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Merge pull request #5 from charmplusplus/ExerciseUpdates
Exercise updates
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‎content/exercises.html

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@@ -41,7 +41,7 @@ <h2>!! Under Construction, the following is currently being updated and will inc
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</ol>
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<li><h2>Load Balancing</h2></li>
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<ol>
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<li>Particle Exercise with multiple types of particles</li>
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<li><a href="loadbalancing">Particle Exercise with multiple types of particles</a></li>
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<ol>
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<li>Add load balancing</li>
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<li>Add projections and show impact of load balancing via time-profile and timeline</li>

‎content/exercises/images/particlescode.png

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‎content/exercises/images/particlescode_lb.png

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‎content/exercises/loadbalancing.html

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---
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title: Load Balancing Particle Exercise (Extension)
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homec: home
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tutorialc: tutorial
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applicationsc: applications
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miniAppsc: miniApps
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downloadc: download
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toolsc: tools
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helpc: help
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---
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<link rel="stylesheet" type="text/css" href="../../tutorial/TutorialStyle.css">
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<h1>Load Balancing: Particle Exercise <em>(Extension)</em></h1>
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<table class="StandardFigure" align="right" border="0">
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<tr><td align="center" valign="middle">
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<a href="../images/particlescode_lb.png">
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<img class="StandardFigure" src="../images/particlescode_lb.png" border="0" width="375">
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</a>
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</td></tr>
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<tr><td align="center" valign="middle"><b>Figure&nbsp;1</b></td></tr>
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</table>
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<h2>Part&nbsp;1 – Introducing Imbalance</h2>
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<p>The very first task is to create measurable imbalance so that load balancers have something meaningful to fix.</p>
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<ol>
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<li>
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Let <code>m&nbsp;×&nbsp;m</code> be the chare-array dimensions and <code>k</code> the baseline number of particles per chare. Update the number of particles per chare to be
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<span lang="latex">N_{x,y} = k + \frac{k(x + y)}{m}</span>.
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</li>
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<li>
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Build with tracing enabled using the <code>-tracemode projections</code> flag.
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</li>
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<li>
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Install <a href="https://github.com/charmplusplus/projections">projections</a>, and focus on the time profile view to see how the imbalance affects performance and efficiency.
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</li>
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</ol>
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<p><u>Variants</u>: Instructors may choose to use a different formula for introducing imbalance</p>
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<!-- =================================================================== -->
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<h2>Part&nbsp;2 – Adding Migration Support &amp; Load-Balancing</h2>
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<p>Now enable dynamic migration so Charm++ balancers can actually move work around.</p>
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<ol>
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<li>
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Implement <code>PUP</code> serialization for your chare array as <a href="https://charm.readthedocs.io/en/latest/charm++/manual.html#arraymigratable">documented</a>.
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</li>
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<li>
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At appropriate intervals, call <code><a href="https://charm.readthedocs.io/en/latest/charm++/manual.html#load-balancing-chare-arrays">AtSync()</a></code> so the runtime knows the chare is ready for potential migration.
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</li>
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<li>
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Select a load-balancing strategy. For this exercise we will use
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<code>GreedyRefineLB</code> (use option <code>-balancer GreedyRefineLB</code>).
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</li>
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<li>
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Re-run the application as in Part&nbsp;1 and compare how the program behaves differently.
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</li>
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</ol>
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<h2>Part&nbsp;3 – Visualising with LiveViz</h2>
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<p>Add colour to emphasise the skew and watch the balancer work.</p>
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<ol>
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<li>
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Extend each chare to contain particles of three different colors. Each color should move at different speeds - green should move at the default speed, red should move twice as fast and green should move half as fast.
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</li>
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<li>
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Distribute colours to reproduce the imbalance as shown in the figure: reds bunched in the centre, greens in the upper-left, blues in the lower-right.
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</li>
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<li>
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Compile with <a href="https://github.com/UIUC-PPL/ccs_tools">LiveViz</a> using the <code>-module liveViz</code> flag.
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</li>
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<li>
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Run your C++ code using <code>++server</code> and <code>++server-port 1234</code>, and then run the liveViz client using <code>liveVize localhost 1234</code>, and observe how the particles behave.
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</li>
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</ol>

‎content/exercises/particle.html

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<li>The pseudocode for the overall algorithm is:</li>
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<code>for(iteration=0; iteration&lt;ITERATION; iteration++){</code>
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<ul>
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<li>For each of the particles that belong to my chare: change its <i>x</i> and <i>y</i> coordinate by a small random amount.</li>
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<li>Move all the particles that do not belong to a chare's bounding box to their correct homes. Since the movement is small, this will mean communication to the eight near neighbor chares. Some of these messages may contain no particles.</li>
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<li>For each of the particles that belong to my chare: change its <i>x</i> and <i>y</i> coordinate by a small random amount.
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If this change causes the particle to move to an invalid coordinate, keep the particle in its original position.</li>
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<li>Move all the particles that do not belong to a chare's bounding box to their correct homes.
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Since the movement is small, this will mean communication to the eight near neighbor chares. Some of these messages may contain no particles.
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For both efficiency and ease of implementation, all communication with neighbor chares should happen at the end of an iteration rather than after each particle.</li>
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<li><code>if(iteration%10 == 0)</code></li>
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<ul>
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<li>Do reductions to calculate average and max number of particles</li>
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</ul>
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</ul>
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<code>}</code>
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</ul>
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<p><u>Instructor's Note</u>: This exercise can be implemented in several ways, feel free to choose from the variations below or create your own based on your learning objectives:</p>
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<ul>
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<li>Choose to move the particle along either the <i>x</i> <i>or</i> <i>y</i> axis using an additional random variable, which simplifies communication as there are now only four neighbor chares.</li>
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<li>If a movement causes a particle to move to an invalid coordinate, move in the opposite direction instead.</li>
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</ul>

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