JVM 64-bit OutOfMemory / Compressed references

If you are running on 64-bit OS and JRockit JVM 64-bit has the heap < 4GB in size and you experience OutOfMemory errors, try disabling compressed references by using the following option in your scripts:

-XXcompressedRefs[=[true|1|false|0]]

or

-XXcompressedRefs:parameter=value

where 1/true stands for enabling the option.

You can also use the following format, when using different heap sizes:

-XXcompressedRefs:enable=true

or

-XXcompressedRefs:size=4GB for Xmx <=2g
-XXcompressedRefs:size=32GB for Xmx <=25g
-XXcompressedRefs:size=64GB for Xmx <=57g

What compressed references does, it limits all pointers (to classes, objects, threads) stored on the heap to 32bits, hence less usage of the heap. 

Define Heap Size/ Memory management

Define Heap Size/ Memory management

HotSpot		JRockit	Function
-Xmx:[size]			Xmx sets the maximum size of the heap. Maximum value should be the maximum available, but in such a way that it does not cause any error, or about 75% of the system physical memory (depends on the OS).
-Xms:[size]			Xms sets the minimum & initial size of the heap. Recommended to set the same value as the Xmx to minimize allocation overhead of the heap.
-Xmn, -XXNewSize, -XXMaxNewSize	-Xns		Xns sets the size of the Young/ Nursery generation heap. This is the place where new objects land in the first place before being promoted to the other space. XX:NewSize - Use   this option to set the New generation Java heap size. Can be set to be one-fourth the size of the maximum heap size. Increase the value of this option for larger numbers of short-lived objects. -XX:MaxNewSize - Use this option to set the maximum New generation Java heap size.
-verbose:gc	-Xverbose:memory		Prints the additional information
-XX:+AggressiveHeap	XXaggressive:memory		Configures the memory system for memory-intensive workloads and sets an expectation to enable large amounts of memory resources to ensure high throughput. The JRockit JVM will also use large pages, if available.
XX:+UseConcMarkSweepGC	Xgc:singlecon		Sets the garbage collector to use a concurrent strategy
XX:+UseParallelGC	Xgc:parallel		Sets the garbage collector to use a parallel strategy
	-XgcPrio		Specifies what to prioritize: even pause times or maximum throughput
	-XpauseTarget		Specifies a suitable pause time for the application
-XX:MaxPermSize			Define Maximum heap for the Perm Genaration. . (Permanent generation) (non-heap) Stores classes, methods and other meta data

Other settings:

• -Xss - If the thread stack size (-Xss) has not been set the default value depends on the threading system and the platform you are running on. When using thin threads the minimum thread stack size is 8 kilobytes and the default is 64 kilobytes. When using native threads the minimum thread stack size is 16 kilobytes. For Windows the default thread stack size when using native threads is 64 kilobytes and for Linux it is 128 kilobytes.

• -XX:SurvivorRatio - The New generation area is divided into several sub-areas: Eden, Survival Space I and Survival Space II. Use the -XX:SurvivorRatio=X option to configure the ratio of the Eden/Survivor Space size. When it is 1:x Eg., -XX:SurvivorRatio=6 - each survivor space will be 1/8 the young generation).

• –XX:NewRatio=n is used divide heap into old and new generation. If we use. -- XX:NewRatio=5, then you create an old-to-new ratio of 5:1; the old generation occupies 5/6 of the heap while the new generation occupies 1/6 of the heap (-XX:NewRatio=3 - the young generation will occupy 1/4 the overall heap).

Tables were taken from the Oracle website, and changed to suit the blog.

JRockit vs HotSpot JVM Difference

Jrockit was developed by Appeal Virtual Machines and acquired by BEA Systems, and later on acquired by Oracle.

HotSpot is a Java Virtual Machine (JVM) currently hold by Oracle Corporation. 

JRockit is based on the HotSpot JVM by Sun Microsystems, and now Oracle. It has been developed in order to make HotSpot more reliable, and faster to used in server side environment. Jrockit has a slightly bit different heap organisation, garbage collector methods, and optimisations. More information about GC for HotSpot can be found here.

