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@@ -98,7 +98,6 @@
<li class="toctree-l2 current"><a class="current reference internal" href="#">Fused Softmax</a><ul>
<li class="toctree-l3"><a class="reference internal" href="#motivations">Motivations</a></li>
<li class="toctree-l3"><a class="reference internal" href="#compute-kernel">Compute Kernel</a></li>
<li class="toctree-l3"><a class="reference internal" href="#torch-bindings">Torch Bindings</a></li>
<li class="toctree-l3"><a class="reference internal" href="#unit-test">Unit Test</a></li>
<li class="toctree-l3"><a class="reference internal" href="#benchmark">Benchmark</a></li>
</ul>
@@ -107,12 +106,14 @@
</ul>
</li>
</ul>
<p class="caption"><span class="caption-text">Language Reference</span></p>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../../language-reference/python-api/index.html">Python API</a></li>
</ul>
<p class="caption"><span class="caption-text">Programming Guide</span></p>
<ul>
<li class="toctree-l1"><a class="reference internal" href="../../programming-guide/chapter-1/introduction.html">Introduction</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../programming-guide/chapter-2/related-work.html">Related Work</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../programming-guide/chapter-3/triton-c.html">The Triton-C Language</a></li>
<li class="toctree-l1"><a class="reference internal" href="../../programming-guide/chapter-4/triton-ir.html">The Triton-IR Intermediate Representation</a></li>
</ul>
@@ -192,8 +193,7 @@ to download the full example code</p>
<p>In this tutorial, you will write a fused softmax operation (that outperforms PyTorch) and learn about:</p>
<ul class="simple">
<li><p>The benefits of kernel fusion for bandwidth-bound operations.</p></li>
<li><p>The syntax and usage of reduction operators in Triton.</p></li>
<li><p>The automatic vectorization capabilities of the Triton compiler.</p></li>
<li><p>The reduction operators in Triton.</p></li>
</ul>
<div class="section" id="motivations">
<h2>Motivations<a class="headerlink" href="#motivations" title="Permalink to this headline"></a></h2>
@@ -220,78 +220,41 @@ Let us consider instead the case of a simple (numerically stabilized) softmax op
</div>
<p>When implemented naively in pytorch, computing <code class="code docutils literal notranslate"><span class="pre">y</span> <span class="pre">=</span> <span class="pre">naive_softmax(x)</span></code> for <span class="math notranslate nohighlight">\(x \in R^{M \times N}\)</span> requires reading <span class="math notranslate nohighlight">\(7MN\)</span> elements from DRAM and writing back <span class="math notranslate nohighlight">\(3MN + 2M\)</span> elements.
This is obviously wasteful; wed prefer to have a custom “fused” kernel that only reads X once and does all the necessary computations on-chip.
In this case, we would be reading and writing back only <span class="math notranslate nohighlight">\(MN\)</span> bytes, so we could expect a theoretical speed-up of ~5x (i.e., <span class="math notranslate nohighlight">\((10MN + 2M) / 2MN\)</span>).
This solution would require reading and writing back only <span class="math notranslate nohighlight">\(MN\)</span> bytes, so we could expect a theoretical speed-up of ~5x (i.e., <span class="math notranslate nohighlight">\((10MN + 2M) / 2MN\)</span>).
In practice, though, we would be getting a bit less as our kernel computes exponentials and internally moves data around in shared memory.</p>
</div>
<div class="section" id="compute-kernel">
<h2>Compute Kernel<a class="headerlink" href="#compute-kernel" title="Permalink to this headline"></a></h2>
<p>Our softmax kernel works as follows: each program loads a row of the input X, normalizes it and writes back the result to the output Y.
<p>Our softmax kernel works as follows: each program loads a row of the input matrix X, normalizes it and writes back the result to the output Y.
