type=page
status=published
title=Tuning the Java Runtime System
next=tuning-os.html
prev=tuning-glassfish.html
~~~~~~

= Tuning the Java Runtime System

[[GSPTG00006]][[abeia]]


[[tuning-the-java-runtime-system]]
== 4 Tuning the Java Runtime System

The following topics are addressed here:

* link:#abeib[Java Virtual Machine Settings]
* link:#gfpzy[Start Options]
* link:#glaat[Tuning High Availability Persistence]
* link:#abeic[Managing Memory and Garbage Collection]
* link:#abeiq[Further Information]

[[abeib]][[GSPTG00069]][[java-virtual-machine-settings]]

=== Java Virtual Machine Settings

Each JDK have own set of JVM options useful for tuning.
The easiest way is to run the following command and then consult the JDK's
documentation to understand all consequences.

[source]
----
JAVA_HOME/bin/java -XX:+UnlockDiagnosticVMOptions -XX:+PrintFlagsFinal -version
----

[[gfpzy]][[GSPTG00070]][[start-options]]

In some situations, performance can be improved by using large page sizes.
For Ultrasparc CMT systems, include the `-XX:+UseLargePages` and
`-XX:LargePageSizeInBytes=256m` arguments with your JVM tuning.

[[glaat]][[GSPTG00071]][[tuning-high-availability-persistence]]

=== Tuning High Availability Persistence

If session `s1` and `s2` need to be replicated to an instance (backup
server), the replication module batches the replication messages to be
sent to that instance instead of sending separate replication messages.
This improves performance. In configurations in which a lot of session
replication is performed, you may find better performance by tuning the
`org.shoal.cache.transmitter.max.batch.size` system property. This
property determines the number of replication messages that constitute
one batch.

The default value for this property is `20`. You can try setting it as
high as `90`, depending on system loads. Like all system properties,
this property is set with the `-D` flag in your Java arguments.

[[abeic]][[GSPTG00072]][[managing-memory-and-garbage-collection]]

=== Managing Memory and Garbage Collection

The efficiency of any application depends on how well memory and garbage
collection are managed. The following sections provide information on
optimizing memory and allocation functions:

* link:#abeid[Tuning the Garbage Collector]
* link:#abeig[Tracing Garbage Collection]
* link:#abeih[Other Garbage Collector Settings]
* link:#abeii[Tuning the Java Heap]
* link:#abeio[Rebasing DLLs on Windows]

[[abeid]][[GSPTG00205]][[tuning-the-garbage-collector]]

==== Tuning the Garbage Collector

Garbage collection (GC) reclaims the heap space previously allocated to
objects no longer needed. The process of locating and removing the dead
objects can stall any application and consume as much as 25 percent
throughput.

Almost all Java Runtime Environments come with a generational object
memory system and sophisticated GC algorithms. A generational memory
system divides the heap into a few carefully sized partitions called
generations. The efficiency of a generational memory system is based on
the observation that most of the objects are short lived. As these
objects accumulate, a low memory condition occurs forcing GC to take
place.

The heap space is divided into the old and the new generation. The new
generation includes the new object space (eden), and two survivor
spaces. The JVM allocates new objects in the eden space, and moves
longer lived objects from the new generation to the old generation.

The young generation uses a fast copying garbage collector which employs
two semi-spaces (survivor spaces) in the eden, copying surviving objects
from one survivor space to the second. Objects that survive multiple
young space collections are tenured, meaning they are copied to the
tenured generation. The tenured generation is larger and fills up less
quickly. So, it is garbage collected less frequently; and each
collection takes longer than a young space only collection. Collecting
the tenured space is also referred to as doing a full generation
collection.

The frequent young space collections are quick (a few milliseconds),
while the full generation collection takes a longer (tens of
milliseconds to a few seconds, depending upon the heap size).

Other GC algorithms, such as the Concurrent Mark Sweep (CMS) algorithm,
are incremental. They divide the full GC into several incremental
pieces. This provides a high probability of small pauses. This process
comes with an overhead and is not required for enterprise web
applications.

When the new generation fills up, it triggers a minor collection in
which the surviving objects are moved to the old generation. When the
old generation fills up, it triggers a major collection which involves
the entire object heap.

Both HotSpot and Solaris JDK use thread local object allocation pools
for lock-free, fast, and scalable object allocation. So, custom object
pooling is not often required. Consider pooling only if object
construction cost is high and significantly affects execution profiles.

[[abeie]][[GSPTG00146]][[choosing-the-garbage-collection-algorithm]]

===== Choosing the Garbage Collection Algorithm

The default collector for Java server class machines will optimize for
throughput but be tolerant of somewhat long pause times. If you would
prefer to have minimal pause times at the expense of some throughput and
increased CPU usage, consider using the CMS collector.

