Java7 Garbage Collector G1

Java 7 Garbage Collector G1Antons Kranga, 3rd May 2012

Introduction to GC

Garbage Collector – reclaims unreachable

removes objects that you don’t need anymore

What is GC?

Picture is the © of Disney

• Java Code • Heap

How does it works?

• Java Code

Object x = new Object();

• Heap

How does it works?

# of ref= 1

• Java Code

Object y = new Object();

• Heap

How does it works?

# of ref= 1

• Java Code

y = x;

• Heap

How does it works?

# of ref= 2

# of ref= 0

• Java Code

y = x;

• Heap

How does it works?

# of ref= 2

# of ref= 0

object y can be collected

• Java Code

y = x;

List z = new ArrayList();

z.add(x);

• Heap

How does it works?

# of ref= 3

# of ref= 0z

# of ref= 1

• Java Code

y = x;

z.add(x);

• Heap

How does it works?

# of ref= 3

# of ref= 0z

# of ref= 1

• Java Code

y = x;

z.add(x);

• Heap

How does it works?

# of ref= 3

# of ref= 0z

# of ref= 1objects in

island of isolation can be collected

• Garbage Collector is non deterministic

• GC carried by separate daemon threat

• Stop the World – pauses all threats during GC

What you need to know about GC

Generational GCs

• Generations are of new or survived during GC objects

Generational GC

Picture taken from OpenFreak.com

• Heap divided in zones by age of the objects

Generational GC

eden survivor tenured

Generational GC

eden survivor

Objects experienced no

GC (age 0)

tenured

Generational GC

eden survivor

experiences several GCs

(2 spaces by default)

tenured

Generational GC

eden survivor

Objects survived multiple GCs

tenured

Generational GC

eden survivor

young generation tenured generation

tenured

Generational GC

eden survivor tenured

young generation tenured generation

objects collected byMinor and

Full GC

objects collected only byFull GC

Objects survival rate statistics

• Serial Collector – always stop-the-world

Types of generational GC in Java

• 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

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:

3. Footprint– current heap size (amount of memory) being used

4. Promptness– how quickly memory from objects no longer needed is freed

GC Metrics

To be continued

Java7 Garbage Collector G1

Technology

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