224 lines
15 KiB
HTML
224 lines
15 KiB
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html
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PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<meta name="DC.Title" content="Java performance considerations" />
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<meta name="abstract" content="Understanding the following considerations can help you improve the performance of your Java applications." />
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<meta name="description" content="Understanding the following considerations can help you improve the performance of your Java applications." />
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<meta name="copyright" content="(C) Copyright IBM Corporation 2006" />
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<!-- GSA ADP Schedule Contract with IBM Corp. -->
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<link rel="stylesheet" type="text/css" href="./ibmdita.css" />
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<link rel="stylesheet" type="text/css" href="./ic.css" />
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<title> Java performance considerations</title>
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</head>
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<body id="runtperf"><a name="runtperf"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1"> Java performance considerations</h1>
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<div><p>Understanding the following considerations can help you improve
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the performance of your Java™ applications.</p>
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<div class="section"><h4 class="sectiontitle">Creating optimized Java programs</h4><p>To greatly improve
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the startup performance of your Java code, use the Create Java Program
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(CRTJVAPGM) control language command before running Java class
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files, JAR files, or ZIP files. The CRTJVAPGM command uses the bytecodes to
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create a Java program object, which contains optimized native
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instructions for the iSeries™ server, and associates the Java program
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object with the class file, JAR file, or ZIP file.</p>
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<p>Subsequent runs are
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much faster because the Java program is saved and remains associated
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with the class file or JAR file. Running the bytecodes interpretively may
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provide acceptable performance during application development, but you may
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want to use the CRTJVAPGM command before running the Java code
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in a production environment.</p>
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<p>When you do not use CRTJVAPGM before running
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a Java class
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file, JAR file, or ZIP file, <span class="keyword">i5/OS™</span> uses
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the Just-In-Time compiler (with the Mixed-Mode Interpreter) instead.</p>
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<p><strong>Selecting
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the optimization level</strong></p>
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<p>When creating your Java program
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object, use the following guidelines to help you select the best optimization
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level for the run mode that you want to use:</p>
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<ul><li>When you want to use direct processing, create the optimized Java program
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object at optimization level 30 or 40.</li>
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<li>When you want to run only with the JIT compiler, create the optimized Java program
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by using the <kbd class="userinput">*Interpret</kbd> optimization parameter. A Java program
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created by using the <kbd class="userinput">*Interpret</kbd> parameter is smaller
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than one created by using optimization level 40.</li>
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<li>When you want to use the default run mode, which is a mix of direct processing
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and the JIT compiler, use the following settings to create your Java program
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objects: <ul><li>For classes that you want to run with direct processing, use either optimization
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level 30 or 40</li>
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<li>For classes that you want to run with the JIT compiler, use the <kbd class="userinput">*Interpret</kbd>
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optimization parameter</li>
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</ul>
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</li>
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</ul>
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<p>For more information, see the following pages:</p>
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<p><a href="../cl/crtjvapgm.htm" target="_blank">Create Java Program
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(CRTJVAPGM) control language command</a></p>
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<p><a href="selmod.htm">Select
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which mode to use when running a Java program</a></p>
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</div>
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<div class="section"><h4 class="sectiontitle">Using the Just-In-Time compiler</h4><p> Using the Just-In-Time
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(JIT) compiler with the Mixed-Mode Interpreter (MMI) results in startup performance
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that almost equals that of compiled code. MMI interprets your Java code
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until reaching the threshold specified by the os400.jit.mmi.threshold Java system
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property. After reaching the threshold, <span class="keyword">i5/OS</span> spends
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the time and resources required to use the JIT compiler to compile a method
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on the most frequently used methods. Using the JIT compiler results in highly
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optimized code that improves runtime performance when compared to precompiled
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code. When you require improved startup performance with the JIT compiler,
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you can use CRTJVAPGM to create an optimized Java program object.</p>
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<p>If your program
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is running slowly, enter the <a href="../cl/dspjvapgm.htm" target="_blank">Display Java Program (DSPJVAPGM)</a> control
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language command to view the attributes of a Java program object. Make sure that the Java program
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object uses the best run mode for your purposes. If you want to change the
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run mode, you may want to delete the Java program object and create a new one