[[gaclx]][[GSPTG00038]][[to-use-the-cms-collector]]

To use the CMS collector

Follow this procedure.

1. Make sure that the system is not using 100 percent of its CPU.
2. Configure the CMS collector in the server instance.
+
To do this, add the following JVM options:

* `-XX:+UseConcMarkSweepGC`
* `-XX:SoftRefLRUPolicyMSPerMB=1`

[[abeif]][[GSPTG00147]][[additional-information]]

===== Additional Information

Use the `jvmstat` utility to monitor HotSpot garbage collection. (See
link:#abeiq[Further Information].)

For detailed information on tuning the garbage collector, see
http://www.oracle.com/technetwork/java/javase/gc-tuning-6-140523.html[Java
SE 6 HotSpot Virtual Machine Garbage Collection Tuning]
(`http://www.oracle.com/technetwork/java/javase/gc-tuning-6-140523.html`).

[[abeig]][[GSPTG00206]][[tracing-garbage-collection]]

==== Tracing Garbage Collection

The two primary measures of garbage collection performance are
throughput and pauses. Throughput is the percentage of the total time
spent on other activities apart from GC. Pauses are times when an
application appears unresponsive due to GC.

Two other considerations are footprint and promptness. Footprint is the
working size of the JVM process, measured in pages and cache lines.
Promptness is the time between when an object becomes dead, and when the
memory becomes available. This is an important consideration for
distributed systems.

A particular generation size makes a trade-off between these four
metrics. For example, a large young generation likely maximizes
throughput, but at the cost of footprint and promptness. Conversely,
using a small young generation and incremental GC will minimize pauses,
and thus increase promptness, but decrease throughput.

JVM diagnostic output will display information on pauses due to garbage
collection. If you start the server in verbose mode (use the command
`asadmin start-domain --verbose` domain), then the command line argument
`-verbose:gc` prints information for every collection. Here is an
example of output of the information generated with this JVM flag:

[source]
----
[GC 50650K->21808K(76868K), 0.0478645 secs]
 [GC 51197K->22305K(76868K), 0.0478645 secs]
 [GC 52293K->23867K(76868K), 0.0478645 secs]
 [Full GC 52970K->1690K(76868K), 0.54789968 secs]
----

On each line, the first number is the combined size of live objects
before GC, the second number is the size of live objects after GC, the
number in parenthesis is the total available space, which is the total
heap minus one of the survivor spaces. The final figure is the amount of
time that the GC took. This example shows three minor collections and
one major collection. In the first GC, 50650 KB of objects existed
before collection and 21808 KB of objects after collection. This means
that 28842 KB of objects were dead and collected. The total heap size is
76868 KB. The collection process required 0.0478645 seconds.

Other useful monitoring options include:

* `-XX:+PrintGCDetails` for more detailed logging information
* `-Xloggc:file` to save the information in a log file

[[abeih]][[GSPTG00207]][[other-garbage-collector-settings]]

==== Other Garbage Collector Settings

To specify the attributes for the Java virtual machine, use the
Administration Console and set the property under config-name > JVM
settings (JVM options).

[[glgkm]][[GSPTG00148]][[setting-the-maximum-permanent-generation]]

===== Setting the Maximum Permanent Generation

For applications that do not dynamically generate and load classes, the
size of the permanent generation does not affect GC performance. For
applications that dynamically generate and load classes (for example,
JSP applications), the size of the permanent generation does affect GC
performance, since filling the permanent generation can trigger a Full
GC. Tune the maximum permanent generation with the `-XX:MaxPermSize`
option.

[[glglk]][[GSPTG00149]][[disabling-explicit-garbage-collection]]

===== Disabling Explicit Garbage Collection

Although applications can explicitly invoke GC with the `System.gc()`
method, doing so is a bad idea since this forces major collections, and
inhibits scalability on large systems. It is best to disable explicit GC
by using the flag `-XX:+DisableExplicitGC`.


[NOTE]
====
On Windows systems, setting the `-XX:+DisableExplicitGC` option might
prevent the renaming or removal of open application files. As a result,
deployment, redeployment, or other operations that attempt to rename or
delete files might fail.

Application files can remain open because the files have been used by
class loaders to find classes or resources, or have been opened
explicitly by {productName} or application code but never explicitly
closed. On Windows systems, open files cannot be renamed or deleted. To
overcome this limitation, {productName} uses the `System.gc()` method
to garbage collect the Java object that corresponds to an open file.