Although JRockit is supposed to have more optimised functions, it is not the rule. Some applications might run faster under HotSpot, and some can be tuned more for JRockit. You don't know it until you try it :)

Additional information:

http://en.wikipedia.org/wiki/JRockit
http://stackoverflow.com/questions/8068717/jrockit-jvm-versus-hotspot-jvm
http://www.oracle.com/technetwork/middleware/jrockit/overview/index.html

http://weblogicserveradministration.blogspot.com/2010/11/differences-between-bea-jrockit-sdk-and.html

Garbage Collectors - JRockit

TL;DR

• Dynamic (default one)

• Throughput – for maximum throughput (default):  –XgcPrio:throughput

• Pausetime – for short and even pause times: –XgcPrio:pausetime

• Deterministic – for very short and deterministic pause times (requires Oracle JRockit Real Time)

• Static (1x Generational, 2x Concurrent, 1x Parallel)

• Generational

• Generational Copy: -Xgc:gencopy

• Concurrent

• Single-spaced mostly concurrent: –Xgc:singlecon

• Generational mostly concurrent: –Xgc:gencon

• Parallel

• General parallel: –Xgc:genpar

Details:

Static:

• Generational Copy:
• best to use with small heap and for testing purposes
• splits heap into generations

• Generational parallel collector:

• causes longer pauses/ stop-the-world
• multiple threads to speed up the process
• segments the heap into nursery and tenured generations (objects that survive two cycles are promoted from nursery to tenured generation)
• use when the application has many short lived objects
• Concurrent:

• Single-spaced mostly concurrent collector:
• almost no pauses
• works concurrently
• uses entire heap
• will not pause the application as long as it can collect dead objects
• Generational mostly concurrent collector:
• frequent stop-the-world
• splits the heap -> parallel on nursery, concurrent on tenured generation
• shorter nursery will cause more objects in tenured, hence more stop-the-world
• Parallel:
• Single-spaced parallel collector:

• causes longer pauses/ stop-the-world
• multiple threads to speed up the process
• does not segment heap into multiple generations
• use when the application has more long lived objects

Dynamic:

Throughput:

"The higher the memory throughput the shorter is the time between the two events. Moreover, the higher the memory throughput the smaller the heap you will need."

 PauseTime:

" The longer the pause, the more unresponsive your system will be. The worst pause time and the average pause time are the two most interesting values you can use for tuning the system."

Garbage Collection (GC)

Garbage Collection is integral part of the JVM (Java Virtual Machine) and it is used to reclaim memory occupied by the unused objects.

When the application runs, it creates objects. Once the object is created it occupies some memory. When the object is not used/ referenced it can be considered as unused and will remain in the memory until the garbage collector claims the memory occupied by this object.

Objects at first are created and put in the Young Generation space. After some time can be moved (Partial/ Minor Garbage Collection) into the Old Generation space. Once the Young and Old Generation is FULL, Full Garbage Collection is run. 

This operation might impact the flow of the application, as it might take a very long time to complete. During this operation application becomes unresponsive.

To avoid this, we can tune the JVM Heap/ other settings. Different JVMs have different GC policy algorithms which will be discussed in the next posts.

Examples:

If we add the following option to the command line for the JVM:

-verbose:gc

we will get the information about the heap and result of the garbage collection for each collection.

[GC 325407K->83000K(776768K), 0.2300771 secs]
[GC 325816K->83372K(776768K), 0.2454258 secs]
[Full GC 267628K->83769K(776768K), 1.8479984 secs]

In this scenario we can see two minor GC and one Full/ Major GC.

• First numbers stand for the size of live objects before and after the GC.
• Numbers in parentheses represent memory space usable for java objects (perm/ survivor spaces not included).
• Secs stands for the amount of time in seconds that was taken to perform the action

[GC [DefNew: 64575K->959K(64576K), 0.0457646 secs] 196016K->133633K(261184K), 0.0459067 secs]

Here the second numbers represent the heap usage reduction (196016K -> 133633K(261184K)



If we want to get even more details on the GC results we can add the following options:

-XX:+PrintGCDetails

If we add:

-XX:+PrintGCTimeStamps

we will get additional information regarding timestamps when the GC occurred  Based on such results we can see how often GC occurs.