(JVM) Garbage Collection & Co.

Brown Bag Session

Jens Hadlich 2015-12-08

Garbage Collection

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Why should I care?

Garbage Collection

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Can have a huge impact

30 seconds

20 seconds

10 seconds

~16 hours

Outline

•  Some Theory

•  Practice & real world examples

•  Tools & live demos

•  Configuration best practices

•  Discussion

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Some Theory

Garbage Collection

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Garbage Collection

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Terminology

Garbage collection … is a form of automatic memory management.

Garbage … memory occupied by objects that are no longer used.

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Pros and Cons

Pros -  Programmer does not need to worry about memory (de-)allocation

(leading to fewer memory leaks)

Cons -  Performance overhead -  Can lead to more complex programming language design

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More Terminology

Collector Heap Mutator Roots

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Roots

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Garbage Collection

Algorithms

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Tracing Reference Counting

Collection Style Batch Incremental

Cost Per Mutation None High

Throughput High Low

Pause Times Long Short

Real Time? No Yes

Collects Cycles? Yes No

Tracing algorithms

Mark & Sweep Copying

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Think of a bookshelf

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Tracing algorithms

Mark & Sweep Copying

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Mark & Sweep

Mark the books you want to keep

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Mark & Sweep

Throw away the books that you don’t want to keep

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Mark & Sweep

Problem: Fragmentation

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This size book won’t fit!

Tracing algorithms

Mark & Sweep Copying

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Copying

Need 2 shelves

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Copying

Move (“copy”) books you want to keep

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Copying

Throw away books you don’t want to keep

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Further readings

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Java

HotSpot JVM Architecture

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http://www.oracle.com/webfolder/technetwork/tutorials/obe/java/gc01/index.html

Java

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Metaspace (Java 8 and later)

(Java 7 and earlier)

Java

Generational Garbage Collection

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Hypothesis: Most objects die young

Generational Garbage Collection

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Tools & Practice

Garbage Collection

Tools (Memory Analyzer) Java VisualVM jClarity Censum Grafana Icinga

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A little bit of GC Tuning

GC Tuning

Performance Triangle

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Memory Footprint

Throughput Latency

M

T L

GC Tuning

Better Performance Triangle

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Compactness

Throughput Responsiveness

C

T R

C x T x R = a Optimization: Increase a

Responsiveness vs. Throughput

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Collectors

Collectors (new generation) -XX:+UseSerialGC (Copy) -XX:+UseParNewGC (used together with old gen CMS) -XX:+UseG1GC (same for old gen)

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Collectors

Collectors (old generation) Throughput: -XX:+UseSerialGC (MarkSweepCompact) -XX:+UseParallelGC -XX:+UseParallelOldGC Low-pause: -XX:+UseConcMarkSweepGC -XX:+UseG1GC

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Concurrent Mark-Sweep

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Fun with JVM Options

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Fun with JVM Options

Most important -server Crucial for all long-running server applications. http://stackoverflow.com/questions/198577/real-differences-between-java-server-and-java-client

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Fun with JVM Options

Most important (cont’d) -Xms (-XX:InitialHeapSize) -Xmx (-XX:MaxHeapSize) Setting Xms = Xmx (slightly) saves some overhead.

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Fun with JVM Options

Enable GC logs -Xloggc:logs/gc-<app>.log -XX:+PrintGCCause -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -XX:+PrintHeapAtGC -XX:+PrintTenuringDistribution -XX:+PrintGCApplicationStoppedTime -XX:+UseGCLogFileRotation -XX:NumberOfGCLogFiles=2 -XX:GCLogFileSize=128M

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Fun with JVM Options

Enable JMX remoting -Dcom.sun.management.jmxremote -Dcom.sun.management.jmxremote.port=<port> -Dcom.sun.management.jmxremote.local.only=false -Dcom.sun.management.jmxremote.authenticate=false -Dcom.sun.management.jmxremote.ssl=false

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Think about your program

https://flink.apache.org/news/2015/05/11/Juggling-with-Bits-and-Bytes.html

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Best practices

Best practices

“Good” JVM options to start with -server -Xms1g -Xmx1g -XX:+AlwaysPreTouch -XX:+UseConcMarkSweepGC -XX:+UseParNewGC -XX:+DisableExplicitGC

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Best practices

Notes on G1GC -XX:+UseG1GC http://www.oracle.com/technetwork/articles/java/g1gc-1984535.html (G1GC shall become the default collector in Java 9)

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Best practices

“Good” JVM options to start with (G1GC) -server -Xms1g -Xmx1g -XX:+AlwaysPreTouch -XX:+UseG1GC -XX:+DisableExplicitGC

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Best practices

Keep the heap size below 30 GB if possible. Why? Compressed oops. https://wiki.openjdk.java.net/display/HotSpot/CompressedOops http://docs.oracle.com/javase/7/docs/technotes/guides/vm/performance-enhancements-7.html (enabled by default since Java 7 for 64-bit)

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Best practices

Look out for specific tuning guides / recommendations. https://www.elastic.co/guide/en/elasticsearch/guide/current/heap-sizing.html

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The End?

The End?

Other things that can go wrong.

Linux OOM Killer

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System (3GB memory)

JVM (Xmx=2g)

/var/log/kern.log jvm.memory.total.used

OOM

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hs_err_pid<pid>.log

Questions?

Add-on slides

Go

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Java Profiling

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