When the Java object that corresponds to an open file is garbage
collected, the object's `finalize` method closes the open channel to the
file. {productName} can then delete or rename the file.
====


[[glglr]][[GSPTG00150]][[setting-the-frequency-of-full-garbage-collection]]

===== Setting the Frequency of Full Garbage Collection

{productName} uses RMI in the Administration module for monitoring.
Garbage cannot be collected in RMI-based distributed applications
without occasional local collections, so RMI forces a periodic full
collection. Control the frequency of these collections with the property
`-sun.rmi.dgc.client.gcInterval`. For example,
`- java -Dsun.rmi.dgc.client.gcInterval=3600000` specifies explicit
collection once per hour instead of the default rate of once per minute.

[[abeii]][[GSPTG00208]][[tuning-the-java-heap]]

==== Tuning the Java Heap

This section discusses topics related to tuning the Java Heap for
performance.

* link:#abeij[Guidelines for Java Heap Sizing]
* link:#abeik[Heap Tuning Parameters]

[[abeij]][[GSPTG00151]][[guidelines-for-java-heap-sizing]]

===== Guidelines for Java Heap Sizing

Maximum heap size depends on maximum address space per process. The
following table shows the maximum per-process address values for various
platforms:

[[sthref10]][[gacna]]

Table 4-1 Maximum Address Space Per Process

[width="100%",cols="<62%,<38%",options="header",]
|===
|Operating System |Maximum Address Space Per Process
|Oracle/Redhat/Ubuntu Linux 32-bit |4 GB
|Oracle/Redhat/Ubuntu Linux 64-bit |Terabytes
|Windows XP/2008/7 |2 GB
|Solaris x86 (32-bit) |4 GB
|Solaris 32-bit |4 GB
|Solaris 64-bit |Terabytes
|===


Maximum heap space is always smaller than maximum address space per
process, because the process also needs space for stack, libraries, and
so on. To determine the maximum heap space that can be allocated, use a
profiling tool to examine the way memory is used. Gauge the maximum
stack space the process uses and the amount of memory taken up libraries
and other memory structures. The difference between the maximum address
space and the total of those values is the amount of memory that can be
allocated to the heap.

You can improve performance by increasing your heap size or using a
different garbage collector. In general, for long-running server
applications, use the Java SE throughput collector on machines with
multiple processors (`-XX:+AggressiveHeap`) and as large a heap as you
can fit in the free memory of your machine.

[[abeik]][[GSPTG00152]][[heap-tuning-parameters]]

===== Heap Tuning Parameters

You can control the heap size with the following JVM parameters:

* ``-Xms``value
* ``-Xmx``value
* ``-XX:MinHeapFreeRatio=``minimum
* ``-XX:MaxHeapFreeRatio=``maximum
* ``-XX:NewRatio=``ratio
* ``-XX:NewSize=``size
* ``-XX:MaxNewSize=``size
* `-XX:+AggressiveHeap`

The `-Xms` and `-Xmx` parameters define the minimum and maximum heap
sizes, respectively. Since GC occurs when the generations fill up,
throughput is inversely proportional to the amount of the memory
available. By default, the JVM grows or shrinks the heap at each GC to
try to keep the proportion of free space to the living objects at each
collection within a specific range. This range is set as a percentage by
the parameters `-XX:MinHeapFreeRatio=`minimum and
`-XX:MaxHeapFreeRatio=`maximum; and the total size bounded by `-Xms` and
`-Xmx`.

Set the values of `-Xms` and `-Xmx` equal to each other for a fixed heap
size. When the heap grows or shrinks, the JVM must recalculate the old
and new generation sizes to maintain a predefined `NewRatio`.

The `NewSize` and `MaxNewSize` parameters control the new generation's
minimum and maximum size. Regulate the new generation size by setting
these parameters equal. The bigger the younger generation, the less
often minor collections occur. The size of the young generation relative
to the old generation is controlled by `NewRatio`. For example, setting
`-XX:NewRatio=3` means that the ratio between the old and young
generation is 1:3, the combined size of eden and the survivor spaces
will be fourth of the heap.

By default, the {productName} is invoked with the Java HotSpot Server
JVM. The default `NewRatio` for the Server JVM is 2: the old generation
occupies 2/3 of the heap while the new generation occupies 1/3. The
larger new generation can accommodate many more short-lived objects,
decreasing the need for slow major collections. The old generation is
still sufficiently large enough to hold many long-lived objects.

To size the Java heap:

* Decide the total amount of memory you can afford for the JVM.
Accordingly, graph your own performance metric against young generation
sizes to find the best setting.
* Make plenty of memory available to the young generation. The default
is calculated from `NewRatio` and the `-Xmx` setting.
