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<meta name="viewport" content="width=device-width, initial-scale=1">
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<title>Taming Sparse Rollout</title>
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<meta name="description" content="Project page for Taming Sparse Rollout: Understanding Stability in RL for Large Language Models with Sparse Attention.">
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<link rel="icon" type="image/png" href="radom/headpic.png">
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<header class="hero">
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<nav class="topbar" aria-label="Page navigation">
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<a class="brand" href="#top" aria-label="Taming Sparse Rollout home">
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<img src="assets/figures/fig-000.png" alt="" aria-hidden="true">
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<img src="radom/headpic.png" alt="" aria-hidden="true">
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<span>Tamed Sparsity</span>
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</a>
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<div class="navlinks">
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<section id="top" class="hero-inner">
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<p class="eyebrow">Sparse rollout with Optimal Stability Efficiency Tradeoff for RLVR</p>
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<h2>Taming Sparse Rollout: Understanding Stability in RL for Large Language Models with Sparse Attention</h2>
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<h2 class="hero-title">
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<span>Taming Sparse Rollout: Understanding Stability in RL for Large Language Models with Sparse Attention</span>
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</h2>
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<p class="authors">Yang Zhou, Ranajoy Sadhukhan, Zhaofeng Sun, Zhuoming Chen, Souvik Kundu, Saket Dingliwal, Sai Muralidhar Jayanthi, Aram Galstyan, Haizhong Zheng, Beidi Chen</p>
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<p class="affiliation">Carnegie Mellon University, Independent Researcher, Intel, Amazon</p>
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<main>
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<section id="desiderata" class="section visual-band">
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<div class="section-heading">
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<span class="section-icon">1</span>
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<span class="section-icon section-icon-image">
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<img src="radom/Telescope.png" alt="" aria-hidden="true">
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</span>
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<h2>Previous Works Limitation and Desirata</h2>
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</div>
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<div class="desiderata-layout">
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<section id="insights" class="section split full-width-section">
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<div>
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<div class="section-heading">
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<span class="section-icon">2</span>
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<span class="section-icon section-icon-image">
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<img src="radom/Idea.png" alt="" aria-hidden="true">
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</span>
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<h2>Insights to Sparse to Dense Actor-Policy Mismatch</h2>
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</div>
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<div class="wide-grid single-figure">
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<section id="control-study-1" class="section visual-band">
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<div class="section-heading">
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<span class="section-icon">3</span>
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<span class="section-icon section-icon-image">
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<img src="radom/Observation.png" alt="" aria-hidden="true">
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</span>
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<h2>Control Study 1: How Mismatch Threshold Varies to Increasing Model Sizes</h2>
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</div>
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<div class="wide-grid">
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Surprisingly, we make the following finding.
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</p>
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<p class="hypothesis-box">
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<strong>Takeaway 1</strong>: across a range of model sizes in Qwen3 thinking family, we find that keeping
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<strong>Takeaway</strong>: across a range of model sizes in Qwen3 thinking family, we find that keeping
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5-percentile mismatch threshold above 0.86 generally leads to stable RL training, supporting our hypothesis.
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</p>
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<p>
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<section id="control-study-2" class="section">
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<div class="section-heading">
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<span class="section-icon">4</span>
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<span class="section-icon section-icon-image">
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<img src="radom/GPU.png" alt="" aria-hidden="true">
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</span>
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<h2>Control Study 2: Finding the Lowest Cost given the Mismatch Threshold</h2>
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</div>
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<div class="results-layout paired-figures">
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<figure class="figure-card">
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<img src="assets/figures/figure106.png" alt="Qwen3-14B sparse rollout and dense rollout AIME2026 results.">
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<figcaption>Qwen3-14B validates stable on-par performance with sparse rollout at the tail threshold.</figcaption>
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<figcaption>From our cost model study, we identified that for block-sparse attention (block-size 16 and above), smaller page size consistently ahead of the tradeoff of distribution alignment to dense and cost.</figcaption>
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</figure>
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<figure class="figure-card">
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<img src="assets/figures/figure103.png" alt="DistillSparse training improves AIME2025 accuracy.">
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<figcaption>LoRA distillation can improve alignment and push beyond the plain sparse threshold.</figcaption>
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</figure>
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</div>
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<div class="metric-strip">
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<div><b>2.2x</b><span>Qwen3-1.7B rollout speedup</span></div>
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<div><b>2.4x</b><span>Qwen3-4B rollout speedup</span></div>
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<div><b>2.0x</b><span>Qwen3-8B rollout speedup</span></div>
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<figcaption>Empirically, we achieve speedup for all different sized models.</figcaption>
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</figure>
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<p class="full-width-text">
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With the mismatch threshold identified, we then look for the lowest cost achievable while meeting the mismatch threshold.
