Java Performance Tuning

Java Performance Tuning Minh Hoang TO Portal Team

Agenda Performance Measure JRE Customization Programming Tips Reading Suggestions

Performance Measure Performance Analytical Tools Determine problem Attack performance problem

Performance Analytical Tools JDK tools: JVisualVM (since Java 6), JConsole Commercial tools: JProfiler

Determine problem Factor to improve? Time or Memory Quantitative analysis to encircle the root causes Use tools to localize memory leak, method call overhead Theoretical argument servers for first-step analysis, but tools provide most convincing information

Attack performance problem Improve execution environment Examine programming aspect

JRE Customization JVM Options Customize Garbage Collection Frequently used JVM options

JVM Options JAVA_HOME/java JAVA_HOME/java -X

Customize Garbage Collection JAVA_HOME/java -X -Xnoclassgc -Xincgc

Frequently used JVM options Heap size options -Xms -Xmx Permanent Generation options -XX:MaxPermSize Class sharing -Xshare

Programming Tips Object Creation String Collection Framework Java IO and NIO (for next time) Concurrency

Object Creation Object scope final modifier Interaction with Garbage Collector

Object Scope String wideScope; for(int i =0; i <10000000; i++) { wideScope = “” + i; } vs for(int i = 0; i < 10000000; i++) { String narrowScope = “” + i; }

Heap space is divided into Young Generation and Old Generation spaces JAVA_HOME/jconsole Garbage Collector has best performance on Young Generation Object with narrow scope has more chance to stay in Young Generation

Final modifier Protect variable against reference reassignment (non-reflect code) final SafeObject safeObject = new SafeObject(); safeObject = null; // compilation error Modularize code with anonymous class final String displayString = “Hello”; Runnable runnable = new Runnable(){ public void run() { System.out.println(displayString); } };

Final modifier Affect of final at execution Look at my IntelliJ IDEA 10

Interaction with Garbage Collector GC (Garbage Collector) reclaims memory occupied by unreferenced object TestObject strongReference = new TestObject(); //variable strongReference //points to memory cells at 0xXXXX strongReference = null; //no reference to cells 0xXXXX System.gc(); //Run the GC GC handles SoftReference , WeakReference and PhantomReference exceptionally JavaDoc from package java.lang.ref

//Object is allocated at 0xXXXX TestObject strongReference = new TestObject(); SoftReference<TestObject> softReference = new SoftReference<TestObject>(strongReference); //Object stored at 0xXXX is reachable from softReference strongReference = null; //No more strong reference to object Object stored at 0xXXXX is softly reachable from softReference, but automatically cleaned by GC when there is lack of memory

Code from LazyList class of GateIn private static class Batch { private final Reference<Object[]> elements; private Batch(Object[] elements) { this.elements = new SoftReference<Object[]>(elements); } }

String String Thread-safe, expensive handling methods StringBuffer Thread-safe, much faster than String StringBuilder Not thread-safe, slightly faster than StringBuffer

String s = new String(); for(int i = 0; i < 10000; i++){ s += “a”; } VS StringBuffer buffer = new StringBuffer(); for(int i = 0; i < 10000; i++){ buffer.append(“a”); } VS StringBuilder builder = new StringBuilder(); for(int i = 0; i < 10000; i++){ builder.append(“a”); }

Collection Framework Random Access vs Sequential Initial allocation HashMap vs TreeMap WeakHashMap

Random Access vs Sequential ArrayList<E> access element: O(1) add element at the end: O(1) or O(n) sort element: O(n.logn) (sort with Collections.sort()) LinkedList<E> access element: O(n) add element at both ends: O(1) sort element: O(n.logn) + O(n) (sort with Collections.sort())

Initial Allocation Code snippet recurring in eXo products List<String> list = new ArrayList<String>(3); Why hard-coded size? Is it better than List<String> list = new LinkedList<String>();

HashMap vs TreeMap HashMap<K,V> fetch <K,V> entry: O(1) put <K,V> entry: O(n) in worst case sort <K,V> entries based on key: Not supported TreeMap<K,V> fetch <K,V> entry: O(1) Put <K,V> entry: O(n) in worst case support sorting entries based on key

WeakHashMap private static class Entry<K,V> extends WeakReference<K> implements Map.Entry<K,V> { private V value; private final int hash; private Entry<K,V> next; … . }

Java IO and NIO In many cases: gainOf( improve_on_programming_logic ) <<<< gainOf( improve_on_input_output ) Java coder should have knowledge on Java NIO

Concurrency Concurrency concepts Intrinsic lock volatile keyword Lock & Lock-free

Concurrency concepts Atomicity Code executing on a thread is not inteferenced with other threads Ex: Code incrementing a counter Visibility Change on shared object is visible to multiple threads Ex: Flag variable

Atomicity yields Visibility Atomicity problems are more complex than Visibility ones Figure out your concurrency problem ( Atomicity or Visibility ) before typing synchronized

Intrinsic lock - synchronized Ensure Atomicity Overhead of synchronized Double-checked pattern 'intrinsic lock extension' in java.util.concurrent.locks

private int counter; private void increaseCounter(){ counter++;} private synchronized void increaseCounterUsingIntrinsicLock(){ counter++;} public static void main(String[] args) { int n = 100000; //Print time to call increaseCounter n times //Print time to call increaseCounterUsingIntrinsicLock n times }

Double-checked pattern synchronized( locked_object ) { if( flag_variable ){ //do something } } VS if( flag_variable ){ synchronized( locked_object ) { if( flag_variable ){ //do something } } }

Double-checked reduces the scope of synchronized block Useful in concurrent initialization problem Requires Visibility on flag_variable

Code from RootContainer class in eXo Kernel public static RootContainer getInstance() { RootContainer result = singleton_; if (result == null) { synchronized (RootContainer.class) {….} } }

Code from Component class in WebUI Framework public Event getUIComponentEventConfig(String eventName) throws Exception { if(eventMap == null) { synchronized(this) { if(eventMap == null) {…} } } }

Volatile keyword Ensure Visibility Work only with atomic operations Usage in double-checked pattern 'volatile extension' in java.util.concurrent.atomic

private volatile long counter; private void nonAtomicOperation(){ counter++;} private void atomicOperation(){ counter = 281283;}

Usage in double-checked pattern Java Memory Model permutes execution order in multi-thread world (JVM Specification) Visibility on flag_variable is mandatory to double-checked pattern volatile boolean flag_variable; //Assumed the type is boolean //Double-checked block

Lock & Lock-free Lock Package java.util.concurrent.locks Lock-free Package java.util.concurrent.atomic

Java Concurrency in Practice – Brian Goetz Java IO Java NIO

Java Performance Tuning

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