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PODoubly nested parallel for loops and their span
 

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  1. PODoubly nested parallel for loops and their span
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    <p>I'm trying to wrap my head around the idea of how parallel-for-loops work, its all rather new to me, and had read this stackoverflow thread, found here <a href="https://stackoverflow.com/questions/5918477/span-and-parallel-loop">span and parallel loop</a>, and found the top answer there really helpful. </p> <p>What I'm wondering though is if I have a parallel-for-lop that has a parallel-for-lop inside it if they will behave in the following manner (for convenience I'll let n=5 and 'to' means up to and including (&lt;=)):</p> <p>Here is the code I have in my mind:</p> <pre><code>parallel for j=1 to n{ parallel for i=1 to j-1{ Do-something } } </code></pre> <p>And this is the behavior of the threads I'm leaning towards, but would like someone to confirm if correct:</p> <pre><code>Time Action 0 Thread0 creates Thread1 1 Thread0 creates Thread2, T1 -&gt; T3 2 T0 -&gt; T4, T1 -&gt; T5 </code></pre> <p>Ok here all the threads for the outer look have been created. And now for the inner. Here subXY means thread number y of thread x:</p> <pre><code>time Action 3 T0 -&gt; SubT0t0, T1 -&gt; subT1t0, T2-&gt; SubT2t0.... //All threads have created a subthread t0 so 5 more threads created. 4 T0 -&gt; subT1t1, subT0t0 -&gt; subT2t1, T1-&gt;subT2t2, subT1t0-&gt;subT3t1... //All the subthreads created more threads AND the 'Older' threads //created subthreads under other 'Older' threads. 11 more created. 5 T0 -&gt; subT5t2, subT0t0 -&gt; subT5t3, T1 -&gt; subT5t4, subT5t5 //All threads have been created. 6 Execute command Do-something </code></pre> <p>I know this might not be the best description of what I'm thinking, but its the best I can come up with.</p> <p>Is this correct? Or would a outer loop thread only make a thread connected to him self? For example T1 would only make subT1XX threads?</p> <p>Also would the threads that do nothing at time 5 execute their commands during that time?</p> <p>It would help me immensely if someone could help me. </p>
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    1. USHrafn
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    1. CODid you have to complicate matters my using the j index as the termination limit of the inside loop? As if this kind of construct is not difficult enough as it is.. :)
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      1. USMartin James
    2. COIt's a thread/task storm. Time is broken here!
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      1. USMartin James
    3. COI'm sorry Martin, not sure what you mean by broken time?
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