Java Myth: Object Pooling

Since first day in industry, I have been told object creation is expensive in java and one need to create pool for object to improve performance. Here is apache's package for http://commons.apache.org/pool/ Today, while reading a java book named "Java Concurrency in Practice", I came to know it is no longer valid concept now. Excerpt from the book:

"11.4.7. Just Say No to Object Pooling
In early JVM versions, object allocation and garbage collection were slow,[13] but their performance has improved substantially since then. In fact, allocation in Java is now faster than malloc is in C: the common code path for new Object in HotSpot 1.4.x and 5.0 is approximately ten machine instructions.


To work around "slow" object lifecycles, many developers turned to object pooling, where objects are recycled instead of being garbage collected and allocated anew when needed. Even taking into account its reduced garbage collection overhead, object pooling has been shown to be a performance loss[14] for all but the most expensive objects (and a serious loss for light- and medium-weight objects) in single-threaded programs (Click, 2005).


In concurrent applications, pooling fares even worse. When threads allocate new objects, very little inter-thread coordination is required, as allocators typically use thread-local allocation blocks to eliminate most synchronization on heap data structures. But if those threads instead request an object from a pool, some synchronization is necessary to coordinate access to the pool data structure, creating the possibility that a thread will block. Because blocking a thread due to lock contention is hundreds of times more expensive than an allocation, even a small amount of pool-induced contention would be a scalability bottleneck. (Even an uncontended synchronization is usually more expensive than allocating an object.) This is yet another technique intended as a performance optimization but that turned into a scalability hazard. Pooling has its uses,[15] but is of limited utility as a performance optimization.

[13] As was everything else synchronization, graphics, JVM startup, reflectionpredictably so in the first version of an experimental technology.

[14] In addition to being a loss in terms of CPU cycles, object pooling has a number of other problems, among them the challenge of setting pool sizes correctly (too small, and pooling has no effect; too large, and it puts pressure on the garbage collector, retaining memory that could be used more effectively for something else); the risk that an object will not be properly reset to its newly allocated state, introducing subtle bugs; the risk that a thread will return an object to the pool but continue using it; and that it makes more work for generational garbage collectors by encouraging a pattern of old-to-young references.

^[15] In constrained environments, such as some J2ME or RTSJ targets, object pooling may still be required for effective memory management or to manage responsiveness."

Java: ArrayList and HashMap Concurrency Issues

"The synchronized collections classes, Hashtable and Vector, and the synchronized wrapper classes, Collections.synchronizedMap and Collections.synchronizedList, provide a basic conditionally thread-safe implementation of Map and List. However, several factors make them unsuitable for use in highly concurrent applications -- their single collection-wide lock is an impediment to scalability and it often becomes necessary to lock a collection for a considerable time during iteration to prevent ConcurrentModificationExceptions. The ConcurrentHashMap and CopyOnWriteArrayList implementations provide much higher concurrency while preserving thread safety, with some minor compromises in their promises to callers. ConcurrentHashMap and CopyOnWriteArrayList are not necessarily useful everywhere you might use HashMap or ArrayList, but are designed to optimize specific common situations. Many concurrent applications will benefit from their use."

Summary from:
http://www.ibm.com/developerworks/java/library/j-jtp07233.html