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<div class="title">C3: Create a Parallel For-loop Graph </div> </div>
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<div class="textblock"><p>Running a for-loop in parallel is the most fundamental building block in parallel programming. In this chapter, we are going to demonstrate how to use Cpp-Taskflow to create a task dependency graph of parallel for-loop.</p>
<h1><a class="anchor" id="RangeBasedForLoop"></a>
Range-based For-loop</h1>
<p>Cpp-Taskflow has a STL-style method <a class="el" href="classtf_1_1FlowBuilder.html#a84d411f16939bc158c5a90256e5d210e" title="constructs a task dependency graph of range-based parallel_for ">tf::Taskflow::parallel_for(I beg, I end, C&& callable, size_t chunk)</a> that takes a range of items and applies a callable to each of the item in parallel. The method constructs a task dependency graph representing this workload and returns a task pair as two synchronization points to this task graph.</p>
<div class="fragment"><div class="line"> 1: <a class="code" href="classtf_1_1BasicTaskflow.html">tf::Taskflow</a> taskflow;</div><div class="line"> 2:</div><div class="line"> 3: <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/container/vector.html">std::vector<int></a> items {1, 2, 3, 4, 5, 6, 7, 8};</div><div class="line"> 4:</div><div class="line"> 5: <span class="keyword">auto</span> [S, T] = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a84d411f16939bc158c5a90256e5d210e">parallel_for</a>(items.begin(), items.end(), [] (<span class="keywordtype">int</span> item) {</div><div class="line"> 6: <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> << <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/thread/get_id.html">std::this_thread::get_id</a>() << <span class="stringliteral">" runs "</span> << item << <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/manip/endl.html">std::endl</a>;</div><div class="line"> 7: });</div><div class="line"> 8:</div><div class="line"> 9: S.work([] () { <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> << <span class="stringliteral">"S\n"</span>; }).name(<span class="stringliteral">"S"</span>);</div><div class="line">10: T.work([] () { <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> << <span class="stringliteral">"T\n"</span>; }).name(<span class="stringliteral">"T"</span>);</div><div class="line">11:</div><div class="line">12: <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> << taskflow.<a class="code" href="classtf_1_1BasicTaskflow.html#adac448e1cc44307856b3116d7ed5490f">dump</a>();</div><div class="line">13:</div><div class="line">14: taskflow.<a class="code" href="classtf_1_1BasicTaskflow.html#a37ef86998f23ee7315be032c40fe815e">wait_for_all</a>();</div></div><!-- fragment --><p>The above code generates the following task dependency graph. The label 0x56* represents an internal task node to execute the callable object. By default (<code>chunk=0</code>), Cpp-Taskflow evenly partitions and distributes the workload to all threads. In our example of eight tasks and four workers, each internal node is responsible for two items.</p>
<div class="image">
<img src="parallel_for1.png" alt="parallel_for1.png" width="60%"/>
</div>
<p>Debrief: </p><ul>
<li>Line 1 creates a taskflow object of four worker threads </li>
<li>Line 3 creates a vector container of eight items </li>
<li>Line 5-7 creates a parallel execution graph using the method <code>parallel_for</code> </li>
<li>Line 9-10 names the synchronization tasks <code>S</code> and <code>T</code> </li>
<li>Line 12 dumps the graph to a dot format which can be visualized through <a href="https://dreampuf.github.io/GraphvizOnline/">GraphViz Online</a> </li>
<li>Line 14 dispatches the graph to execution and blocks until the graph finishes</li>
</ul>
<p>Here is one possible output of this program:</p>
<div class="fragment"><div class="line">S</div><div class="line">139931471636224 runs 1</div><div class="line">139931471636224 runs 2</div><div class="line">139931480028928 runs 7</div><div class="line">139931480028928 runs 8</div><div class="line">139931496814336 runs 3</div><div class="line">139931496814336 runs 4</div><div class="line">139931488421632 runs 5</div><div class="line">139931488421632 runs 6</div><div class="line">T</div></div><!-- fragment --><h1><a class="anchor" id="PartitionTheWorkloadExplicitly"></a>
Partition the Workload Explicitly</h1>
<p>By default, Cpp-Taskflow partitions the workload evenly across the workers. In some cases, it is useful to disable this feature and apply user-specified partition. The method <code>parallel_for</code> tasks an unsigned integer <code>chunk</code> as the number of items in each partition.</p>
<div class="fragment"><div class="line"><span class="keyword">auto</span> [S, T] = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a84d411f16939bc158c5a90256e5d210e">parallel_for</a>(items.begin(), items.end(), [] (<span class="keywordtype">int</span> item) {</div><div class="line"> <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> << <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/thread/get_id.html">std::this_thread::get_id</a>() << <span class="stringliteral">" runs "</span> << item << <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/manip/endl.html">std::endl</a>;</div><div class="line">}, 1);</div></div><!-- fragment --><p>The above example will force each partition to run exactly one item. This can be useful when you have unbalanced workload and would like to enable more efficient parallelization.</p>
