2001-08-09 10:48:12 +00:00
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<meta http-equiv="Content-Language" content="en-us">
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2002-02-03 22:00:42 +00:00
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<title>Parallel Task</title>
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2001-08-09 10:48:12 +00:00
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</head>
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<body>
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<h2>Parallel</h2>
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<h3>Description</h3>
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<p>Parallel is a container task - it can contain other Ant tasks. Each nested
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task within the parallel task will be executed in its own thread. </p>
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2003-02-19 05:11:34 +00:00
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<h3>Parameters</h3>
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<table border="1" cellpadding="2" cellspacing="0">
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<tr>
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<td valign="top"><b>Attribute</b></td>
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<td valign="top"><b>Description</b></td>
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<td align="center" valign="top"><b>Required</b></td>
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</tr>
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<tr>
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<td valign="top">threadCount</td>
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<td valign="top">Maximum numbers of thread to use.</td>
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<td align="center" valign="top">No</td>
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</tr>
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<tr>
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<td valign="top">threadsPerProcessor</td>
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<td valign="top">Maximum number of threads to use per available processor
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(Requires JDK 1.4)</td>
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<td align="center" valign="top">No, defers to threadCount</td>
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</tr>
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<tr>
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<td valign="top">pollInterval</td>
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<td valign="top">Maximum number of milliseconds to wait for before checking
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when waiting for available threads.</td>
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<td align="center" valign="top">No, default is 1000</td>
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</tr>
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</table>
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2001-08-09 10:48:12 +00:00
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2001-09-08 01:05:18 +00:00
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<p>Parallel tasks have a number of uses in an Ant build file including:</p>
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<ul>
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<li>Taking advantage of available processing resources to reduce build time</li>
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<li>Testing servers, where the server can be run in one thread and the test
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harness is run in another thread.</li>
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</ul>
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<p>Care must be taken when using multithreading to ensure the tasks within the
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threads do not interact. For example, two javac compile tasks which write
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classes into the same destination directory may interact where one tries to
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2001-08-30 13:23:14 +00:00
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read a class for dependency information while the other task is writing the
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2001-08-09 10:48:12 +00:00
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class file. Be sure to avoid these types of interactions within a
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<parallel> task</p>
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<p>The parallel task has no attributes and does not support any nested
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elements apart from Ant tasks. Any valid Ant task may be embedded within a
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parallel task, including other parallel tasks.</p>
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<p>Note that while the tasks within the parallel task are being run, the main
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thread will be blocked waiting for all the child threads to complete.</p>
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<p>If any of the tasks within the <parallel> task fails, the remaining
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tasks in other threads will continue to run until all threads have completed.
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In this situation, the parallel task will also fail.</p>
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2001-08-09 11:03:39 +00:00
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<p>The parallel task may be combined with the <a href="sequential.html">
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sequential</a> task to define sequences of tasks to be executed on each thread
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within the parallel block</p>
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2003-02-19 05:11:34 +00:00
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<p>The threadCount attribute can be used to place a maximum number of available
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threads for the execution. When not present all child tasks will be executed at
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once. When present then the maximum number of concurrently executing tasks will
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not exceed the number of threads specified. Furthermore, each task will be
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started in the order they are given. But no guarantee is made as to the speed
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of execution or the order of completion of the tasks, only that each will be
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started before the next.<p>
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<p>If you are using J2RE 1.4 or later you can also use the threadsPerProcessor
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and the number of available threads will be the stated multiple of the number of
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processors (there is no affinity to a particular processor however). This will
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override the value in threadCount. If threadsPerProcessor is specified using
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any version prior to 1.4 then the value in threadCount will be used as is.</p>
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<p>When using threadCount and threadsPerProcessor care should be taken to insure
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that the build does not deadlock. This can be caused by tasks such as waitFor
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takeing up all available threads before the tasks that would unlock the waitfor
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would occur. This is not a repalcement for Java Language level thread
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semantics and is best used for "embarasingly parallel" tasks.</p>
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2001-08-09 10:48:12 +00:00
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<h3>Examples</h3>
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<pre>
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<parallel>
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2002-09-04 11:05:19 +00:00
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<wlrun ... >
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<sequential>
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<sleep seconds="30"/>
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2002-09-04 11:05:19 +00:00
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<junit ... >
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<wlstop/>
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</sequential>
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</parallel>
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</pre>
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<p>This example represents a typical pattern for testing a server application.
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In one thread the server is started (the wlrun task). The other thread consists
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of a three tasks which are performed in sequence. The sleep task is used to
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give the server time to come up. Another task which is capable of validating
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that the server is available could be used in place of the sleep task. The
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test harness is then run. Once the tests are complete, the server is stopped
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(using wlstop in this example), allowing both threads to complete. The
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parallel task will also complete at this time and the build will then
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continue.</p>
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<pre>
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<parallel>
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<javac ...> <!-- compiler servlet code -->
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<wljspc ...> <!-- precompile JSPs -->
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</parallel>
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</pre>
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<p>This example shows two independent tasks being run to achieve better
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resource utilization during the build. In this instance, some servlets are being
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compiled in one thead and a set of JSPs is being precompiled in another. As
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noted above, you need to be careful that the two tasks are independent, both in
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terms of their dependencies and in terms of their potential interactions in
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Ant's external environment.</p>
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2003-02-19 05:11:34 +00:00
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<pre>
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<parallel threadCount='4'>
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<ant target='TargetThatConsumesLotsOfCPUTimeAndMemory'>
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<param name='file' value='one.txt'/>
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</ant>
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<ant target='TargetThatConsumesLotsOfCPUTimeAndMemory'>
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<param name='file' value='two.txt'/>
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</ant>
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<ant target='TargetThatConsumesLotsOfCPUTimeAndMemory'>
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<param name='file' value='three.txt'/>
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</ant>
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<!-- repeated about 40 times -->
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</parallel>
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</pre>
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<p>This example represents a typical need for use of the threadCount and
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threadsPerProcessor attributes. Spinning up all 40 of those tasks could cripple
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the JVM for memory and the CPU for available time. By limiting the number of
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concurrent executions you can get the task done in about the same assuming
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infinite memory time without needing infinite memory. This is also a good
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candidiate for use of threadCount (and possibly threadsPerProcessor) because
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each task (in this hypothetical case) is independent and has no dependencies on
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the other tasks.</p>
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2001-08-09 10:48:12 +00:00
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<hr>
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2003-02-19 07:34:48 +00:00
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<p align="center">Copyright © 2001-2003 Apache Software Foundation. All rights
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Reserved.</p>
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</body>
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</html>
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