Note that one important limitation of Triton is that each block must have a power-of-two number of elements,
so we need to internally “pad” tiles and guard the memory operations properly if we want to handle any possible input shapes:</p>
<blockquote>
<div><div class="highlight-C notranslate"><div class="highlight"><pre><span></span><span class="n">__global__</span> <span class="kt">void</span> <span class="n">softmax</span><span class="p">(</span><span class="kt">float</span><span class="o">*</span> <span class="n">Y</span><span class="p">,</span> <span class="kt">float</span><span class="o">*</span> <span class="n">X</span><span class="p">,</span> <span class="kt">int</span> <span class="n">stride_xm</span><span class="p">,</span> <span class="kt">int</span> <span class="n">stride_ym</span><span class="p">,</span> <span class="kt">int</span> <span class="n">M</span><span class="p">,</span> <span class="kt">int</span> <span class="n">N</span><span class="p">){</span>
<span class="c1">// row index</span>
<span class="kt">int</span> <span class="n">m</span> <span class="o">=</span> <span class="n">get_program_id</span><span class="p">(</span><span class="mi">0</span><span class="p">);</span>
<span class="c1">// column indices</span>
<span class="kt">int</span> <span class="n">n</span> <span class="p">[</span><span class="n">BLOCK</span><span class="p">]</span> <span class="o">=</span> <span class="mi">0</span> <span class="p">...</span> <span class="n">BLOCK</span><span class="p">;</span>
<span class="c1">// the memory address of all the elements</span>
<span class="c1">// that we want to load can be computed as follows</span>
<span class="kt">float</span><span class="o">*</span> <span class="n">px</span> <span class="p">[</span><span class="n">BLOCK</span><span class="p">]</span> <span class="o">=</span> <span class="n">X</span> <span class="o">+</span> <span class="n">m</span><span class="o">*</span><span class="n">stride_xm</span> <span class="o">+</span> <span class="n">n</span><span class="p">;</span>
<span class="c1">// because BLOCK has to be a power of two</span>
<span class="c1">// (per Triton-C specs), it is important</span>
<span class="c1">// to guard each memory operation with predicates</span>
<span class="c1">// or we will read out of bounds</span>
<span class="kt">bool</span> <span class="n">check</span><span class="p">[</span><span class="n">BLOCK</span><span class="p">]</span> <span class="o">=</span> <span class="n">n</span> <span class="o">&lt;</span> <span class="n">N</span><span class="p">;</span>
<span class="kt">float</span> <span class="n">x</span> <span class="p">[</span><span class="n">BLOCK</span><span class="p">]</span> <span class="o">=</span> <span class="n">check</span> <span class="o">?</span> <span class="o">*</span><span class="nl">px</span> <span class="p">:</span> <span class="o">-</span><span class="n">F32_INFINITY</span><span class="p">;</span>
<span class="c1">// syntax for reduction in Triton is:</span>
<span class="c1">// x[:, :, OPERATOR, :, :]</span>
<span class="c1">// ^</span>
<span class="c1">// index</span>
<span class="c1">// where operator is in {min, max, +}</span>
<span class="c1">// for 1D vectors, this is just x[OPERATOR].</span>
<span class="kt">float</span> <span class="n">z</span> <span class="p">[</span><span class="n">BLOCK</span><span class="p">]</span> <span class="o">=</span> <span class="n">x</span> <span class="o">-</span> <span class="n">x</span><span class="p">[</span><span class="n">max</span><span class="p">];</span>
<span class="c1">// Note that exponentials in Triton are fast</span>
<span class="c1">// but approximate (i.e., think __expf in CUDA)</span>
<span class="kt">float</span> <span class="n">num</span> <span class="p">[</span><span class="n">BLOCK</span><span class="p">]</span> <span class="o">=</span> <span class="n">exp</span><span class="p">(</span><span class="n">z</span><span class="p">);</span>
<span class="kt">float</span> <span class="n">denom</span> <span class="o">=</span> <span class="n">num</span><span class="p">[</span><span class="o">+</span><span class="p">];</span>
<span class="c1">// The result of the reduction is now stored in y</span>
<span class="kt">float</span> <span class="n">y</span> <span class="p">[</span><span class="n">BLOCK</span><span class="p">]</span> <span class="o">=</span> <span class="n">num</span> <span class="o">/</span> <span class="n">denom</span><span class="p">;</span>
<span class="c1">// We write it back</span>
