Java7 Garbage Collector G1

Java 7 Garbage Collector G1
Antons Kranga, 3rd May 2012

What is GC?

Garbage Collector – reclaims unreachable
removes objects that you don’t need
anymore

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How does it works?

• Java Code • Heap

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How does it works?


Object x = new Object();

x
# of ref= 1


How does it works?


Object y = new Object();
x
# of ref= 1
y
# of ref= 1


How does it works?


y = x; x
# of ref= 2
y
# of ref= 0


How does it works?


y = x; x
# of ref= 2
y
# of ref= 0

object y can
be collected


How does it works?


y = x; x
List z = new ArrayList();
# of ref= 3
z.add(x); y
# of ref= 0
z
# of ref= 1


How does it works?

{
y = x; x
# of ref= 3
z.add(x); y
}
# of ref= 0
z
# of ref= 1


How does it works?

{
y = x; x
# of ref= 3
z.add(x); y
}
# of ref= 0
z
# of ref= 1
objects in
island of isolation
can be collected


What you need to know about GC

• Garbage Collector is non deterministic

• GC carried by separate daemon threat

• Stop the World – pauses all threats during GC


Generational GC

• Generations are of new or survived during GC objects

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Generational GC


• Heap divided in zones by age of the objects

eden survivor tenured


Generational GC


Objects
experienced no
GC (age 0)



Generational GC


experiences several GCs
(2 spaces by default)



Generational GC


Objects survived
multiple GCs



Generational GC


young generation tenured generation



Generational GC


young generation tenured generation

objects objects
collected by collected
Minor and only by
Full GC Full GC



Objects survival rate statistics


Types of generational GC in Java

• Serial Collector – always stop-the-world



• Throughput Collector
– Concurrent for minor GC
– Stop-the-world for major GC



• Concurrent Low Pause Collector –
– Tenured gen: application paused for short periods during
GC
– Young gen: runs concurrently
– the app. will have a large number of long-lived objects,
and there is a pause time constraint


GC Metrics

Primary Parameters
1. Throughput
– percentage of the total run time not spent performing GC
2. Pauses
– times when the application code stops running while GC is
performed
– number of pauses, their average duration and their maximum
duration

Secondary Parameters:
1. Footprint
– current heap size (amount of memory) being used
2. Promptness
– how quickly memory from objects no longer needed is freed


Java7 Garbage Collector G1

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Java7 Garbage Collector G1