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<h2>Parallel</h2>
<h3>Description</h3>
<p>
    Executes nested tasks in parallel with no guarantees of thread safety.
    Every task will run in its own thread, with the likelihood of 
    concurrency problems scaling with the number of CPUs on the host system.
</p>
<p><b>Warning:</b> While the Apache Ant core is believed to be thread safe, no such 
    guarantees are made about tasks, which are not tested for thread safety during
    Ant's test process. 
    Third party tasks may or may not be thread safe, and some of Ant's core tasks, such as
    <code>&lt;javac&gt;</code> are definitely not re-entrant. This is because they use libraries that
    were never designed to be used in a multithreaded environment.
</p>
<p>
    The primary use case for <code>&lt;parallel&gt;</code> is to run external programs
    such as an application server, and the JUnit or TestNG test suites at the
    same time. Anyone trying to run large Ant task sequences in parallel, such
    as javadoc and javac at the same time, is implicitly taking on the task
    of identifying and fixing all concurrency bugs the tasks that they run.
    
</p>
<p>
    Accordingly, while this task has uses, it should be considered an advanced
    task which should be used in certain batch-processing or testing situations,
    rather than an easy trick to speed up build times on a multiway CPU. 
</p>

<h3>Parameters</h3>
<table border="1" cellpadding="2" cellspacing="0">
  <tr>
    <td valign="top"><b>Attribute</b></td>
    <td valign="top"><b>Description</b></td>
    <td align="center" valign="top"><b>Required</b></td>
  </tr>
  <tr>
    <td valign="top">threadCount</td>
    <td valign="top">Maximum numbers of thread to use.</td>
    <td align="center" valign="top">No</td>
  </tr>
  <tr>
    <td valign="top">threadsPerProcessor</td>
    <td valign="top">Maximum number of threads to use per available processor
(Java 1.4+)</td>
    <td align="center" valign="top">No, defers to threadCount</td>
  </tr>
  <tr>
    <td valign="top">timeout</td>
    <td valign="top">Number of milliseconds before execution is terminated</td>
    <td align="center" valign="top">No</td>
  </tr>
  <tr>
    <td valign="top">failonany</td>
    <td valign="top">If any of the nested tasks fails, execution of the task completes
                     at that point without waiting for any other tasks to complete.</td>
    <td align="center" valign="top">No, default is false.</td>
  </tr>
  <tr>
    <td valign="top">pollInterval</td>
    <td valign="top">Currently has no effect</td>
    <td align="center" valign="top">No, default is 1000</td>
  </tr>
</table>

<p>Parallel tasks have a number of uses in an Ant build file including:</p>
<ul>
<li>Taking advantage of available processing resources to execute external
 programs simultaneously.</li>
<li>Testing servers, where the server can be run in one thread and the test
harness is run in another thread.</li>
</ul>

<p>Any valid Ant task may be embedded within a
parallel task, including other parallel tasks, though there is no guarantee that
the tasks will be thread safe in such an environment.</p>

<p>While the tasks within the parallel task are being run, the main
thread will be blocked waiting for all the child threads to complete. If
execution is terminated by a timeout or a nested task failure when the
<code>failonany</code>
flag is set, the parallel task will complete without waiting for other nested
tasks to complete in other threads.
</p>

<p>If any of the tasks within the <code>&lt;parallel&gt;</code> task fails and failonany is
not set, the remaining tasks in other threads will continue to run until
all threads have completed. In this situation, the parallel task will also fail.</p>

<p>The parallel task may be combined with the <a href="sequential.html">
sequential</a> task to define sequences of tasks to be executed on each thread
within the parallel block</p>

<p>The <code>threadCount</code> attribute can be used to place a maximum number of available
threads for the execution.  When not present all child tasks will be executed at
once.  When present then the maximum number of concurrently executing tasks will
not exceed the number of threads specified.  Furthermore, each task will be
started in the order they are given.  But no guarantee is made as to the speed
of execution or the order of completion of the tasks, only that each will be
started before the next.<p>

<p>If you are using Java 1.4 or later you can also use the <code>threadsPerProcessor</code>
and the number of available threads will be the stated multiple of the number of
processors (there is no affinity to a particular processor however).  This will
override the value in <code>threadCount</code>.  If <code>threadsPerProcessor</code>
is specified on any older JVM, then the value in <code>threadCount</code> will be used as is.</p>

