The JVM is your friendKai Koenig @AgentK

Web/Mobile Developer since the late 1990s

Interested in: Java, CFML, Functional

Programming, Go, JS, Mobile, Raspberry Pi

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I’ve already showed you where I live :)

Me

- The JVM and Java Memory Management- What’s the big deal with Garbage Collection? - GC Strategies for various situations & JVMs- How to approach JVM “tuning”?- CFML specifics

Agenda

The JVM and Java Memory Management

JVM Architecture (“What’s that JVM thing?”)

The JVM Architecture

History

First JVM implementations were rather simple:

- Weak JMM (Java Memory Model)

- Issues with concepts like final, volatile etc.

- Very simple, non-generational memory

- “Mark-and-Sweep” Garbage Collection

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History

Hotspot JVM was introduced in Java 1.2 as an add-on — became part of the default setup in Java 1.3 (~ mid 2000).

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Also be aware of notational flukes:

1.0 -> 1.1 -> Java 2 (1.2) -> Java 2 (1.3) -> Java 2 (1.4) -> Java 5 -> Java 6 -> Java 7 -> Java 8

Modern JVMs

Generational Memory Management

Generational Garbage Collection

Hotspot JVM

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What’s the big deal with Garbage Collection?

Garbage(“Who made that mess?”)

JVM Garbage

Over time the JVM accumulates a lot of objects that are not needed anymore.

If we didn’t clean up, we’d get Out-Of-Memory errors sooner or later.

Q: What’s being cleaned up?

A: “Dead” objects

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JVM Garbage Collection

Every single GC algorithm starts with some kind of “marking”: identifying the objects that are not necessary anymore.

The Collector would start with a Root Set and follow along object references it can reach.

Everything else is Garbage and can go!

Q: What’s the Root Set?

The Root Set

References on the Call Stacks of the JVM’s threads

Global References, e.g. static fields in Classes

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The Root Set are entry points into the reference graph of “alive” objects.

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Root Set and Reference Graphs

Root Set and Reference Graphs

Root Set and Reference Graphs

Root Set and Reference Graphs

Root Set and Reference Graphs

What we’ve looked at is a basic “Mark-and-Sweep” algorithm.

The “Free List” could in the easiest form just be used to mark memory as free.

Problem: Fragmentation and therefore inability to assign memory for new, fresh objects.

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Fragmentation

Generations(“OK, let’s make this stuff really complicated”)

Basics of Generational Memory: Heap

Stores your objects and classes at the JVM’s runtime.

Usually the following basic assumptions are true:

- Lots of short-lived objects

- Very few (or: fewer) long-lived objects

Also function-local objects are created here.

Lifetime vs. # of objects

Heap management

The JVM can’t know in advance what the lifespan of a certain object would be.

Generational Memory Management is a solution to overcome this issue and fragmentation:

- Young Generation

- Old Generation / Tenured Generation

- Permanent Generation (special case…)

Generations

Young Generation - for new objects

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Typical short-lived objects:

- Objects local to a function

- Loop iterators, StringBuilders etc.

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Young Generation - for new objects

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Typical medium-lived objects:

- Objects tied to a session

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Young Generation - for new objects

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Typical long-lived objects:

- Thread Pools

- Singletons

- Certain framework objects

Young Generation - what happens next?

In general and following the theory of Generational Memory Management:

- YG fills up -> Garbage Collection happens

- YG collection is supposed to be fast

If an object survives a certain amount of collections in the YG, the JVM will assume the object is medium- or long-lived and move it into the Old Generation.

Old Generation

Over time, more long-lived objects end up in the Old Generation and at some point it’s going to be full.

In general and following the theory of Generational Memory Management:

- OG fills up -> Garbage Collection happens

- OG collection is usually slower than YG

- Size of OG

Why is Generational Memory good?

Lots of garbage - cleaning it up fast is worthwhile

Generational Memory Management:

- YG GC often -> space for new objects

- Each generation focusses on “type” of objects

- GC doesn’t have to search the whole heap

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Permanent Generation

Not a “Generation” as such, but still needs to be managed appropriately.

Stores:

- Classes

- Internal JVM objects

- JIT information

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GC Strategies for various situations & JVMs

Generation Strategy (optional)

Young Generation Strategies

Generally, the YG is smaller than the OG.

The YG consists of sub-sections:

- Eden (new objects)

- Survivor 1 and Survivor 2

One Survivor space is always empty and during a YG collection the JVM will copy survivors from Eden and S1 to S2 and vice versa.

Old Generation Strategies

The amount of survived GCs in the YG is called “Tenuring Threshold”

If Eden and Survivor Spaces are too small, sometimes objects might get instant-promoted to the OG (because there’s no space in the YG).

Old Generation Collections are usually expensive (slow, long)!

