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<title>CUDA Standard Algorithms &raquo; Parallel Find | Taskflow QuickStart</title>
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<h1>
<span class="m-breadcrumb"><a href="cudaStandardAlgorithms.html">CUDA Standard Algorithms</a> &raquo;</span>
Parallel Find
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<h3>Contents</h3>
<ul>
<li><a href="#CUDASTDFindIncludeTheHeader">Include the Header</a></li>
<li><a href="#CUDASTDFindItems">Find an Element in a Range</a></li>
<li><a href="#CUDASTDFindMinItems">Find the Minimum Element in a Range</a></li>
<li><a href="#CUDASTDFindMaxItems">Find the Maximum Element in a Range</a></li>
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<p>Taskflow provides standalone template methods for finding elements in the given ranges using GPU.</p><section id="CUDASTDFindIncludeTheHeader"><h2><a href="#CUDASTDFindIncludeTheHeader">Include the Header</a></h2><p>You need to include the header file, <code>taskflow/cuda/algorithm/find.hpp</code>, for using the parallel-find algorithm.</p><pre class="m-code"><span class="cp">#include</span><span class="w"> </span><span class="cpf">&lt;taskflow/cuda/algorithm/find.hpp&gt;</span></pre></section><section id="CUDASTDFindItems"><h2><a href="#CUDASTDFindItems">Find an Element in a Range</a></h2><p><a href="namespacetf.html#a5f9dabd7c5d0fa5166cf76d9fa5a038e" class="m-doc">tf::<wbr />cuda_find_if</a> finds the index of the first element in the range <code>[first, last)</code> that satisfies the given criteria. This is equivalent to the parallel execution of the following loop:</p><pre class="m-code"><span class="kt">unsigned</span><span class="w"> </span><span class="n">idx</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="k">for</span><span class="p">(;</span><span class="w"> </span><span class="n">first</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n">last</span><span class="p">;</span><span class="w"> </span><span class="o">++</span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="o">++</span><span class="n">idx</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">p</span><span class="p">(</span><span class="o">*</span><span class="n">first</span><span class="p">))</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">idx</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
<span class="k">return</span><span class="w"> </span><span class="n">idx</span><span class="p">;</span></pre><p>If no such an element is found, the size of the range is returned. The following code finds the index of the first element that is dividable by <code>17</code> over a range of one million elements.</p><pre class="m-code"><span class="k">const</span><span class="w"> </span><span class="kt">size_t</span><span class="w"> </span><span class="n">N</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1000000</span><span class="p">;</span>
<span class="k">auto</span><span class="w"> </span><span class="n">vec</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">cuda_malloc_shared</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="p">(</span><span class="n">N</span><span class="p">);</span><span class="w"> </span><span class="c1">// vector</span>
<span class="k">auto</span><span class="w"> </span><span class="n">idx</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">cuda_malloc_shared</span><span class="o">&lt;</span><span class="kt">unsigned</span><span class="o">&gt;</span><span class="p">(</span><span class="mi">1</span><span class="p">);</span><span class="w"> </span><span class="c1">// index</span>
<span class="c1">// initializes the data</span>
<span class="k">for</span><span class="p">(</span><span class="kt">size_t</span><span class="w"> </span><span class="n">i</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">&lt;</span><span class="n">N</span><span class="p">;</span><span class="w"> </span><span class="n">vec</span><span class="p">[</span><span class="n">i</span><span class="o">++</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">rand</span><span class="p">());</span>
<span class="c1">// create an execution policy</span>
<span class="n">tf</span><span class="o">::</span><span class="n">cudaDefaultExecutionPolicy</span><span class="w"> </span><span class="n">policy</span><span class="p">;</span>
<span class="c1">// finds the index of the first element that is a multiple of 17</span>
<span class="n">tf</span><span class="o">::</span><span class="n">cuda_find_if</span><span class="p">(</span>
<span class="w"> </span><span class="n">policy</span><span class="p">,</span><span class="w"> </span><span class="n">vec</span><span class="p">,</span><span class="w"> </span><span class="n">vec</span><span class="o">+</span><span class="n">N</span><span class="p">,</span><span class="w"> </span><span class="n">idx</span><span class="p">,</span><span class="w"> </span><span class="p">[]</span><span class="w"> </span><span class="n">__device__</span><span class="w"> </span><span class="p">(</span><span class="k">auto</span><span class="w"> </span><span class="n">v</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">v</span><span class="o">%</span><span class="mi">17</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="p">}</span>
<span class="p">);</span>
<span class="c1">// wait for the find operation to complete</span>
<span class="n">stream</span><span class="p">.</span><span class="n">synchronize</span><span class="p">();</span>
<span class="c1">// verifies the result</span>