<span class="kt">float</span><span class="o">*</span> <span class="n">py</span> <span class="p">[</span><span class="n">BLOCK</span><span class="p">]</span> <span class="o">=</span> <span class="n">Y</span> <span class="o">+</span> <span class="n">m</span><span class="o">*</span><span class="n">stride_ym</span> <span class="o">+</span> <span class="n">n</span><span class="p">;</span>
<span class="o">*?</span><span class="p">(</span><span class="n">check</span><span class="p">)</span><span class="n">py</span> <span class="o">=</span> <span class="n">y</span><span class="p">;</span>
<span class="p">}</span>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">triton</span>
<span class="nd">@triton</span><span class="o">.</span><span class="n">jit</span>
<span class="k">def</span> <span class="nf">_softmax</span><span class="p">(</span><span class="n">Y</span><span class="p">,</span> <span class="n">X</span><span class="p">,</span> <span class="n">stride_xm</span><span class="p">,</span> <span class="n">stride_ym</span><span class="p">,</span> <span class="n">M</span><span class="p">,</span> <span class="n">N</span><span class="p">,</span> <span class="o">**</span><span class="n">meta</span><span class="p">):</span>
<span class="c1"># row index</span>
<span class="n">m</span> <span class="o">=</span> <span class="n">triton</span><span class="o">.</span><span class="n">program_id</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
<span class="c1"># col indices</span>
<span class="n">n</span> <span class="o">=</span> <span class="n">triton</span><span class="o">.</span><span class="n">arange</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="n">meta</span><span class="p">[</span><span class="s1">&#39;BLOCK&#39;</span><span class="p">])</span>
<span class="c1"># the memory address of all the elements</span>
<span class="c1"># that we want to load can be computed as follows</span>
<span class="n">X</span> <span class="o">=</span> <span class="n">X</span> <span class="o">+</span> <span class="n">m</span> <span class="o">*</span> <span class="n">stride_xm</span> <span class="o">+</span> <span class="n">n</span>
<span class="n">x</span> <span class="o">=</span> <span class="n">triton</span><span class="o">.</span><span class="n">load</span><span class="p">(</span><span class="n">X</span><span class="p">,</span> <span class="n">mask</span><span class="o">=</span><span class="n">n</span> <span class="o">&lt;</span> <span class="n">N</span><span class="p">,</span> <span class="n">other</span><span class="o">=-</span><span class="nb">float</span><span class="p">(</span><span class="s1">&#39;inf&#39;</span><span class="p">))</span>
<span class="c1"># Substract maximum for numerical stability</span>
<span class="n">z</span> <span class="o">=</span> <span class="n">x</span> <span class="o">-</span> <span class="n">triton</span><span class="o">.</span><span class="n">max</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">axis</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span>
<span class="c1"># Note that exponentials in Triton are fast</span>
<span class="c1"># but approximate (i.e., think __expf in CUDA)</span>
<span class="n">num</span> <span class="o">=</span> <span class="n">triton</span><span class="o">.</span><span class="n">exp</span><span class="p">(</span><span class="n">z</span><span class="p">)</span>
<span class="n">denom</span> <span class="o">=</span> <span class="n">triton</span><span class="o">.</span><span class="n">sum</span><span class="p">(</span><span class="n">num</span><span class="p">,</span> <span class="n">axis</span><span class="o">=</span><span class="mi">0</span><span class="p">)</span>
<span class="n">y</span> <span class="o">=</span> <span class="n">num</span> <span class="o">/</span> <span class="n">denom</span>
<span class="c1"># Write back to Y</span>
<span class="n">Y</span> <span class="o">=</span> <span class="n">Y</span> <span class="o">+</span> <span class="n">m</span> <span class="o">*</span> <span class="n">stride_ym</span> <span class="o">+</span> <span class="n">n</span>
<span class="n">triton</span><span class="o">.</span><span class="n">store</span><span class="p">(</span><span class="n">Y</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">mask</span><span class="o">=</span><span class="n">n</span> <span class="o">&lt;</span> <span class="n">N</span><span class="p">)</span>
</pre></div>
</div>
</div></blockquote>
</div>
<div class="section" id="torch-bindings">
<h2>Torch Bindings<a class="headerlink" href="#torch-bindings" title="Permalink to this headline"></a></h2>
<p>Here our torch bindings is quite similar to that of the vector addition mentioned in the previous tutorial.
We just need to make sure that BLOCK is the smallest power of two greater than the number of columns N of the input matrix.
