JVM and Garbage Collection

by marcos.garcia@savoirfairelinux.com, Jan 2013

JVM?

Bytecode interpreter

http://viralpatel.net/blogs/java-virtual-machine-an-inside-story/

JVM Memory Vocabulary

Runtime data areasPC register: One for each thread

Points to current execution code of each thread

JVM stacksOne dedicated stack for each thread

Holds many stack frames, one for each invoked methodEvery frame has 3 registers

HeapShared among all threads

Stores Java objects.

Cleaned up by GC

Method areaShared among all threads

Contains method code (class implementations)

Has a Runtime constant pool

Native method stack (just for native methods

http://docs.oracle.com/javase/specs/jvms/se7/html/jvms-2.html#jvms-2.5

JVM Stack

Stack FrameUsed to store data and partial results, as well as to perform dynamic linking, return values for methods, and dispatch exceptions.

Describes all the needed information to run a method, including current method stack as well as other JVM shared spaces

Stack Frame registers (one per method being executed)Local Vars, used by current method invocation

Frame data / Execution environment, used for internal stack operations

Optop / Operand stack (for bytecode), reference to runtime constant poolIt is a LIFO stack, need push/pop operations

http://performeister.tistory.com/38

JVM Heap

Two heap memory typesYoung Generation MemoryTo/From Survivor spaces

Eden space

Old Generation Memory

Other memoryPermanent Generation

Native memory (because of native memory references, to the underlying OS)

Garbage collection

Principle: Generational collection or weak generational hypothesisMost allocated objects are not referenced (considered live) for long, that is, they die young

Few references from older to younger objects exist

Minor GCYoung generation collections occur relatively frequently and are efficient and fast because the young generation space is usually small and likely to contain a lot of objects that are no longer referenced.

Major GCObjects that survive some number of young generation collections are eventually promoted, or tenured, to the old generation

http://www.oracle.com/technetwork/java/javase/tech/memorymanagement-whitepaper-1-150020.pdf

Young/Old Memory explained

Young SpaceEdenAny object created inside a method or as a class variable

2 survivor spaces (From/To)Objects in use, that have survived at least one Minor GC

1 space empty, other full. Swaps on every GC pass

Old (Tenured)Objects referenced since a long time ago, still being used

Very big objects are created directly in Old space

http://java-espresso.blogspot.ca/2011/05/heap-structure-in-jvm.html

GC example

Serial Collector (not currently used in modern JVM)

http://www.oracle.com/technetwork/java/javase/tech/memorymanagement-whitepaper-1-150020.pdf

Parallel and CMS GC

CMS is the most used GC implementation

Heap fragmentation

We need contiguous space for memory allocation

GC also relocates (called compactation) objects to reduce fragmentation

G1 collector (JDK 6 & 7)

Garbage-First (G1) collector: server-style garbage collector, targeted for multi-processor machines with large memories.

It meets garbage collection (GC) pause time goals with a high probability, while achieving high throughput. Designed for applications that:Can operate concurrently with applications threads like the CMS collector.

Compact free space without lengthy GC induced pause times.

Need more predictable GC pause durations.

Do not want to sacrifice a lot of throughput performance.

Do not require a much larger Java heap.

G1 is planned as the long term replacement for the Concurrent Mark-Sweep Collector (CMS).

Better than CMS. G1 is a compacting collector. G1 compacts sufficiently to completely avoid the use of fine-grained free lists for allocation, and instead relies on regions. This considerably simplifies parts of the collector, and mostly eliminates potential fragmentation issues.

G1 offers more predictable garbage collection pauses than the CMS collector, and allows users to specify desired pause targets.

http://www.oracle.com/webfolder/technetwork/tutorials/obe/java/G1GettingStarted/index.html

G1 Heap Fragmentation

YoungGC

Heap evolution

Regular case

OOM case

Tool: jconsole

Heap evolution

Regular case

OOM case

Tool: websphere resource analyzer