<span class="k">if</span><span class="p">(</span><span class="o">*</span><span class="n">idx</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n">N</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">assert</span><span class="p">(</span><span class="n">vec</span><span class="p">[</span><span class="o">*</span><span class="n">idx</span><span class="p">]</span><span class="w"> </span><span class="o">%</span><span class="mi">17</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="mi">0</span><span class="p">);</span>
<span class="p">}</span>
<span class="c1">// deletes the memory</span>
<span class="n">cudaFree</span><span class="p">(</span><span class="n">vec</span><span class="p">);</span>
<span class="n">cudaFree</span><span class="p">(</span><span class="n">idx</span><span class="p">);</span></pre><p>The find-if algorithm runs <em>asynchronously</em> through the stream specified in the execution policy. You need to synchronize the stream to obtain the correct result.</p></section><section id="CUDASTDFindMinItems"><h2><a href="#CUDASTDFindMinItems">Find the Minimum Element in a Range</a></h2><p><a href="namespacetf.html#a572c13198191c46765264f8afabe2e9f" class="m-doc">tf::<wbr />cuda_min_element</a> finds the index of the minimum element in the given range <code>[first, last)</code> using the given comparison function object. This is equivalent to a parallel execution of the following loop:</p><pre class="m-code"><span class="k">if</span><span class="p">(</span><span class="n">first</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">last</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">auto</span><span class="w"> </span><span class="n">smallest</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">first</span><span class="p">;</span>
<span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="o">++</span><span class="n">first</span><span class="p">;</span><span class="w"> </span><span class="n">first</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n">last</span><span class="p">;</span><span class="w"> </span><span class="o">++</span><span class="n">first</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">op</span><span class="p">(</span><span class="o">*</span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="o">*</span><span class="n">smallest</span><span class="p">))</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">smallest</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">first</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
<span class="k">return</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">distance</span><span class="p">(</span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="n">smallest</span><span class="p">);</span></pre><p>The following code finds the index of the minimum element in a range of one millions elements using GPU computing:</p><pre class="m-code"><span class="k">const</span><span class="w"> </span><span class="kt">size_t</span><span class="w"> </span><span class="n">N</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1000000</span><span class="p">;</span>
<span class="k">auto</span><span class="w"> </span><span class="n">vec</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">cuda_malloc_shared</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="p">(</span><span class="n">N</span><span class="p">);</span><span class="w"> </span><span class="c1">// vector</span>
<span class="k">auto</span><span class="w"> </span><span class="n">idx</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">cuda_malloc_shared</span><span class="o">&lt;</span><span class="kt">unsigned</span><span class="o">&gt;</span><span class="p">(</span><span class="mi">1</span><span class="p">);</span><span class="w"> </span><span class="c1">// index</span>
<span class="c1">// initializes the data</span>
<span class="k">for</span><span class="p">(</span><span class="kt">size_t</span><span class="w"> </span><span class="n">i</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">&lt;</span><span class="n">N</span><span class="p">;</span><span class="w"> </span><span class="n">vec</span><span class="p">[</span><span class="n">i</span><span class="o">++</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">rand</span><span class="p">());</span>
<span class="c1">// create an execution policy</span>
<span class="n">tf</span><span class="o">::</span><span class="n">cudaStream</span><span class="w"> </span><span class="n">stream</span><span class="p">;</span>
<span class="n">tf</span><span class="o">::</span><span class="n">cudaDefaultExecutionPolicy</span><span class="w"> </span><span class="nf">policy</span><span class="p">(</span><span class="n">stream</span><span class="p">);</span>
<span class="c1">// queries the required buffer size to find the minimum element over N element</span>
<span class="k">auto</span><span class="w"> </span><span class="n">bytes</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">policy</span><span class="p">.</span><span class="n">min_element_bufsz</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="p">(</span><span class="n">N</span><span class="p">);</span>
<span class="k">auto</span><span class="w"> </span><span class="n">buffer</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">cuda_malloc_device</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">byte</span><span class="o">&gt;</span><span class="p">(</span><span class="n">bytes</span><span class="p">);</span>