<div class="image">
<img src="parallel_for2.png" alt="parallel_for2.png" width="100%"/>
</div>
<p>You can leave the chunk size to 0 to use our default partition strategy.</p>
<h1><a class="anchor" id="ConstructTheGraphExplicitly"></a>
Construct the Graph Explicitly</h1>
<p>You can explicitly construct a dependency graph that represents a parallel execution of a for-loop using only the basic methods <a class="el" href="classtf_1_1FlowBuilder.html#a4d52a7fe2814b264846a2085e931652c" title="creates a task from a given callable object ">tf::Taskflow::emplace</a> and <a class="el" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588" title="adds precedence links from this to other tasks ">tf::Task::precede</a>.</p>
<div class="fragment"><div class="line"><a class="code" href="classtf_1_1Task.html">tf::Task</a> S = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a4d52a7fe2814b264846a2085e931652c">emplace</a>([] () {}).name(<span class="stringliteral">"S"</span>);</div><div class="line"><a class="code" href="classtf_1_1Task.html">tf::Task</a> T = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a4d52a7fe2814b264846a2085e931652c">emplace</a>([] () {}).name(<span class="stringliteral">"T"</span>);</div><div class="line"></div><div class="line"><span class="keywordflow">for</span>(<span class="keyword">auto</span> item : items) {</div><div class="line"> <a class="code" href="classtf_1_1Task.html">tf::Task</a> task = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a4d52a7fe2814b264846a2085e931652c">emplace</a>([item] () {</div><div class="line"> <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> << <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/thread/get_id.html">std::this_thread::get_id</a>() << <span class="stringliteral">" runs "</span> << item << <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/manip/endl.html">std::endl</a>;</div><div class="line"> });</div><div class="line"> S.<a class="code" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(task);</div><div class="line"> task.<a class="code" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(T);</div><div class="line">}</div></div><!-- fragment --><h1><a class="anchor" id="IndexBasedForLoop"></a>
Index-based For-loop</h1>
<p>Cpp-Taskflow provides an overload <a class="el" href="classtf_1_1FlowBuilder.html#abe18f8824cba16cde03e0dea8fae14ac" title="constructs a task dependency graph of index-based parallel_for ">tf::Taskflow::parallel_for(I beg, I end, I step, C&& callable, size_t chunk)</a> to parallelize an index-based for-loop. It takes three numbers <code>beg</code>, <code>end</code>, and <code>step</code> of the same type <code>I</code> and applies <code>callable</code> to each index in the range <code>[beg, end)</code> with the step size <code>step</code>.</p>
<div class="fragment"><div class="line">1: taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a84d411f16939bc158c5a90256e5d210e">parallel_for</a>(0, 10, 2, [] (<span class="keywordtype">int</span> i) {</div><div class="line">2: <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> << <span class="stringliteral">"parallel on "</span> << i << <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/manip/endl.html">std::endl</a>;</div><div class="line">3: });</div><div class="line">4: <span class="comment">// print 0, 2, 4, 6, 8</span></div></div><!-- fragment --><p>It also works on the opposite order with negative step size.</p>
<div class="fragment"><div class="line">1: taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a84d411f16939bc158c5a90256e5d210e">parallel_for</a>(10, 0, -2, [] (<span class="keywordtype">int</span> i) {</div><div class="line">2: <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> << <span class="stringliteral">"parallel on "</span> << i << <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/manip/endl.html">std::endl</a>;</div><div class="line">3: });</div><div class="line">4: <span class="comment">// print 10, 8, 6, 4, 2</span></div></div><!-- fragment --><p>Similarly, you can specify a chunk size to group the works per thread.</p>
<div class="fragment"><div class="line">1: taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a84d411f16939bc158c5a90256e5d210e">parallel_for</a>(0, 10, 2, [] (<span class="keywordtype">int</span> i) {</div><div class="line">2: <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> << <span class="stringliteral">"parallel on "</span> << i << <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/manip/endl.html">std::endl</a>;</div><div class="line">3: }, 2);</div><div class="line">4: <span class="comment">// print 0, 2, 4, 6, 8 (three groups {0, 2}, {4, 6}, {8})</span></div></div><!-- fragment --><p>By default, Cpp-Taskflow performs even partition across the number of available threads if no group size is given.</p>
<h1><a class="anchor" id="Chapter3Example1"></a>
Example 1: Parallel Map</h1>
<p>This example demonstrates how to use <a class="el" href="classtf_1_1FlowBuilder.html#a84d411f16939bc158c5a90256e5d210e" title="constructs a task dependency graph of range-based parallel_for ">tf::Taskflow::parallel_for</a> to create a parallel map pattern. The map operator modifies each item in the container to one if it is an odd number, or zero if it is an even number.</p>
