My Workday

java -XX:+PrintFlagsFinal -version| grep -i Pre
     intx AllocatePrefetchDistance                  = 192             {product}           
     intx AllocatePrefetchInstr                     = 0               {product}           
     intx AllocatePrefetchLines                     = 4               {product}           
     intx AllocatePrefetchStepSize                  = 64              {product}           
     intx AllocatePrefetchStyle                     = 1               {product}           
     bool AlwaysPreTouch                            = false           {product}           
    uintx CMSAbortablePrecleanMinWorkPerIteration   = 100             {product}           
     intx CMSAbortablePrecleanWaitMillis            = 100             {manageable}        
    uintx CMSMaxAbortablePrecleanLoops              = 0               {product}           
     intx CMSMaxAbortablePrecleanTime               = 5000            {product}           
     bool CMSPermGenPrecleaningEnabled              = true            {product}           
    uintx CMSPrecleanDenominator                    = 3               {product}           
    uintx CMSPrecleanIter                           = 3               {product}           
    uintx CMSPrecleanNumerator                      = 2               {product}           
     bool CMSPrecleanRefLists1                      = true            {product}           
     bool CMSPrecleanRefLists2                      = false           {product}           
     bool CMSPrecleanSurvivors1                     = false           {product}           
     bool CMSPrecleanSurvivors2                     = true            {product}           
    uintx CMSPrecleanThreshold                      = 1000            {product}           
     bool CMSPrecleaningEnabled                     = true            {product}           
     bool CompilerThreadHintNoPreempt               = true            {product}           
    uintx G1HeapRegionSize                          = 0               {product}           
     intx InterpreterProfilePercentage              = 33              {product}           
     intx PreBlockSpin                              = 10              {product}           
     intx PreInflateSpin                            = 10              {pd product}

G1 collector is concurrent and parallel with more predictability and better useability. It splits the heap into regions and young and old generational spaces are organized in sets of regions. This eliminates the need to fine tune “NewSize/MaxNewSize”. G1 also provides incremental compaction in those regions by “evacuating” the regions that are full and fragmented. To try this G1 collector, you’ll need the aforementioned Fix Pack/Update and the following JVM options:

-XX:+UnlockExperimentalVMOptions -XX:+UseG1GC

In addition, the following options can be used to adjust G1’s characteristics:

To set the max GC pause time goal in milliseconds that G1 will attempt to meet:

-XX:MaxGCPauseMillis

To set the time interval over which GC pauses totaling up to MaxGCPauseMillis may take place:

-XX:GCPauseIntervalMillis

For more information about G1 (how it operates, more options, etc.) see Java HotSpot Garbage Collection.

The Java Virtual Machine (JVM)* has the following types of memory: heap, non-heap, and native.

Heap memory is the runtime data area from which memory for all class instances and arrays is allocated. Non-heap memory includes the method area and memory required for the internal processing or optimization of the JVM. It stores per-class structures such as a runtime constant pool, field and method data, and the code for methods and constructors. Native memory is the virtual memory managed by the operating system. When the memory is insufficient for an application to allocate, a java.lang.OutOfMemoryError will be thrown.

Following are the possible error messages for OutOfMemoryErrors in each type of memory:

Heap memory error: When an application creates a new object but the heap does not have sufficient space and cannot be expanded further, an OutOfMemoryError will be thrown with the following error message:

java.lang.OutOfMemoryError: Java heap space

Non-heap memory error: The permanent generation is a non-heap memory area in the HotSpot VM implementation that stores per-class structures as well as interned strings. When the permanent generation is full, the application will fail to load a class or to allocate an interned string, and an OutOfMemoryError will be thrown with the following error message:

java.lang.OutOfMemoryError: PermGen space

Native memory error: The Java Native Interface (JNI) code or the native library of an application and the JVM implementation allocate memory from the native heap. An OutOfMemoryError will be thrown when an allocation in the native heap fails. For example, the following error message indicates insufficient swap space, which could be caused by a configuration issue in the operating system or by another process in the system that is consuming much of the memory:

java.lang.OutOfMemoryError: request bytes for .
Out of swap space?

An insufficient memory problem could be due either to a problem with the configuration — the application really needs that much memory — or to a performance problem in the application that requires you to profile and optimize to reduce the memory use.

You can find more information at http://java.sun.com/developer/technicalArticles/J2SE/monitoring/

ConcurrentHashMap is hash table supporting full concurrency of retrievals and adjustable expected concurrency for updates. I recently came across this code during testing, and one part really got my attention. To generate the hash, ConcurrentHashMap uses an algorithm based on bitshifting and bitwise operations.

========================================
Variant of single-word Wang/Jenkins hash
========================================
private static int hash(int h) {

// Spread bits to regularize both segment and index locations,
// using variant of single-word Wang/Jenkins hash.
h += (h <<  15) ^ 0xffffcd7d;
h ^= (h >>> 10);
h += (h <<   3);
h ^= (h >>>  6);
h += (h <<   2) + (h << 14);

return h ^ (h >>> 16);
}

According to the comment in the code, this method applies a supplemental hash function to a given hashCode, which defends against poor quality hash functions.

Good hash functions are important as a hash table effectively turns from a map to a linked list, in the worst case, all keys in the same bucket. There are also other considerations that come into play such as the performance of hash calculation and the number of buckets. Dr. Heinz M. Kabutz explains the power of “power-of-two number of buckets” which gives us some good starting point to understand what is really going on here.

Let’s look at the code above and see how things change, line-by-line. To make things simple, I use int 1 to perform all the operations.

In Java, the int data type is a 32-bit signed two’s complement integer. To represent int 1 in binary code, we have the following:

h=1 > 0000-0000-0000-0000-0000-0000-0001

Now, let’s dissect the following line:

h += (h << 15) ^ 0xffffcd7d

First, let's re-write this into an easier-to-read format.. at least for me :).

h1 = h << 15      =  0000-0000-0000-0000-1000-0000-0000-0000
hex = 0xffffcd7d  =  1111-1111-1111-1111-1100-1101-0111-1101
h2 = h1 ^ hex     =  1111-1111-1111-1111-0100-1101-0111-1101
h2 + h            =  1111-1111-1111-1111-0100-1101-0111-1110

Using the same thought processing and applying it to each line, we end-up with:

h += (h << 15) ^ 0xffffcd7d = 1111-1111-1111-1111-0100-1101-0111-1110
h ^= (h >>> 10)	            = 1111-1111-1100-0000-1011-0010-1010-1101
h += (h << 3)		    = 1111-1101-1100-0110-0100-1000-0001-0101
h ^= (h >>> 6)              = 1111-1110-0011-0001-0101-0001-0011-0101
h += (h << 2) + (h << 14)   = 0100-1011-0100-0011-1101-0110-0000-1001
h ^= (h >>> 16)             = 0100-1011-0100-0011-1001-1101-0100-1010

Result:
Bin = 0100-1011-0100-0011-1001-1101-0100-1010
Decimal = 1,262,722,378

JProfiler helps you find performance bottlenecks, pin down memory leaks and resolve threading issues in your Java application. JProfiler combines CPU, Memory and Thread profiling in one application and is developed by ej-technologies. The latest version at the time of this article is 6.0.

Verbose GC options

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