<span class="c1">// finds the minimum element using the less comparator</span>
<span class="n">tf</span><span class="o">::</span><span class="n">cuda_min_element</span><span class="p">(</span>
<span class="w"> </span><span class="n">policy</span><span class="p">,</span><span class="w"> </span><span class="n">vec</span><span class="p">,</span><span class="w"> </span><span class="n">vec</span><span class="o">+</span><span class="n">N</span><span class="p">,</span><span class="w"> </span><span class="n">idx</span><span class="p">,</span><span class="w"> </span><span class="p">[]</span><span class="w"> </span><span class="n">__device__</span><span class="w"> </span><span class="p">(</span><span class="k">auto</span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">b</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">a</span><span class="o">&lt;</span><span class="n">b</span><span class="p">;</span><span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="n">buffer</span>
<span class="p">);</span>
<span class="c1">// wait for the min-element operation completes</span>
<span class="n">stream</span><span class="p">.</span><span class="n">synchronize</span><span class="p">();</span>
<span class="c1">// verifies the result</span>
<span class="n">assert</span><span class="p">(</span><span class="n">vec</span><span class="p">[</span><span class="o">*</span><span class="n">idx</span><span class="p">]</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="o">*</span><span class="n">std</span><span class="o">::</span><span class="n">min_element</span><span class="p">(</span><span class="n">vec</span><span class="p">,</span><span class="w"> </span><span class="n">vec</span><span class="o">+</span><span class="n">N</span><span class="p">,</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">less</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="p">{}));</span>
<span class="c1">// deletes the memory</span>
<span class="n">cudaFree</span><span class="p">(</span><span class="n">vec</span><span class="p">);</span>
<span class="n">cudaFree</span><span class="p">(</span><span class="n">idx</span><span class="p">);</span>
<span class="n">cudaFree</span><span class="p">(</span><span class="n">buffer</span><span class="p">);</span></pre><p>Since the GPU min-element algorithm may require extra buffer to store the temporary results, you need to provide a buffer of size at least larger or equal to the value returned from <code><a href="classtf_1_1cudaExecutionPolicy.html#abcafb001cd68c1135392f4bcda5a2a05" class="m-doc">tf::<wbr />cudaDefaultExecutionPolicy::<wbr />min_element_bufsz</a></code>.</p><aside class="m-note m-warning"><h4>Attention</h4><p>You must keep the buffer alive before the <a href="namespacetf.html#a572c13198191c46765264f8afabe2e9f" class="m-doc">tf::<wbr />cuda_min_element</a> completes.</p></aside></section><section id="CUDASTDFindMaxItems"><h2><a href="#CUDASTDFindMaxItems">Find the Maximum Element in a Range</a></h2><p>Similar to <a href="namespacetf.html#a572c13198191c46765264f8afabe2e9f" class="m-doc">tf::<wbr />cuda_min_element</a>, <a href="namespacetf.html#a3fc577fd0a8f127770bcf68bc56c073e" class="m-doc">tf::<wbr />cuda_max_element</a> finds the index of the maximum element in the given range <code>[first, last)</code> using the given comparison function object. This is equivalent to a parallel execution of the following loop:</p><pre class="m-code"><span class="k">if</span><span class="p">(</span><span class="n">first</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="n">last</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span>
<span class="p">}</span>
<span class="k">auto</span><span class="w"> </span><span class="n">largest</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">first</span><span class="p">;</span>
<span class="k">for</span><span class="w"> </span><span class="p">(</span><span class="o">++</span><span class="n">first</span><span class="p">;</span><span class="w"> </span><span class="n">first</span><span class="w"> </span><span class="o">!=</span><span class="w"> </span><span class="n">last</span><span class="p">;</span><span class="w"> </span><span class="o">++</span><span class="n">first</span><span class="p">)</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="n">op</span><span class="p">(</span><span class="o">*</span><span class="n">largest</span><span class="p">,</span><span class="w"> </span><span class="o">*</span><span class="n">first</span><span class="p">))</span><span class="w"> </span><span class="p">{</span>
<span class="w"> </span><span class="n">largest</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">first</span><span class="p">;</span>
<span class="w"> </span><span class="p">}</span>
<span class="p">}</span>
<span class="k">return</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">distance</span><span class="p">(</span><span class="n">first</span><span class="p">,</span><span class="w"> </span><span class="n">largest</span><span class="p">);</span></pre><p>The following code finds the index of the maximum element in a range of one millions elements using GPU computing:</p><pre class="m-code"><span class="k">const</span><span class="w"> </span><span class="kt">size_t</span><span class="w"> </span><span class="n">N</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">1000000</span><span class="p">;</span>