<p>When using <code>threadCount</code> and  <code>threadsPerProcessor</code> 
    care should be taken to ensure that the build does not deadlock.  
    This can be caused by tasks such as <code>waitfor</code>
    taking up all available threads before the tasks that would unlock the 
    <code>waitfor</code>
would occur.  This is not a replacement for Java Language level thread
semantics and is best used for "embarassingly parallel" tasks.</p>


<h3>Parameters specified as nested elements</h3>

<h4>daemons</h4>
<p>
The parallel task supports a <code>&lt;daemons&gt;</code> nested element. This is a list of tasks
which are to be run in parallel daemon threads. The parallel task will not wait for
these tasks to complete. Being daemon threads, however, they will not prevent Ant from
completing, whereupon the threads are terminated. Failures in daemon threads which
occur before the parallel task itself finishes will be reported and can cause
parallel to throw an exception. Failures which occur after parallel has completed are not
reported.
</p>

<p>Daemon tasks can be used, for example, to start test servers which might not be easily
terminated from Ant. By using <code>&lt;daemons&gt;</code> such servers do not halt the build.
</p>


<h3>Examples</h3>
<pre>
&lt;parallel&gt;
  &lt;wlrun ... &gt;
  &lt;sequential&gt;
    &lt;sleep seconds=&quot;30&quot;/&gt;
    &lt;junit fork="true" forkmode="once" ... &gt;
    &lt;wlstop/&gt;
  &lt;/sequential&gt;
&lt;/parallel&gt;
</pre>
<p>This example represents a typical pattern for testing a server application.
In one thread the server is started (the <code>&lt;wlrun&gt;</code> task). 
The other thread consists
of a three tasks which are performed in sequence. The <code>&lt;sleep&gt;</code> task is used to
give the server time to come up. Another task which is capable of validating
that the server is available could be used in place of the <code>&lt;sleep&gt;</code> task. The
<code>&lt;junit&gt;</code> test harness then runs, again in its own JVM. Once the tests are complete, the server is stopped
(using <code>&lt;wlstop&gt;</code> in this example), allowing both threads to complete. The
<code>&lt;parallel&gt;</code> task will also complete at this time and the build will then
continue.</p>

<pre>
&lt;parallel&gt;
  &lt;javac fork="true"...&gt; &lt;!-- compiler servlet code --&gt;
  &lt;wljspc ...&gt; &lt;!-- precompile JSPs --&gt;
&lt;/parallel&gt;
</pre>

<p>This example shows two independent tasks being run to achieve better
resource utilization during the build. In this instance, some servlets are being
compiled in one thead and a set of JSPs is being precompiled in another. Developers
need to be careful that the two tasks are independent, both in
terms of their dependencies and in terms of their potential interactions in
Ant's external environment. Here we set <code>fork="true"</code> for the 
<code>&lt;javac&gt;</code> task, so that it runs in a new process; 
if the <code>&lt;wljspc&gt;</code> task used the javac compiler in-VM
(it may), concurrency problems may arise.
</p>

<pre>
 &lt;macrodef name="dbpurge"&gt;
    &lt;attribute file="file"/&gt;
   &lt;sequential&gt;
      &lt;java jar="utils/dbpurge.jar" fork="true" &gt;
        &lt;arg file="@{file} /&gt;
      &lt;/java&gt;
   &lt;/sequential&gt;
&lt;/macrodef&gt;
    
&lt;parallel threadCount="4"&gt;
  &lt;dbpurge file="db/one" /&gt;
  &lt;dbpurge file="db/two" /&gt;
  &lt;dbpurge file="db/three" /&gt;
  &lt;dbpurge file="db/four" /&gt;
  &lt;dbpurge file="db/five" /&gt;
  &lt;dbpurge file="db/six" /&gt;
  &lt;dbpurge file="db/seven" /&gt;
  &lt;dbpurge file="db/eight" /&gt;
  &lt;!-- repeated about 40 times --&gt;
&lt;/parallel&gt;
</pre>

<p>This example represents a typical need for use of the threadCount and
threadsPerProcessor attributes.  Spinning up all 40 of those tasks could cripple
the system for memory and CPU time.  By limiting the number of
concurrent executions you can reduce contention for CPU, memory and disk IO, 
and so actually finish faster.  This is also a good
candidate for use of threadCount (and possibly threadsPerProcessor) because
each task is independent (every new JVM is forked) and has no dependencies on
the other tasks.</p>


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