This means that different values of BLOCK will result in different kernels</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="kn">import</span> <span class="nn">torch</span>
<span class="kn">import</span> <span class="nn">triton</span>
<span class="c1"># Source code for the Triton kernel</span>
<span class="n">_src</span> <span class="o">=</span> <span class="s2">&quot;&quot;&quot;</span>
<span class="s2">__global__ void softmax(float* Y, float* X, int stride_ym, int stride_xm, int M, int N){</span>
<span class="s2"> int m = get_program_id(0);</span>
<span class="s2"> int n [BLOCK] = 0 ... BLOCK;</span>
<span class="s2"> float* px [BLOCK] = X + m*stride_xm + n;</span>
<span class="s2"> bool check[BLOCK] = n &lt; N;</span>
<span class="s2"> float x [BLOCK] = check ? *px : -F32_INFINITY;</span>
<span class="s2"> float z [BLOCK] = x - x[max];</span>
<span class="s2"> float num [BLOCK] = exp(z);</span>
<span class="s2"> float denom = num[+];</span>
<span class="s2"> float y [BLOCK] = num / denom;</span>
<span class="s2"> float* py [BLOCK] = Y + m*stride_ym + n;</span>
<span class="s2"> *?(check)py = y;</span>
<span class="s2">}</span>
<span class="s2">&quot;&quot;&quot;</span>
<span class="c1"># helper function to get the smaller power-of-two larger than a given number</span>
<span class="k">def</span> <span class="nf">next_power_of_2</span><span class="p">(</span><span class="n">n</span><span class="p">):</span>
<p>We can create a helper function that enqueues the kernel and its (meta-)arguments for any given input tensor.</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="k">def</span> <span class="nf">next_power_of_2</span><span class="p">(</span><span class="n">n</span><span class="p">):</span>
<span class="n">n</span> <span class="o">-=</span> <span class="mi">1</span>
<span class="n">n</span> <span class="o">|=</span> <span class="n">n</span> <span class="o">&gt;&gt;</span> <span class="mi">1</span>
<span class="n">n</span> <span class="o">|=</span> <span class="n">n</span> <span class="o">&gt;&gt;</span> <span class="mi">2</span>
@@ -302,11 +265,9 @@ This means that different values of BLOCK will result in different kernels</p>
<span class="k">return</span> <span class="n">n</span>
<span class="c1"># kernel caching mechanism</span>
<span class="k">def</span> <span class="nf">make_kernel</span><span class="p">(</span><span class="n">N</span><span class="p">,</span> <span class="n">device</span><span class="p">):</span>
<span class="n">cache</span> <span class="o">=</span> <span class="n">make_kernel</span><span class="o">.</span><span class="n">cache</span>
<span class="c1"># Now are kernels are indexed not only by the provided device but also</span>
<span class="c1"># by the rounded number of columns in the input matrix</span>
<span class="k">def</span> <span class="nf">softmax</span><span class="p">(</span><span class="n">x</span><span class="p">):</span>
<span class="n">M</span><span class="p">,</span> <span class="n">N</span> <span class="o">=</span> <span class="n">x</span><span class="o">.</span><span class="n">shape</span>
<span class="c1"># The block size is the smallest power of two greater than the number of columns in `x`</span>
<span class="n">BLOCK</span> <span class="o">=</span> <span class="n">next_power_of_2</span><span class="p">(</span><span class="n">N</span><span class="p">)</span>
<span class="c1"># Another trick we can use is to ask the compiler to parallelize each</span>
<span class="c1"># row-normalization more aggressively -- i.e., with more warps -- vectors</span>
@@ -316,36 +277,13 @@ This means that different values of BLOCK will result in different kernels</p>
<span class="n">num_warps</span> <span class="o">=</span> <span class="mi">4</span>
<span class="k">if</span> <span class="n">BLOCK</span> <span class="o">&gt;=</span> <span class="mi">2048</span><span class="p">:</span> <span class="n">num_warps</span> <span class="o">=</span> <span class="mi">8</span>
<span class="k">if</span> <span class="n">BLOCK</span> <span class="o">&gt;=</span> <span class="mi">4096</span><span class="p">:</span> <span class="n">num_warps</span> <span class="o">=</span> <span class="mi">16</span>
<span class="c1"># Each (BLOCK, num_warps, device) results in a different kernel</span>
<span class="n">key</span> <span class="o">=</span> <span class="p">(</span><span class="n">BLOCK</span><span class="p">,</span> <span class="n">num_warps</span><span class="p">,</span> <span class="n">device</span><span class="p">)</span>
<span class="k">if</span> <span class="n">key</span> <span class="ow">not</span> <span class="ow">in</span> <span class="n">cache</span><span class="p">:</span>
<span class="n">defines</span> <span class="o">=</span> <span class="p">{</span><span class="s1">&#39;BLOCK&#39;</span><span class="p">:</span> <span class="n">BLOCK</span><span class="p">}</span>