<div class="fragment"><div class="line"><span class="preprocessor">#include <taskflow/taskflow.hpp></span></div><div class="line"></div><div class="line"><span class="keywordtype">int</span> main() {</div><div class="line"></div><div class="line"> <a class="code" href="classtf_1_1BasicTaskflow.html">tf::Taskflow</a> taskflow;</div><div class="line"></div><div class="line"> <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/container/vector.html">std::vector<int></a> items{1, 2, 3, 4, 5, 6, 7, 8};</div><div class="line"></div><div class="line"> taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a84d411f16939bc158c5a90256e5d210e">parallel_for</a>(items.begin(), items.end(), [] (<span class="keywordtype">int</span>& item) {</div><div class="line"> item = (item & 1) ? 1 : 0;</div><div class="line"> });</div><div class="line"></div><div class="line"> taskflow.<a class="code" href="classtf_1_1BasicTaskflow.html#a37ef86998f23ee7315be032c40fe815e">wait_for_all</a>();</div><div class="line"></div><div class="line"> <span class="keywordflow">for</span>(<span class="keyword">auto</span> item : items) {</div><div class="line"> <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> << item << <span class="stringliteral">" "</span>;</div><div class="line"> }</div><div class="line"></div><div class="line"> <span class="keywordflow">return</span> 0;</div><div class="line">}</div></div><!-- fragment --><p>The program outputs the following:</p>
<div class="fragment"><div class="line">1 0 1 0 1 0 1 0 </div></div><!-- fragment --><h1><a class="anchor" id="Chapter3Example2"></a>
Example 2: Pipeline a Parallel For-loop</h1>
<p>This example demonstrates how to pipeline a parallel-for workload with other tasks.</p>
<div class="fragment"><div class="line"> 1: #include <taskflow/taskflow.hpp></div><div class="line"> 2:</div><div class="line"> 3: <span class="keywordtype">int</span> main() {</div><div class="line"> 4:</div><div class="line"> 5: <a class="code" href="classtf_1_1BasicTaskflow.html">tf::Taskflow</a> taskflow;</div><div class="line"> 6:</div><div class="line"> 7: <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/container/vector.html">std::vector<int></a> items(1024);</div><div class="line"> 8: <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/atomic/atomic.html">std::atomic<int></a> sum {0};</div><div class="line"> 9:</div><div class="line">10: <a class="code" href="classtf_1_1Task.html">tf::Task</a> T1 = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a4d52a7fe2814b264846a2085e931652c">emplace</a>([&] () { <span class="comment">// create a modifier task</span></div><div class="line">11: <span class="keywordflow">for</span>(<span class="keyword">auto</span>& item : items) {</div><div class="line">12: item = 1;</div><div class="line">13: }</div><div class="line">14: }).name(<span class="stringliteral">"Create Items"</span>);</div><div class="line">15:</div><div class="line">16: <span class="keyword">auto</span> [S, T] = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a84d411f16939bc158c5a90256e5d210e">parallel_for</a>(items.begin(), items.end(), [&] (<span class="keywordtype">int</span> item) {</div><div class="line">17: sum.fetch_add(item, std::memory_order_relaxed);</div><div class="line">18: }, 128);</div><div class="line">19:</div><div class="line">20: <a class="code" href="classtf_1_1Task.html">tf::Task</a> T2 = taskflow.<a class="code" href="classtf_1_1FlowBuilder.html#a4d52a7fe2814b264846a2085e931652c">emplace</a>([&] () { <span class="comment">// create a output task</span></div><div class="line">21: <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/basic_ostream.html">std::cout</a> << <span class="stringliteral">"sum is: "</span> << sum << <a class="codeRef" doxygen="/home/twhuang/PhD/Code/cpp-taskflow/docs/cppreference-doxygen-web.tag.xml:http://en.cppreference.com/w/" href="http://en.cppreference.com/w/cpp/io/manip/endl.html">std::endl</a>;</div><div class="line">22: }).name(<span class="stringliteral">"Print Sum"</span>);</div><div class="line">23:</div><div class="line">24: T1.<a class="code" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(S); <span class="comment">// modifier precedes parallel-for</span></div><div class="line">25: T.<a class="code" href="classtf_1_1Task.html#a8c78c453295a553c1c016e4062da8588">precede</a>(T2); <span class="comment">// parallel-for precedes the output task</span></div><div class="line">26:</div><div class="line">27: taskflow.<a class="code" href="classtf_1_1BasicTaskflow.html#a37ef86998f23ee7315be032c40fe815e">wait_for_all</a>();</div><div class="line">28:</div><div class="line">29: <span class="keywordflow">return</span> 0;</div><div class="line">30: }</div></div><!-- fragment --><div class="image">
<img src="parallel_for_example2.png" alt="parallel_for_example2.png" width="100%"/>
</div>
<p>The output of this programs is:</p>
<div class="fragment"><div class="line">sum is: 1024</div></div><!-- fragment --><p>Debrief: </p><ul>
<li>Line 5 creates a taskflow object </li>
<li>Line 7 creates a vector of 1024 uninitialized integers </li>
<li>Line 8 creates an atomic integer variable </li>
<li>Line 10-14 creates a task that captures the vector to initialize all items to one </li>
<li>Line 16-18 sums up all items with each thread running on a partition of 128 items (total 1024/128=8 partitions) </li>
<li>Line 20-22 creates a task that outputs the summation value </li>
<li>Line 24-25 pipelines the parallel-for workload with the two tasks </li>
<li>Line 27 dispatches the graph to threads and blocks until the execution completes </li>
</ul>
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