<span class="k">auto</span><span class="w"> </span><span class="n">vec</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">cuda_malloc_shared</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="p">(</span><span class="n">N</span><span class="p">);</span><span class="w"> </span><span class="c1">// vector</span>
<span class="k">auto</span><span class="w"> </span><span class="n">idx</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">cuda_malloc_shared</span><span class="o">&lt;</span><span class="kt">unsigned</span><span class="o">&gt;</span><span class="p">(</span><span class="mi">1</span><span class="p">);</span><span class="w"> </span><span class="c1">// index</span>
<span class="c1">// initializes the data</span>
<span class="k">for</span><span class="p">(</span><span class="kt">size_t</span><span class="w"> </span><span class="n">i</span><span class="o">=</span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="n">i</span><span class="o">&lt;</span><span class="n">N</span><span class="p">;</span><span class="w"> </span><span class="n">vec</span><span class="p">[</span><span class="n">i</span><span class="o">++</span><span class="p">]</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">rand</span><span class="p">());</span>
<span class="c1">// create an execution policy</span>
<span class="n">tf</span><span class="o">::</span><span class="n">cudaStream</span><span class="w"> </span><span class="n">stream</span><span class="p">;</span>
<span class="n">tf</span><span class="o">::</span><span class="n">cudaDefaultExecutionPolicy</span><span class="w"> </span><span class="nf">policy</span><span class="p">(</span><span class="n">stream</span><span class="p">);</span>
<span class="c1">// queries the required buffer size to find the maximum element over N element</span>
<span class="k">auto</span><span class="w"> </span><span class="n">bytes</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">policy</span><span class="p">.</span><span class="n">max_element_bufsz</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="p">(</span><span class="n">N</span><span class="p">);</span>
<span class="k">auto</span><span class="w"> </span><span class="n">buffer</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="n">tf</span><span class="o">::</span><span class="n">cuda_malloc_device</span><span class="o">&lt;</span><span class="n">std</span><span class="o">::</span><span class="n">byte</span><span class="o">&gt;</span><span class="p">(</span><span class="n">bytes</span><span class="p">);</span>
<span class="c1">// finds the maximum element using the less comparator</span>
<span class="n">tf</span><span class="o">::</span><span class="n">cuda_max_element</span><span class="p">(</span>
<span class="w"> </span><span class="n">policy</span><span class="p">,</span><span class="w"> </span><span class="n">vec</span><span class="p">,</span><span class="w"> </span><span class="n">vec</span><span class="o">+</span><span class="n">N</span><span class="p">,</span><span class="w"> </span><span class="n">idx</span><span class="p">,</span><span class="w"> </span><span class="p">[]</span><span class="w"> </span><span class="n">__device__</span><span class="w"> </span><span class="p">(</span><span class="k">auto</span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="k">auto</span><span class="w"> </span><span class="n">b</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="n">a</span><span class="o">&lt;</span><span class="n">b</span><span class="p">;</span><span class="w"> </span><span class="p">},</span><span class="w"> </span><span class="n">buffer</span>
<span class="p">);</span>
<span class="c1">// wait for the max-element operation to complete</span>
<span class="n">stream</span><span class="p">.</span><span class="n">synchronize</span><span class="p">();</span>
<span class="c1">// verifies the result</span>
<span class="n">assert</span><span class="p">(</span><span class="n">vec</span><span class="p">[</span><span class="o">*</span><span class="n">idx</span><span class="p">]</span><span class="w"> </span><span class="o">==</span><span class="w"> </span><span class="o">*</span><span class="n">std</span><span class="o">::</span><span class="n">max_element</span><span class="p">(</span><span class="n">vec</span><span class="p">,</span><span class="w"> </span><span class="n">vec</span><span class="o">+</span><span class="n">N</span><span class="p">,</span><span class="w"> </span><span class="n">std</span><span class="o">::</span><span class="n">less</span><span class="o">&lt;</span><span class="kt">int</span><span class="o">&gt;</span><span class="p">{}));</span>
<span class="c1">// deletes the memory</span>
<span class="n">cudaFree</span><span class="p">(</span><span class="n">vec</span><span class="p">);</span>
<span class="n">cudaFree</span><span class="p">(</span><span class="n">idx</span><span class="p">);</span>
<span class="n">cudaFree</span><span class="p">(</span><span class="n">buffer</span><span class="p">);</span></pre><p>Since the GPU max-element algorithm may require extra buffer to store the temporary results, you need to provide a buffer of size at least larger or equal to the value returned from <code><a href="classtf_1_1cudaExecutionPolicy.html#a31fe75c4b0765df3035e12be49af88aa" class="m-doc">tf::<wbr />cudaDefaultExecutionPolicy::<wbr />max_element_bufsz</a></code>.</p><aside class="m-note m-warning"><h4>Attention</h4><p>You must keep the buffer alive before <a href="namespacetf.html#a3fc577fd0a8f127770bcf68bc56c073e" class="m-doc">tf::<wbr />cuda_max_element</a> completes.</p></aside></section>
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