JVM Memory Management

Java Virtual Machine (JVM)

Memory Management & Monitoring

Java Performance

• Application Algorithm Inefficiencies

• In-process / Multi-Threads Inter-locking

• I/O Bound

• Memory Management (Garbage Collection)

Memory Management

JVM Heap Memory• Different Version JVM, Different Default Heap Size

• Initial Heap = Physical Memory / 64

• 4GB Physical = 64MB

• Max Heap = Physical Memory / 4

• 4GB Physical = 1GB

• Can be Override by -Xms and -Xmx

-Xms 1.5g -Xmx 3g

-Xms 1.5g -Xmx 3g

-Xms 1.5g -Xmx 3g

Out of Memory (OOM)Outcomes

• Extremely Slow JVM or Not Responding

• High CPU Load

• Memory/Disk Swapping

• A lot of Available Memory

• JVM Crashed

Garbage Collection

Memory Pools(Not G1GC)

Memory Pools

• Different Version JVM = Different Default Size Memory Pools

• Sizing by Ratio, Not Absolute Number

• Different Garbage Collector = Different Memory Pools

Garbage Collection

• All new allocations in Eden (Super Fast)

• When Eden has no space

• Stop the World

• Copy Collect the data into Survivor

• After several collection, Survivor will promote to Old Gen

• Old Gen Fragmentation

Garbage Collectors

• Young Gen Collectors

• -XX:+UseSerialGC

• -XX:+UseParallelGC (Java 6 Default, GC in other Thread)

• -XX:+UseParallelOldGC

• -XX:+UseParNewGC

• -XX:+UseG1GC (Java 7 Experimental)

Garbage Collectors

• Old Gen Collectors

• MarkSweepCompact (enabled with -XX:UseSerialGC)

• PS MarkSweep (enabled with -XX:UseParallelOldGC)

• -XX:ConcMarkSweepGC (Default Java 7)

• G1 Mixed Generation (enabled with -XX:+UseG1GC)

Garbage Collectors Combinations

• 2 Garbage Collections for 2 Types of Garbage Collections

• Young Gen => Survivor

• Survivor => Old Gen

• See http://www.fasterj.com/articles/oraclecollectors1.shtml

Throughput Collectors

• Long Pause

• Can Tune Automatically

• -XX:+UseAdaptiveSizePolicy

• -XX:MaxGCPauseMillis=…

• -XX:GCTimeRatio=…

Low Pause Collectors• Use these if Application response time is sensitive

• Works best with multiple CPU cores (4 cores min. for Java apps)

• 2 Cores for JVM + Garbage Collectors

• 2+ Cores for Application

• -XX:+UseConcMarkSweepGC

• Best Garbage Collectors for Web Services

• -XX:+UseG1GC

• Auto Memory Pools Size Adjustment

• Very Fast, No Old Gen Fragmentation

GC Tuning

• Do not touch anything unless you know what you are doing!

• Default settings for Modern JVM is usually very good

• Common Problems

• Mix Bag of Multiple Garbage Collectors Configurations

• Different JVM Version Different Default Settings

• Old Settings have No Meaning Anymore

Examples

Example Service

-Xms256M -Xmx1536M !-XX:NewSize=128m -XX:MaxNewSize=512m !-XX:+CMSPermGenSweepingEnabled !-XX:PermSize=128m -XX:MaxPermSize=256m !-XX:+CMSClassUnloadingEnabled !-XX:+UseConcMarkSweepGC !-XX:+HeapDumpOnOutOfMemoryError

Tips• For Java server application, -Xms should be the same

as -Xms

• Give JVM Heap as much as possible

• 4GB or below has the best performance

• 4GB+ may need JVM Heap Tuning

• Bigger the Young Gen

• Less Minor GC => More Full GC

Tips• Bigger the Young Gen

• Less Minor GC => More Full GC

• Most Concurrent GC (other than G1)

• -XX:+UseConcMarkSweepGC -XX:+UseParNewGC

• -XX:+UseParallelGC should not be used with -XX:+UseConcMarkSweepGC

Best JVM Heap Configuration

NONE

Example-Xms2048m -Xmx2048m !-XX:PermSize=512m -XX:MaxPermSize=512m!-XX:+UseConcMarkSweepGC -XX:+UseParNewGC!-XX:MaxGCPauseMillis=2000!-XX:+CMSPermGenSweepingEnabled !-XX:+CMSClassUnloadingEnabled !-XX:+UseCompressedOops!-XX:+HeapDumpOnOutOfMemoryError!-XX:+PrintGCTimeStamps !-XX:+PrintGCApplicationStoppedTime !-XX:+PrintGCApplicationConcurrentTime!!

JVM Tuning• Don’t adjust memory pool size unless

• Too many Full GC

• GC Takes Too long

• GC does not have high yield

• Adjust NewRatio, NewSize, MaxNewSize based on performance testing

• Considering using G1 instead of CMS