Memory Management: What You Need to Know When Moving to Java 8

Memory Management
What You Need To Know When Moving to Java 8
Todd Rader, Architect / Sales Engineer
AppDynamics

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Agenda
• Memory Basics
• …with a dive into metaspace
• Fun With Garbage Collectors
• GC bake-off
• Summary/Conclusions
• Q&A

Memory Basics: Types of Memory
STACK
• Each thread has its own call stack
• Adjust with -Xss
• 64-bit JVMs need more stack size (8-byte references)
• Default stack size: ????
Java7, jdk1.7.0_51, 64-bit on MacOS 10.9.6
• Call depth before StackOverflowError: 10,827 (consistently)
Java8, jdk1.8.0_05, 64-bit on MacOS 10.9.6
• Call depth before StackOverflowError: 20,000-ish (huh?)

Stack (cont’d)
FUN WITH NUMBERS….
• With Java8 default stack size, call depth = 20,000-ish
• Using –Xss1024k, same result (SO: default –Xss=1024k)
• Using –XX:ThreadStackSize=1024, same result (-
XX:ThreadStackSize is another way of expressing –Xss)
Okay, but…
• -Xss512k result: 9000-ish
• Not quite linear!
Need more memory space? Use a smaller –Xss value!
• 200 threads * 512kb/thread = 100 MB saved!
• What is your true stack size requirement?

Memory Basics: Types of Memory (cont’d)
HEAP
• Eden/New vs. Survivor vs. Old/Tenured
• “Ideal Object Death Rate”
• Eden >> Old >> Survivor

Heap (cont’d)
HEAP
• What is your object survival demography?
• Caching!
• Find out with:
• -XX:+PrintGCDetails
• -XX:+PrintTenuringDistribution
• -XX:+PrintGCTimestamps
• Adjust with:
• -XX:NewRatio=n
– Example: NewRatio=2  (oldGenSize) = 2 * (newGenSize)
• -XX:NewSize=n (minimum)
• -XX:MaxNewSize=n
• -XX:MinFreeHeapRatio=n
• -XX:MaxFreeHeapRatio=n

Heap: Java 6 HotSpot

META
• PermGen in Java 6
• PermGen (sort of) in Java 7
• Metaspace in Java 8
while (true) {
int length = rnd.nextInt(100);
StringBuilder builder = new StringBuilder();
String chars = "abcdefghijklmnopqrstuvwxyz";
for (int i = 0; i < length; i++) {
builder.append(chars.charAt(rnd.nextInt(chars.length())));
}
interned.add(builder.toString().intern());
}
What happens when this code is run?

Running the code…
JAVA 6
Exception in thread "main" java.lang.OutOfMemoryError: PermGen space
• …in about 8 seconds!
JAVA 7
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
• …and it takes a long time to happen!
JAVA 8
• Similar result as with Java 7

What happened???
JAVA 6
PermGen contains:
• Class meta data
• Interned strings
JAVA 7
• Interned strings moved to Metaspace
JAVA 8
• Class meta data also moved to Metaspace

Metaspace
REPLACEMENT FOR PERMGEN
• NOT JVM memory – native memory!
• BEA/Oracle JRockit
• IBM JVM
SOME NEW STARTUP FLAGS
• -XX:MetaSpaceSize=n (initial size)
• -XX:MaxMetaSpaceSize=n
• -XX:MaxMetaspaceExpansion=n
• -XX:MaxMetaspaceFreeRatio=n
• No default values for size! Max metaspace size is unlimited :-0
• Say goodbye to PermGen problems……right? Right?

More Metaspace
OBSOLETES SOME OLD STARTUP FLAGS
• All of the “-XX:*Perm*” flags
NEW MEMORY POOL OBJECTS
• No more matching on “Perm” for pool name

What does this mean?
NO MORE OOM/PERMGEN
• But metaspace errors don’t go away 
• Monitor metaspace, not PermGen
• Use –XX:MaxMetaSpaceSize?
• No  you will never run out of metaspace
• …but your use of native memory can grow unbounded!
• Yes  you merely replace “OutOfMemory: PermGen space” with
“OutOfMemory: Metaspace”
MONITOR METASPACE
• Apps that leak metadata will still leak metadata
• Gobbling up metaspace affects the entire system, not just the JVM
• Remember: native memory!