<span class="n">cache</span><span class="p">[</span><span class="n">key</span><span class="p">]</span> <span class="o">=</span> <span class="n">triton</span><span class="o">.</span><span class="n">kernel</span><span class="p">(</span><span class="n">_src</span><span class="p">,</span> <span class="n">device</span><span class="o">=</span><span class="n">device</span><span class="p">,</span> <span class="n">defines</span><span class="o">=</span><span class="n">defines</span><span class="p">,</span> <span class="n">num_warps</span><span class="o">=</span><span class="n">num_warps</span><span class="p">)</span>
<span class="k">return</span> <span class="n">cache</span><span class="p">[</span><span class="n">key</span><span class="p">]</span>
<span class="n">make_kernel</span><span class="o">.</span><span class="n">cache</span> <span class="o">=</span> <span class="nb">dict</span><span class="p">()</span>
<span class="k">class</span> <span class="nc">_softmax</span><span class="p">(</span><span class="n">torch</span><span class="o">.</span><span class="n">autograd</span><span class="o">.</span><span class="n">Function</span><span class="p">):</span>
<span class="nd">@staticmethod</span>
<span class="k">def</span> <span class="nf">forward</span><span class="p">(</span><span class="n">ctx</span><span class="p">,</span> <span class="n">x</span><span class="p">):</span>
<span class="c1"># constraints of the op</span>
<span class="k">assert</span> <span class="n">x</span><span class="o">.</span><span class="n">dtype</span> <span class="o">==</span> <span class="n">torch</span><span class="o">.</span><span class="n">float32</span>
<span class="n">y</span> <span class="o">=</span> <span class="n">torch</span><span class="o">.</span><span class="n">empty_like</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="c1"># The launch grid is simple: we have one kernel instance per row of the input matrix</span>
<span class="n">M</span><span class="p">,</span> <span class="n">N</span> <span class="o">=</span> <span class="n">y</span><span class="o">.</span><span class="n">shape</span>
<span class="n">grid</span> <span class="o">=</span> <span class="k">lambda</span> <span class="n">opt</span><span class="p">:</span> <span class="p">(</span><span class="n">M</span><span class="p">,</span> <span class="p">)</span>
<span class="c1"># Launch kernel</span>
<span class="n">kernel</span> <span class="o">=</span> <span class="n">make_kernel</span><span class="p">(</span><span class="n">N</span><span class="p">,</span> <span class="n">y</span><span class="o">.</span><span class="n">device</span><span class="p">)</span>
<span class="n">kernel</span><span class="p">(</span><span class="n">y</span><span class="o">.</span><span class="n">data_ptr</span><span class="p">(),</span> <span class="n">x</span><span class="o">.</span><span class="n">data_ptr</span><span class="p">(),</span> <span class="n">y</span><span class="o">.</span><span class="n">stride</span><span class="p">(</span><span class="mi">0</span><span class="p">),</span> <span class="n">x</span><span class="o">.</span><span class="n">stride</span><span class="p">(</span><span class="mi">0</span><span class="p">),</span> <span class="n">M</span><span class="p">,</span> <span class="n">N</span><span class="p">,</span> <span class="n">grid</span><span class="o">=</span><span class="n">grid</span><span class="p">)</span>
<span class="k">return</span> <span class="n">y</span>
<span class="n">softmax</span> <span class="o">=</span> <span class="n">_softmax</span><span class="o">.</span><span class="n">apply</span>
<span class="c1"># Allocate output</span>
<span class="n">y</span> <span class="o">=</span> <span class="n">torch</span><span class="o">.</span><span class="n">empty_like</span><span class="p">(</span><span class="n">x</span><span class="p">)</span>
<span class="c1"># Enqueue kernel. The launch grid is simple: we have one kernel instance per row of the input matrix</span>
<span class="n">_softmax</span><span class="p">[(</span><span class="n">M</span><span class="p">,</span> <span class="p">)](</span><span class="n">y</span><span class="p">,</span> <span class="n">x</span><span class="p">,</span> <span class="n">x</span><span class="o">.</span><span class="n">stride</span><span class="p">(</span><span class="mi">0</span><span class="p">),</span> <span class="n">y</span><span class="o">.</span><span class="n">stride</span><span class="p">(</span><span class="mi">0</span><span class="p">),</span> <span class="n">M</span><span class="p">,</span> <span class="n">N</span><span class="p">,</span> <span class="n">BLOCK</span><span class="o">=</span><span class="n">BLOCK</span><span class="p">)</span>
<span class="k">return</span> <span class="n">y</span>
</pre></div>
</div>
<p>We can use the above softmax function to compute the row-wise softmax of a given matrix.</p>
</div>
<div class="section" id="unit-test">
<h2>Unit Test<a class="headerlink" href="#unit-test" title="Permalink to this headline"></a></h2>
@@ -405,7 +343,7 @@ This means that when temporary data is too large to fit entirely in the GPU
Note that our Triton kernel is not only faster than PyTorchs CUDA kernel, it is also <strong>easier to read, understand and maintain</strong>.</p></li>
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