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This is what your heap really looks like

Collector Selection

Selection criteria

Efficiency / Throughput

Concurrency

Overhead

JVM version you’re on

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Ergonomics

Since Java 5 (and much improved in Java 6-land), the JVM comes pre-setup with certain criteria for selecting GC strategies and settings (“Ergonomics”). Most can be changed.

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JRockit/Apple JVMs — similar mechanisms

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Young Generation(optional)

YG Collectors: Serial

Mark-and-Sweep: Marking phase gets all reachable objects, Sweeping cleans up the leftovers

Problems:

- Fragmentation

YG Collectors: Serial

Mark-and-Copy: Marking phase gets all reachable objects, Copy moves those into a new (empty) space.

Problems:

- Slightly more expensive than MaS

- Copying and References have to be shifted

- “Intergenerational References” -> homework

YG Collectors: Serial

Both MaS and MaC need exclusive access to the Reference Graph.

Stop-the-World: stops all threads, the collection was traditionally done by a single “Reaper Thread”.

Problems:

- Long Pauses

- Inefficient

YG Collectors: Parallel

Parallel MaC (since Java 1.4.2) distributes the Marking and Copying phases over multiple threads.

The actual collecting is still Stop-the-World, but for a much shorter period of time.

YG default since Java 5 if machine has 2+ cores or CPUs, otherwise: -XX:+UseParallelGC

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YG Collectors: Parallel

Default: 1 GC thread per CPU/Core

8+ CPUs/Cores: 5/8 * CPUs/Cores

Explicit: -XX:+UseParallelGCThread=n

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Old Generation (optional)

OG Collectors

Many objects and low mortality means MaC would be inefficient. Instead we use Mark-and-Compact.

MaCo is a variation of MaS with lower fragmentation

4 Phases: Marking, Calculation of new Locations, Reference Adjustments and Moving

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OG Collectors

MaCo is a Full Collection algorithm - there’s no “Pure OG collection”.

Doesn’t run often, but if it runs it’ll take a while.

Performance issues:

- All objects are visited multiple times

- Serial collector, stops all the threads

Enable via -XX:+UseSerialGC

OG Collectors: parallel

ParallelOld: Parallel and more efficient version of MaCo, still Stop-the-World though - but shorter StW pause than MaCo.

Idea:

- Marking and Compacting are multi-threaded

- Algorithm operates on 2 segments per thread

OG default since Java 6 on server profiles or via -XX:+UseParallelOldGC

OG Collectors: concurrent

CMS: concurrent version of MaS, does NOT need to stop threads for the majority parts of its work.

4 Phases: Initial Marking, Concurrent Marking, Remarking, Concurrent Sweep.

Stop-the-World: Initial Marking & Remarking

CMS via -XX:+UseConcMarkSweepGC

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OG Collectors: concurrent

Concurrent Mark-and-Sweep is the preferred OG collector if you want to minimise Stop-the-World collections.

Overall throughput slightly less than ParallelOld, but much better suited for web/server apps.

Well suited for large heaps (but be aware of fragmentation), there’s an “incremental” mode for systems with 1-2 cores.

OG Collectors: G1 (Garbage First)

G1 is a replacement for CMS (experimental in later Java 6 release, full support in Java 7+)

Benefits:

- Low-pause

- Adaptable

- Much less fragmentation than CMS

- Better collector for full heap

OG Collectors: G1 (Garbage First)

Heap is split into regions (1-32MB)

Collector is controlled by min time between GC pauses and min length of GC pause

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In Java 6/7 (6u14, 7u4) set via -XX:+UseG1GC

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How to approach JVM “tuning”?

Tuning

Preamble

Do not trust consultants, blog posts, mailing list discussions etc. telling you what the “best” JVM settings for you would be.

(That’s including myself!)

There is no such thing as the “best” settings.

It solely depends on the application and your usage.

Typical reasons for tuning

Application Growth

Change in available Resources (memory, CPU etc)

Actual Performance issues (unresponsiveness…)

JVM-level error messages in log files

Tools

Process

Make an assumption for load, memory and GC settings.

Run Load tests, monitor and measure results.

Change one setting, rinse and repeat.

JVM settings and logging

How do you find out what’s happening in your JVM?