Monitoring Metaspace
COMMERCIAL SOLUTIONS
• Expect (demand!) metaspace monitoring support to go along with
Java 8 support
JAVA MANAGEMENT BEANS
• Update any querying of MBeans to search for Metaspace memory
pool instead of PermGen pools.
JVISUALVM
• Metaspace monitoring supported
• …at least in the jvisualvm that is bundled with Jdk1.8.0_05 on MacOS!

DIRECT
• sun.misc.Unsafe
• Used by the JDK (nio classes, for example)
• Out of the scope of this talk!

Let’s talk garbage…
PARALLEL GC
• Default on Java 7, Java 8
• ParNewGC is the default on Java 6
• Does parallel compaction in Java 6 and 7
• -XX:+UseParallelOldGC option in Java 5
CONCURRENT MARK AND SWEEP GC
• NOT compacting!
• Can be prone to fragmentation of old gen
•  very bad full GCs result
G1GC
• Only use after jdk1.7.0_u4!
• “Experimental” in previous releases

The GC Bake-off
GC KILLER PROGRAM
• Allocates lots of java.lang.Objects
• Allocates fewer long Strings
• Keeps objects around for varying lengths of times
• Checks heap memory availability
• When low, thousands of memory references are let go of
• No GC tuning options used
• 15-minute runs (killed with Ctrl-C)
• Java 7 vs. Java 8

Java 7, Parallel GC

Java 7, Parallel GC
HIGHLIGHTS
• No “-XX” option to enable (default)
• Heap usage varied between 1500-2750 MB
• Reasonable time spent in GC (1.6 seconds/minute)
• No major collections

Java 7, ConcMarkSweepGC

HIGHLIGHTS
• Enabled with –XX:+UseConcMarkSweepGC
• Heap usage held steady around 2000 MB
• Huge amount of time spent in GC (24 seconds/minute!)

Java 7, G1GC

Java 7, G1GC
HIGHLIGHTS
• Enabled with –XX:+UseG1GC
• Heap usage varied between 750-2800 MB
• Very reasonable time spent in GC (0.6 seconds/minute)

The winner for Java 7
G1!
• Less time spent in GC
• Heap usage max barely more than max using Parallel
• CMS max heap usage was only 2000
• CMS min was also near 2000 (very steady)
• Heap usage min best of all three
• BUT:
• Remember, no GC tuning parameters were used
• DO NOT USE G1GC before jdk1.7.0_04
• “Experimental” in prior releases

Java 8, Parallel GC

Java 8, Parallel GC
HIGHLIGHTS
• Incrementally worse than ParallelGC on Java 7
• Higher max heap (2950 vs. 2750)
• Higher min heap (1700MB vs. 1500MB)
• Roughly the same GC time (1.6 seconds/minute)

HIGHLIGHTS
• Just……lousy
• HUGE amount of initial time spent in gc
• 55 seconds/minute!
• Heap usage steadily declined
• From 3000MB to 2000MB
• With all that time spent, you’d think a major collection would be
avoided
• You’d be wrong 
• The only run with a major collection

Java 8, G1GC

Java 8, G1GC
HIGHLIGHTS
• Incrementally worse than G1/Java 7
• Heap usage max 2850MB vs. 2800MB
• GC time .8 seconds/minute vs. .6 seconds/minute

The winner for Java 8, and overall
G1 AGAIN!
• CMS not even close
• G1 on Java 7 overall winner
• Caveats:
• No GC tuning parameters used
• Heap-only: very little metadata memory used
• MacOS 10.9.4 (Maverick’s)
• 2.8 GHz Intel Core i7 processor
• 16 GB 1600 MHz memory
• Java 7: jdk 1.7.0_51 for MacOS
• Java 8: jdk 1.8.0_05 for MacOS

GC Tuning
OUT OF SCOPE 
• 600+ “-XX:” options
• 83 for CMS in Java 7
• 82 for CMS in Java 8
• No love for CMSTriggerPermRatio!
• 24 for G1 in Java 7 and 8
• Too many to cover!

Summary
• Know your object demography
• Monitor your metaspace
• Interned strings
• Class metadata
• Research the best GC
• Watch out for outdated information
• G1GC tests before jdk1.7.0_04
• Re-think CMS GC?
• Un-tuned CMS results far worse

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Memory Management: What You Need to Know When Moving to Java 8

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