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| 3 | <head>
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| 4 | <meta http-equiv="Content-Language" content="en-us">
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| 5 | <title>Parallel Task</title>
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| 6 | <link rel="stylesheet" type="text/css" href="../stylesheets/antmanual.css">
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| 7 | </head>
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| 8 |
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| 9 | <body>
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| 10 |
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| 11 | <h2>Parallel</h2>
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| 12 | <h3>Description</h3>
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| 13 | <p>Parallel is a container task - it can contain other Ant tasks. Each nested
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| 14 | task within the parallel task will be executed in its own thread. </p>
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| 15 | <h3>Parameters</h3>
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| 16 | <table border="1" cellpadding="2" cellspacing="0">
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| 17 | <tr>
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| 18 | <td valign="top"><b>Attribute</b></td>
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| 19 | <td valign="top"><b>Description</b></td>
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| 20 | <td align="center" valign="top"><b>Required</b></td>
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| 21 | </tr>
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| 22 | <tr>
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| 23 | <td valign="top">threadCount</td>
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| 24 | <td valign="top">Maximum numbers of thread to use.</td>
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| 25 | <td align="center" valign="top">No</td>
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| 26 | </tr>
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| 27 | <tr>
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| 28 | <td valign="top">threadsPerProcessor</td>
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| 29 | <td valign="top">Maximum number of threads to use per available processor
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| 30 | (Requires JDK 1.4)</td>
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| 31 | <td align="center" valign="top">No, defers to threadCount</td>
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| 32 | </tr>
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| 33 | <tr>
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| 34 | <td valign="top">pollInterval</td>
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| 35 | <td valign="top">Currently has no effect</td>
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| 36 | <td align="center" valign="top">No, default is 1000</td>
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| 37 | </tr>
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| 38 | <tr>
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| 39 | <td valign="top">timeout</td>
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| 40 | <td valign="top">Number of milliseconds before execution is terminated</td>
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| 41 | <td align="center" valign="top">No</td>
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| 42 | </tr>
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| 43 | <tr>
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| 44 | <td valign="top">failonany</td>
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| 45 | <td valign="top">If any of the nested tasks fails, execution of the task completes
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| 46 | at that point without waiting for any other tasks to complete.</td>
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| 47 | <td align="center" valign="top">No</td>
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| 48 | </tr>
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| 49 | </table>
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| 50 |
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| 51 | <p>Parallel tasks have a number of uses in an Ant build file including:</p>
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| 52 | <ul>
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| 53 | <li>Taking advantage of available processing resources to reduce build time</li>
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| 54 | <li>Testing servers, where the server can be run in one thread and the test
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| 55 | harness is run in another thread.</li>
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| 56 | </ul>
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| 57 |
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| 58 | <p>Care must be taken when using multithreading to ensure the tasks within the
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| 59 | threads do not interact. For example, two javac compile tasks which write
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| 60 | classes into the same destination directory may interact where one tries to
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| 61 | read a class for dependency information while the other task is writing the
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| 62 | class file. Be sure to avoid these types of interactions within a
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| 63 | <code><parallel></code> task</p>
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| 64 |
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| 65 | <p>Any valid Ant task may be embedded within a
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| 66 | parallel task, including other parallel tasks.</p>
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| 67 |
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| 68 | <p>Note that while the tasks within the parallel task are being run, the main
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| 69 | thread will be blocked waiting for all the child threads to complete. If
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| 70 | execution is terminated by a timeout or a nested task failure when the failonany
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| 71 | flag is set, the parallel task will complete without waiting for other nested
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| 72 | tasks to complete in other threads.
