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132 lines
6.9 KiB
HTML
132 lines
6.9 KiB
HTML
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<?xml version="1.0" encoding="UTF-8" standalone="no"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
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<title>Sizing the Cache</title>
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<link rel="stylesheet" href="gettingStarted.css" type="text/css" />
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<meta name="generator" content="DocBook XSL Stylesheets V1.73.2" />
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<link rel="start" href="index.html" title="Getting Started with Berkeley DB Java Edition" />
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<link rel="up" href="administration.html" title="Chapter 12. Administering Berkeley DB Java Edition Applications" />
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<link rel="prev" href="backgroundthreads.html" title="Managing the Background Threads" />
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<link rel="next" href="diskthreshold.html" title="Setting Disk Thresholds" />
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</head>
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<body>
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<div xmlns="" class="navheader">
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<div class="libver">
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<p>Library Version 12.2.7.5</p>
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</div>
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<table width="100%" summary="Navigation header">
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<tr>
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<th colspan="3" align="center">Sizing the Cache</th>
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</tr>
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<tr>
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<td width="20%" align="left"><a accesskey="p" href="backgroundthreads.html">Prev</a> </td>
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<th width="60%" align="center">Chapter 12. Administering Berkeley DB Java Edition Applications</th>
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<td width="20%" align="right"> <a accesskey="n" href="diskthreshold.html">Next</a></td>
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</tr>
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</table>
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<hr />
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</div>
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<div class="sect1" lang="en" xml:lang="en">
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<div class="titlepage">
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<div>
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<div>
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<h2 class="title" style="clear: both"><a id="cachesize"></a>Sizing the Cache</h2>
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</div>
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</div>
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</div>
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<p>
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By default, your cache is limited to a percentage of the JVM maximum
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memory as specified by the <code class="literal">-Xmx</code> parameter. You can
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change this percentage by using the <code class="literal">je.maxMemoryPercent</code> property
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or through <code class="methodname">EnvironmentMutableConfig.setCachePercent()</code>.
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That is, the maximum amount of memory available to your cache is
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normally calculated as:
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</p>
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<pre class="programlisting">je.maxMemoryPercent * JVM_maximum_memory</pre>
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<p>
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You can find out what the value for this property is by using
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<code class="methodname">EnvironmentConfig.getCachePercent()</code>.
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</p>
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<p>
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Note that you can cause JE to use a fixed maximum cache size by
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using <code class="literal">je.maxMemory</code> or by using
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<code class="methodname">EnvironmentConfig.setCacheSize()</code>.
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</p>
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<p>
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Also, not every JVM is capable of identifying the amount of memory requested via
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the <code class="literal">-Xmx</code> parameter. For those JVMs you must use
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<code class="literal">je.maxMemory</code> to change your maximum cache size. The
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default maximum memory available to your cache in this case is 38M.
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</p>
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<p>
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Of the amount of memory allowed for your cache, 93% is used for the internal BTree and the other 7% is
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used for internal buffers. When your application first starts up, the 7% for buffers is immediately allocated.
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The remainder of the cache grows lazily as your application reads and writes data.
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</p>
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<p>
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In order for your application to start up successfully, the Java virtual machine must have enough memory
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available to it (as identified by the <code class="literal">-Xmx</code> command line switch) for both your application and
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7% of your maximum cache value. In order for your application to run continuously (all the while loading data
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into the cache), you must make sure your JVM has enough memory for your application plus the maximum cache size.
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</p>
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<p>
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The best way to determine how large your cache needs to be is to put your application into a production
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environment and watch to see how much disk I/O is occurring. If the application is going to disk quite a lot to
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retrieve database records, then you should increase the size of your cache (provided that you have enough memory
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to do so).
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</p>
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<p>
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You can also use the
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<code class="methodname">com.sleepycat.je.util.DbCacheSize</code> utility
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to obtain a rough estimate of how large your cache needs to be for
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a given number of records and record characteristics. The utility
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returns an estimate of the cache size to hold
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the specified number of records in memory. See the
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<a class="ulink" href="../java/com/sleepycat/je/util/DbCacheSize.html" target="_top">DbCacheSize javadoc</a>
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for information on the utility's usage.
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</p>
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<p>
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In order to determine how frequently your application is going to disk for database records not found in the
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cache, you can examine the value returned by <code class="methodname">EnvironmentStats.getNCacheMiss()</code>.
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</p>
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<p>
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<code class="methodname">EnvironmentStats.getNCacheMiss()</code> identifies the total number of requests for
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database objects that were
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not serviceable from the cache. This value is cumulative since the application started. The faster this number grows,
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the more your application is going to disk to service database operations. Upon application startup you can
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expect this value to grow quite rapidly. However, as time passes and your cache is seeded with your most
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frequently accessed database records, what you want is for this number's growth to be zero or at least very
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small.
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</p>
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<p>
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Note that this statistic can only be collected from within the application itself or using the JMX extension
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(see <a class="xref" href="jmx.html" title="JConsole and JMX Support">JConsole and JMX Support</a>).
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</p>
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<p>
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For more information on collecting this statistic, see <a class="xref" href="envStats.html" title="Environment Statistics">Environment Statistics</a>.
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</p>
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</div>
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<div class="navfooter">
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<hr />
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<table width="100%" summary="Navigation footer">
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<tr>
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<td width="40%" align="left"><a accesskey="p" href="backgroundthreads.html">Prev</a> </td>
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<td width="20%" align="center">
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<a accesskey="u" href="administration.html">Up</a>
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</td>
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<td width="40%" align="right"> <a accesskey="n" href="diskthreshold.html">Next</a></td>
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</tr>
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<tr>
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<td width="40%" align="left" valign="top">Managing the Background Threads </td>
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<td width="20%" align="center">
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<a accesskey="h" href="index.html">Home</a>
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</td>
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<td width="40%" align="right" valign="top"> Setting Disk Thresholds</td>
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</tr>
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</table>
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</div>
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</body>
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</html>
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