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using different optimization parameters.</p>
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<p>For more information, see the
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following:</p>
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<blockquote> <p><a href="../cl/dspjvapgm.htm" target="_blank">Display Java Program
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(DSPJVAPGM) Control Language command</a></p>
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</blockquote>
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</div>
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<div class="section"><h4 class="sectiontitle">Using caches for user class loaders</h4><p>Using the <span class="keyword">i5/OS</span> Java virtual machine (JVM) cache for user
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class loaders improves startup performance for classes that you load from
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a user class loader. The cache stores the optimized Java program
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objects, which enables the JVM to reuse them. Reusing stored Java programs
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improves performance by avoiding both recreating the cached Java program
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objects and verifying the bytecode.</p>
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<p> Use the following properties to
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control caches for user class loaders: </p>
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<dl><dt class="dlterm">os400.define.class.cache.file</dt>
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<dd> The value of this property specifies the name (with the full path) of
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a valid Java ARchive (JAR) file. At a minimum, the specified
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JAR file must contain a valid JAR directory (as built by the <samp class="codeph">jar</samp> QSH
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command) and the single member required for the <samp class="codeph">jar</samp> command
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to function. Do not include the specified JAR file in any Java CLASSPATH.
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The default value of this property is /QIBM/ProdData/Java400/QDefineClassCache.jar.
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To disable caching, specify this property with no value.</dd>
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<dt class="dlterm">os400.define.class.cache.hours</dt>
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<dd> The value of this property specifies how long (in hours) that you want
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a Java program
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object to persist in the cache. When the JVM does not use a cached Java program
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object by the specified length of time, <span class="keyword">i5/OS</span> removes
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the Java program
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object from the cache. The default value of this property is 768 hours (33
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days). The maximum value is 9999 (about 59 weeks). When you specify either
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a value of 0 or a value that <span class="keyword">i5/OS</span> does
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not recognize as a valid decimal number, <span class="keyword">i5/OS</span> uses
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the default value.</dd>
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<dt class="dlterm">os400.define.class.cache.maxpgms</dt>
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<dd> The value of this property specifies the maximum number of Java program
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objects that the cache can hold. When the cache exceeds this limit, <span class="keyword">i5/OS</span> removes the oldest Java program
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object from the cache. <span class="keyword">i5/OS</span> determines
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which cached program is oldest by comparing times when the JVM last referenced
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the Java program
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objects. The default value is 5000, and the maximum value is 40000. When
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you specify either a value of 0 or a value that <span class="keyword">i5/OS</span> does
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not recognize as a valid decimal number, <span class="keyword">i5/OS</span> uses
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the default value.</dd>
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</dl>
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<p>Use DSPJVAPGM on the JAR file, which you specify in the
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os400.define.class.cache.file property, to determine the number of cached Java program
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objects.</p>
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<ul><li>The <strong>Java programs</strong> field of the DSPJVAPGM display indicates
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the number of cached Java program objects.</li>
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<li>The <strong>Java program size</strong> field indicates the amount of
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storage used by the cached Java program objects.</li>
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<li>Other fields of the DSPJVAPGM display are meaningless when you use the
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command on a JAR file that you are using for caching.</li>
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</ul>
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<p><strong>Cache performance</strong></p>
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<p>Running some Java applications
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can cache a large number of Java program objects. Use DSPJVAPGM to determine
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if the number of cached Java programs approaches the maximum value
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before the application finishes running. Application performance can degrade
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when the cache gets full because <span class="keyword">i5/OS</span> may
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remove from the cache some programs that the application requires.</p>
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<p>You
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can prevent performance degradation that results when the cache becomes full.
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For example, you can set up applications to use separate caches for applications
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that run frequently but load different programs into the cache. Using separate
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caches can prevent the cache from getting full and thus prevent <span class="keyword">i5/OS</span> from
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removing Java programs from the cache. Alternatively, you can
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increase the number that you specify for the os400.define.class.cache.maxpgms
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property.</p>
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<p>You can use Change Java Program (CHGJVAPGM) control language
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command on the JAR file to change the optimization of the classes in the cache.