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| 73 | </p>
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| 74 |
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| 75 | <p>If any of the tasks within the <code><parallel></code> task fails and failonany is
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| 76 | not set, the remaining tasks in other threads will continue to run until
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| 77 | all threads have completed. In this situation, the parallel task will also fail.</p>
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| 78 |
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| 79 | <p>The parallel task may be combined with the <a href="sequential.html">
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| 80 | sequential</a> task to define sequences of tasks to be executed on each thread
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| 81 | within the parallel block</p>
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| 82 |
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| 83 | <p>The threadCount attribute can be used to place a maximum number of available
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| 84 | threads for the execution. When not present all child tasks will be executed at
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| 85 | once. When present then the maximum number of concurrently executing tasks will
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| 86 | not exceed the number of threads specified. Furthermore, each task will be
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| 87 | started in the order they are given. But no guarantee is made as to the speed
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| 88 | of execution or the order of completion of the tasks, only that each will be
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| 89 | started before the next.<p>
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| 90 |
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| 91 | <p>If you are using J2RE 1.4 or later you can also use the threadsPerProcessor
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| 92 | and the number of available threads will be the stated multiple of the number of
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| 93 | processors (there is no affinity to a particular processor however). This will
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| 94 | override the value in threadCount. If threadsPerProcessor is specified using
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| 95 | any version prior to 1.4 then the value in threadCount will be used as is.</p>
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| 96 |
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| 97 | <p>When using threadCount and threadsPerProcessor care should be taken to ensure
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| 98 | that the build does not deadlock. This can be caused by tasks such as waitFor
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| 99 | taking up all available threads before the tasks that would unlock the waitfor
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| 100 | would occur. This is not a repalcement for Java Language level thread
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| 101 | semantics and is best used for "embarassingly parallel" tasks.</p>
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| 102 |
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| 103 |
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| 104 | <h3>Parameters specified as nested elements</h3>
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| 105 |
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| 106 | <h4>daemons</h4>
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| 107 | <p>
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| 108 | The parallel task supports a <code><daemons></code> nested element. This is a list of tasks
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| 109 | which are to be run in parallel daemon threads. The parallel task will not wait for
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| 110 | these tasks to complete. Being daemon threads, however, they will not prevent Ant from
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| 111 | completing, whereupon the threads are terminated. Failures in daemon threads which
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| 112 | occur before the parallel task itself finishes will be reported and can cause
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| 113 | parallel to throw an exception. Failures which occur after parallel has completed are not
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| 114 | reported.
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| 115 | </p>
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| 116 |
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| 117 | <p>Daemon tasks can be used, for example, to start test servers which might not be easily
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| 118 | terminated from Ant. By using <code><daemons></code> such servers do not halt the build.
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| 119 | </p>
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| 120 |
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| 121 |
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| 122 | <h3>Examples</h3>
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| 123 | <pre>
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| 124 | <parallel>
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| 125 | <wlrun ... >
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| 126 | <sequential>
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| 127 | <sleep seconds="30"/>
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| 128 | <junit ... >
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| 129 | <wlstop/>
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| 130 | </sequential>
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| 131 | </parallel>
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| 132 | </pre>
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| 133 | <p>This example represents a typical pattern for testing a server application.
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| 134 | In one thread the server is started (the wlrun task). The other thread consists
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| 135 | of a three tasks which are performed in sequence. The sleep task is used to
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| 136 | give the server time to come up. Another task which is capable of validating
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| 137 | that the server is available could be used in place of the sleep task. The
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| 138 | test harness is then run. Once the tests are complete, the server is stopped
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| 139 | (using wlstop in this example), allowing both threads to complete. The
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| 140 | parallel task will also complete at this time and the build will then
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| 141 | continue.</p>
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| 142 |
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| 143 | <pre>
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| 144 | <parallel>
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| 145 | <javac ...> <!-- compiler servlet code -->
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| 146 | <wljspc ...> <!-- precompile JSPs -->
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| 147 | </parallel>
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| 148 | </pre>
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| 149 |
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| 150 | <p>This example shows two independent tasks being run to achieve better
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| 151 | resource utilization during the build. In this instance, some servlets are being
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| 152 | compiled in one thead and a set of JSPs is being precompiled in another. As
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| 153 | noted above, you need to be careful that the two tasks are independent, both in
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| 154 | terms of their dependencies and in terms of their potential interactions in
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| 155 | Ant's external environment.</p>
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| 156 |
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| 157 | <pre>
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| 158 | <parallel threadCount='4'>
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| 159 | <ant target='TargetThatConsumesLotsOfCPUTimeAndMemory'>
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| 160 | <param name='file' value='one.txt'/>
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| 161 | </ant>
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| 162 | <ant target='TargetThatConsumesLotsOfCPUTimeAndMemory'>
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| 163 | <param name='file' value='two.txt'/>
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| 164 | </ant>
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| 165 | <ant target='TargetThatConsumesLotsOfCPUTimeAndMemory'>
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| 166 | <param name='file' value='three.txt'/>
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| 167 | </ant>
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| 168 | <!-- repeated about 40 times -->
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| 169 | </parallel>
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| 170 | </pre>
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| 171 |
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| 172 | <p>This example represents a typical need for use of the threadCount and
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| 173 | threadsPerProcessor attributes. Spinning up all 40 of those tasks could cripple
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| 174 | the JVM for memory and the CPU for available time. By limiting the number of
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| 175 | concurrent executions you can get the task done in about the same assuming
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| 176 | infinite memory time without needing infinite memory. This is also a good
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| 177 | candidiate for use of threadCount (and possibly threadsPerProcessor) because
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| 178 | each task (in this hypothetical case) is independent and has no dependencies on
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| 179 | the other tasks.</p>
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| 180 |
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| 181 | <hr>
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| 182 | <p align="center">Copyright © 2001-2004 The Apache Software Foundation. All rights
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| 183 | Reserved.</p>
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| 184 | </body>
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| 185 | </html>
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| 186 |
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