-verbose:GC

-XX:+PrintGCDetails

-XX:+PrintGCTimeStamps

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[GC 64781K->22983K(71360K), 0.0242084 secs] [GC 68487K->25003K(77888K), 0.0194041 secs] [Full GC 25003K->20302K(89600K), 0.1713420 secs] [GC 70670K->21755K(90048K), 0.0054093 secs] [GC 71913K->46558K(94912K), 0.0295257 secs] [Full GC 46558K->45267K(118336K), 0.2144038 secs] [GC 88214K->84651K(133056K), 0.0674443 secs] [Full GC 84651K->84633K(171648K), 0.1739369 secs] [GC 117977K->115114K(180736K), 0.0623399 secs] [GC 158613K->157136K(201152K), 0.0591171 secs] [Full GC 157136K->157098K(254784K), 0.1868453 secs] [GC 160678K->160455K(261184K), 0.0536678 secs] 01/24 19:36:22 Debug [scheduler-1] - Next mail spool run in 15 seconds. [GC 202912K->200819K(268288K), 0.0625820 secs] [Full GC 200819K->200776K(332224K), 0.2121724 secs] [GC 213293K->212423K(339520K), 0.0426462 secs] [GC 259465K->256115K(340288K), 0.0645039 secs] [Full GC 256115K->255462K(418432K), 0.3226731 secs] [GC 281947K->279651K(421760K), 0.0530268 secs] [GC 331073K->323785K(422720K), 0.0695117 secs] [Full GC 323785K->323697K(459264K), 0.2139458 secs] [Full GC 364365K->361525K(459264K), 0.2180439 secs] [Full GC 400859K->400859K(459264K), 0.1702890 secs] [Full GC 400859K->43989K(274112K), 0.2642407 secs] [GC 95197K->93707K(273216K), 0.0338568 secs] [GC 146978K->140363K(276032K), 0.0664380 secs] [GC 193696K->189635K(277952K), 0.0630006 secs] [Full GC 189635K->189604K(425920K), 0.1913979 secs] [GC 219773K->205157K(426048K), 0.0442126 secs]

GC tuning process

Let’s look at a real world case

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GC tuning results/criteria

Demo of some tools

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GC tuning results/criteria

More often than not you’d want to optimise for low GC pauses.

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GC Throughput: 95%+ are good.

Optimising for Throughput usually leads to longer GC pauses, still useful for batch operations.

Memory sizing concerns

Initial and maximum heap size:

-Xms4096m, -Xmx6144m

PermGen size:

-XX:MaxPermSize=256m

YG size:

-XX:NewSize=768m, -XX:MaxNewSize=768m

Memory sizing concerns

32bit JVM: theoretically 4GB

- In reality under Windows: ~1.2-1.4GB

Switching to a 64bit JVM creates ~20-30% memory overhead due to longer pointer references.

Also: easier to multi-threaded create new objects than clean them up multi-threaded.

Example

Setup of extremely high volume/traffic site, optimisation goal low pause times -Xms6144m -Xmx6144m -XX:NewSize=2500m -XX:MaxNewSize=2500m -XX:+CMSIncrementalMode -XX:+ExplicitGCInvokesConcurrent -XX:+CMSPermGenSweepingEnabled -XX:+CMSClassUnloadingEnabled -XX:MaxPermSize=384m -XX:PermSize=384m -XX:+UseConcMarkSweepGC

Overview Java 6

Max throughput Min pause time

2+ cores 1 core 2+ cores 1 core

YG par YG ser YG par YG par YG

OldGen par OG ser OG CMS iCMS

JVM Flags defaults-XX:

+UseSerialGC

-XX:+UseConcMarkSweepGC

(implicitly using: -XX:+UseParNewGC for YG)

-XX:+UseConcMarkSweepGC

-XX:+CMSIncrementalMode (implicitly using: -XX:

+UseParNewGC for YG)

Overview Java 7

Max throughput Min pause time

2+ cores 1 core 2+ cores 1 core

YG par YG ser YG G1 par YG

OldGen par OG ser OG G1 iCMS

JVM Flags defaults-XX:

+UseSerialGC-XX:+UseG1GC

-XX:+UseConcMarkSweepGC

-XX:+CMSIncrementalMode (implicitly using: -XX:

+UseParNewGC for YG)

(Oracle Java 7 JVMs also incorporate some JRockit features)

Photo credits

http://www.flickr.com/photos/aigle_dore/6973012997/

http://www.flickr.com/photos/thanh_tan/2903192937

http://www.flickr.com/photos/museemccordmuseum/3294656277/

http://www.flickr.com/photos/wwarby/3297205226/

https://www.flickr.com/photos/nationalmuseumofamericanhistory/9607222709/

https://www.flickr.com/photos/mr_zorgman/6396087451/

http://www.flickr.com/photos/thomashawk/3958193579/

https://www.flickr.com/photos/24742305@N00/5589187752

https://www.flickr.com/photos/39587102@N07/3821835981

https://www.flickr.com/photos/21160499@N04/5437070898

https://www.flickr.com/photos/65694112@N05/6147388744

Get in touch

Kai Koenig

Email: kai@ventego-creative.co.nz

www.ventego-creative.co.nz

Blog: www.bloginblack.de

Twitter: @AgentK