	O
is	O
a	O
server-style	O
garbage	B-General_Concept
collector	I-General_Concept
,	O
targeted	O
for	O
multiprocessors	B-Operating_System
with	O
large	O
memories	B-General_Concept
,	O
that	O
meets	O
a	O
soft	O
real-time	B-General_Concept
goal	O
with	O
high	O
probability	O
,	O
while	O
achieving	O
high-throughput	B-Operating_System
.	O
</s>
<s>
G1	B-Language
preferentially	O
collects	O
regions	O
with	O
the	O
least	O
amount	O
of	O
live	O
data	O
,	O
or	O
"	O
garbage	O
first	O
"	O
.	O
</s>
<s>
G1	B-Language
is	O
the	O
long	O
term	O
replacement	O
of	O
CMS	O
.	O
</s>
<s>
Whole-heap	O
operations	O
,	O
such	O
as	O
global	O
marking	O
,	O
are	O
performed	O
concurrently	O
with	O
the	O
application	O
threads	B-Operating_System
,	O
to	O
prevent	O
interruptions	O
proportional	O
to	O
heap	O
or	O
live-data	O
size	O
.	O
</s>
<s>
Concurrent	B-Operating_System
marking	O
provides	O
both	O
collection	O
completeness	O
and	O
identifies	O
regions	O
ripe	O
for	O
reclamation	O
via	O
compacting	O
evacuation	O
.	O
</s>
<s>
This	O
evacuation	O
is	O
performed	O
in	O
parallel	B-Operating_System
on	O
multiprocessors	B-Operating_System
,	O
to	O
decrease	O
pause	O
times	O
and	O
increase	O
throughput	O
.	O
</s>
<s>
G1	B-Language
was	O
first	O
introduced	O
as	O
an	O
experimental	O
option	O
in	O
Java	B-Language
SE	I-Language
6	O
Update	O
14	O
,	O
where	O
it	O
can	O
be	O
enabled	O
with	O
the	O
following	O
two	O
command-line	O
parameters	O
:	O
-XX	O
:	O
+UnlockExperimentalVMOptions	O
and	O
-XX	O
:	O
+UseG1GC	O
With	O
JDK	O
7	O
,	O
G1	B-Language
was	O
planned	O
to	O
replace	O
CMS	O
in	O
the	O
Hotspot	B-Language
JVM	B-Language
.	O
</s>
<s>
There	O
are	O
two	O
major	O
differences	O
between	O
CMS	O
and	O
G1	B-Language
.	O
</s>
<s>
The	O
first	O
is	O
that	O
G1	B-Language
is	O
a	O
compacting	O
collector	O
.	O
</s>
<s>
G1	B-Language
compacts	O
sufficiently	O
to	O
completely	O
avoid	O
the	O
use	O
of	O
fine-grain	O
free	O
lists	O
for	O
allocation	O
,	O
which	O
considerably	O
simplifies	O
parts	O
of	O
the	O
collector	O
and	O
mostly	O
eliminates	O
potential	O
fragmentation	B-Architecture
issues	O
.	O
</s>
<s>
As	O
well	O
as	O
compacting	O
,	O
G1	B-Language
offers	O
more	O
predictable	O
garbage	B-General_Concept
collection	I-General_Concept
pauses	O
than	O
the	O
CMS	O
collector	O
and	O
allows	O
users	O
to	O
set	O
their	O
desired	O
pause	O
targets	O
.	O
</s>
<s>
In	O
Java	B-Device
9	O
G1	B-Language
was	O
made	O
the	O
default	O
garbage	B-General_Concept
collector	I-General_Concept
,	O
in	O
spite	O
of	O
Google	B-Application
counter	O
proposing	O
the	O
well-known	O
CMS	O
as	O
the	O
standard	O
,	O
claiming	O
the	O
modified	O
CMS	O
it	O
uses	O
performs	O
better	O
than	O
G1	B-Language
.	O
</s>
<s>
Since	O
then	O
,	O
Oracle	B-Application
has	O
greatly	O
improved	O
G1	B-Language
's	O
throughput	O
,	O
latency	O
and	O
memory	O
footprint	O
.	O
</s>
<s>
Guaranteed	O
real-time	B-General_Concept
behavior	O
even	O
with	O
garbage	B-General_Concept
collection	I-General_Concept
requires	O
a	O
real-time	B-General_Concept
garbage	B-General_Concept
collector	I-General_Concept
such	O
as	O
those	O
that	O
come	O
with	O
Sun	O
's	O
Java	B-Device
RTS	O
or	O
IBM	O
’s	O
WebSphere	B-Language
RT	O
.	O
</s>