* Larger eden or younger generation spaces increase the spacing between
full GCs. But young space collections could take a proportionally longer
time. In general, keep the eden size between one fourth and one third
the maximum heap size. The old generation must be larger than the new
generation.

For up-to-date defaults, see
http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html[Java
HotSpot VM Options]
(`http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html`).

[[GSPTG00032]][[fxxpw]]
Example 4-1 Heap Configuration on Solaris

This is an exmple heap configuration used by {productName} on Solaris
for large applications:

[source]
----
-Xms3584m
-Xmx3584m
-verbose:gc
-Dsun.rmi.dgc.client.gcInterval=3600000
----

[[abeil]][[GSPTG00031]][[survivor-ratio-sizing]]

Survivor Ratio Sizing

The `SurvivorRatio` parameter controls the size of the two survivor
spaces. For example, `-XX:SurvivorRatio=6` sets the ratio between each
survivor space and eden to be 1:6, each survivor space will be one
eighth of the young generation. The default for Solaris is 32. If
survivor spaces are too small, copying collection overflows directly
into the old generation. If survivor spaces are too large, they will be
empty. At each GC, the JVM determines the number of times an object can
be copied before it is tenured, called the tenure threshold. This
threshold is chosen to keep the survivor space half full.

Use the option `-XX:+PrintTenuringDistribution` to show the threshold
and ages of the objects in the new generation. It is useful for
observing the lifetime distribution of an application.

[[abeio]][[GSPTG00209]][[rebasing-dlls-on-windows]]

==== Rebasing DLLs on Windows

When the JVM initializes, it tries to allocate its heap using the `-Xms`
setting. The base addresses of {productName} DLLs can restrict the
amount of contiguous address space available, causing JVM initialization
to fail. The amount of contiguous address space available for Java
memory varies depending on the base addresses assigned to the DLLs. You
can increase the amount of contiguous address space available by
rebasing the {productName} DLLs.

To prevent load address collisions, set preferred base addresses with
the rebase utilty that comes with Visual Studio and the Platform SDK.
Use the rebase utility to reassign the base addresses of the {productName} DLLs to prevent relocations at load time and increase the
available process memory for the Java heap.

There are a few {productName} DLLs that have non-default base
addresses that can cause collisions. For example:

* The `nspr` libraries have a preferred address of 0x30000000.
* The `icu` libraries have the address of 0x4A?00000.

Move these libraries near the system DLLs (`msvcrt.dll` is at
`0x78000000`) to increase the available maximum contiguous address space
substantially. Since rebasing can be done on any DLL, rebase to the DLLs
after installing the {productName}.

[[gacmt]][[GSPTG00039]][[to-rebase-the-glassfish-servers-dlls]]

===== To rebase the {productName}'s DLLs

[[sthref11]]

Before You Begin

To perform rebasing, you need:

* Windows 2000
* Visual Studio and the Microsoft Framework SDK rebase utility

1. Make as-install\ `bin` the default directory.
+
[source]
----
cd as-install\bin
----

2. Enter this command:
+
[source]
----
rebase -b 0x6000000 *.dll
----

3. Use the `dependencywalker` utility to make sure the DLLs were
rebased correctly.
+
For more information, see the http://www.dependencywalker.com[Dependency
Walker website] (`http://www.dependencywalker.com`).

4. Increase the size for the Java heap, and set the JVM Option
accordingly on the JVM Settings page in the Admin Console.

5. Restart the {productName}.

[[GSPTG00033]][[fxxpz]]
Example 4-2 Heap Configuration on Windows

This is an example heap configuration used by {productName}
for heavy server-centric applications, on Windows, as set in the
`domain.xml` file.

[source,xml]
----
<jvm-options> -Xms1400m </jvm-options>
<jvm-options> -Xmx1400m </jvm-options>
----

[[sthref12]]

See Also

For more information on rebasing, see
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/tools/tools/rebase.asp[MSDN
documentation for rebase utility]
(`http://msdn.microsoft.com/library/default.asp?url=/library/en-us/tools/tools/rebase.asp`).

[[abeiq]][[GSPTG00073]][[further-information]]

=== Further Information

For more information on tuning the JVM, see:

* http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html[
Java HotSpot VM Options]
(`http://www.oracle.com/technetwork/java/javase/tech/vmoptions-jsp-140102.html`)
* http://www.oracle.com/technetwork/java/hotspotfaq-138619.html[
Frequently Asked Questions About the Java HotSpot Virtual Machine]
(`http://www.oracle.com/technetwork/java/hotspotfaq-138619.html`)
* http://www.oracle.com/technetwork/java/javase/tech/index-jsp-136373.html[