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CHGJVAPGM affects only programs that the cache currently holds. After you
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make changes to the optimization levels, the os400.defineClass.optLevel property
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specifies how to optimize any classes that are added to the cache.</p>
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<p>For
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example, to use the shipped cache JAR with a maximum of 10000 Java program
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objects, where each Java program has a maximum life of 1 year,
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set the following values for the cache properties:</p>
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<pre>os400.define.class.cache.file /QIBM/ProdData/Java400/QDefineClassCache.jar
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os400.define.class.cache.hours 8760
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os400.define.class.cache.maxpgms 10000</pre>
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</div>
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</div>
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<div>
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<ul class="ullinks">
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<li class="ulchildlink"><strong><a href="selmod.htm">Select which mode to use when running a Java program</a></strong><br />
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When you run a Java program, you can select which mode
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you would like to use. All modes verify the code and create a Java program
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object to hold the preverified form of the program.</li>
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<li class="ulchildlink"><strong><a href="interprt.htm">Java interpreter</a></strong><br />
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The Java interpreter is the part of the Java virtual
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machine that interprets Java class files for a particular hardware
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platform. The Java interpreter decodes each bytecode and runs a series
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of machine instructions for that bytecode.</li>
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<li class="ulchildlink"><strong><a href="transfor.htm">Static compilation</a></strong><br />
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The Java transformer is an IBM<sup>®</sup> <span class="keyword">i5/OS</span> component
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that preprocesses class files to prepare them to run using the iSeries Java virtual
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machine. The Java transformer creates an optimized program object
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that is persistent and is associated with the class file.</li>
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<li class="ulchildlink"><strong><a href="jit.htm">Just-In-Time compiler</a></strong><br />
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A Just-In-Time (JIT) compiler is a platform-specific compiler that generates machine instructions for each method as needed.</li>
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</ul>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="tuning.htm" title="You should take several aspects of Java application performance into consideration when building a Java application for your iSeries server.">Tune Java program performance with IBM Developer Kit for Java</a></div>
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</div>
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<div class="relconcepts"><strong>Related concepts</strong><br />
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<div><a href="evenperf.htm" title="The iSeries Java virtual machine enables the trace of certain Java events.">Java event trace performance tools</a></div>
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<div><a href="gc.htm" title="Garbage collection is the process of freeing storage that is used by objects that are no longer referred to by a program. With garbage collection, programmers no longer have to write error prone code to explicitly "free" or "delete" their objects. This code frequently results in "memory leak" program errors. The garbage collector automatically detects an object or group of objects that the user program can no longer reach. It does this because there are no references to that object in any program structure. Once the object has been collected, you can allocate the space for other uses.">Java garbage collection</a></div>
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<div><a href="jnmiperf.htm" title="Native method invocation on an iSeries server may not perform as well as native method invocation on other platforms.">Java Native Method Invocation performance considerations</a></div>
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<div><a href="jmiperf.htm" title="Method inlining can significantly improve method call performance. Any method that is final is a potential candidate for inlining.">Java method inlining performance considerations</a></div>
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<div><a href="excperf.htm" title="The iSeries exception architecture allows versatile interrupt and retry capabilities. It also allows mixed language interaction. Throwing Java exceptions on an iSeries server may be more expensive than on other platforms. This should not affect overall application performance unless Java exceptions are routinely used in the normal application path.">Java exception performance considerations</a></div>
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<div><a href="callperf.htm" title="Java method call traces provide significant performance information about the time that is spent in each Java method.">Java call trace performance tools</a></div>
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<div><a href="profperf.htm" title="System wide central processing unit (CPU) profiling calculates the relative amount of CPU time that is spent in each Java method and all system functions in use by your Java program.">Java profiling performance tools</a></div>
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</div>
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<div class="reltasks"><strong>Related tasks</strong><br />
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<div><a href="colpdc.htm" title="To collect Java performance data on an iSeries server, follow these steps.">Collect Java performance data</a></div>
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</div>
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</div>
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</body>
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</html>
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