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To make sure our analysis is general and can transfer to different hardware, we use a cost model for our analysis.
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</p>
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<details class="formula cost-model full-width-text">
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<summary>Full explanation of the cost model</summary>
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<p>
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We follow Sadhukhan et al. (2025)
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and model rollout cost from model size and hardware memory bandwidth. For repeated sampling <code>N</code> times,
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let <code>P</code> be the number of model parameters, <code>L<sub>in</sub></code> the input prompt length,
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<code>L<sub>out</sub></code> the output length, <code>D</code> the Key/Value dimension, <code>r</code> the GQA ratio,
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and <code>I</code> one over the GPU SRAM memory bandwidth.
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</p>
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<div class="cost-equation">
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<p class="formula-label">Dense attention</p>
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<pre><code>C<sub>comp</sub> = 2 * P * N * L<sub>out</sub> + r * (2 * L<sub>in</sub> + L<sub>out</sub>) * L<sub>out</sub> * N * D
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C<sub>mem</sub> = 2 * L<sub>in</sub> * L<sub>out</sub> * D + N * L<sub>out</sub><sup>2</sup> * D
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C<sub>dense</sub> = C<sub>comp</sub> + I * C<sub>mem</sub></code></pre>
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</div>
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<p>
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For block-sparse attention, we assume Top-k kernels incur only minimal overhead for page size
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<code>&ge; 16</code>. With KV budget <code>B</code> and page size <code>pagesize</code>, the sparse cost is:
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</p>
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<div class="cost-equation">
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<p class="formula-label">Block-sparse attention</p>
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<pre><code>C<sub>sparse,no scoring</sub> = 2 * N * P * L<sub>out</sub> + 2 * r * N * D * B * L<sub>out</sub> + 2 * I * N * D * B * L<sub>out</sub>
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C<sub>scoring</sub> = 2 * N * L<sub>in</sub> * D * L<sub>out</sub> + (r * N * D * L<sub>out</sub><sup>2</sup>) / (2 * pagesize)
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+ 2 * I * L<sub>in</sub> * D * L<sub>out</sub> + (I * N * D * L<sub>out</sub><sup>2</sup>) / (2 * pagesize)
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C<sub>sparse</sub> = C<sub>sparse,no scoring</sub> + C<sub>scoring</sub></code></pre>
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</div>
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</details>
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<p class="full-width-text">
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Using this cost model, we find that for <span class="text-highlight">block-sparse attention with page size 16 and above</span>,
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smaller page sizes consistently dominate larger page sizes in the tradeoff between
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<span class="text-highlight">dense-policy distribution alignment</span> and cost. In the paper, we provide full details
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for other generation-length regimes, and the same conclusion holds across them. Although the cost model does
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not explicitly account for <span class="text-highlight">Top-k kernel overhead</span>, our page-size measurements show that when
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the Top-k kernels are well implemented, as in Vortex, sparse decoding cost is not highly sensitive to page size.
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We therefore use <span class="text-highlight">page size 16</span> for sparsity scheduling, since it gives the strongest
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cost-alignment tradeoff while remaining practical for efficient sparse rollout.
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</p>
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</div>
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</section>
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<section id="empirical-generalization" class="section visual-band">
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<div class="section-heading">
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<span class="section-icon">$</span>
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<span class="section-icon section-icon-image">
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<img src="radom/iss.webp" alt="" aria-hidden="true">
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</span>
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<h2>Generalization to Larger Model and Other RL domains (Coding RL)</h2>
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</div>
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<div class="wide-grid single-figure">
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<section id="distillsparse" class="section">
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<div class="section-heading">
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<span class="section-icon">PDF</span>
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<span class="section-icon section-icon-image">
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<img src="radom/cosmonautllama.png" alt="" aria-hidden="true">
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</span>
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<h2>DistillSparse: A Technique Pushes for More Aggressive Sparsity and Higher Speedup</h2>
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</div>
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<div class="results-layout paired-figures balanced-figures">

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