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    <li>模型参数检查点（Checkpointing）</li>
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  <div class="section" id="checkpointing">
<span id="checkpointing"></span><h1>模型参数检查点（Checkpointing）<a class="headerlink" href="#checkpointing" title="永久链接至标题">¶</a></h1>
<p>模型数据检查点的实现，可以有效的避免parameter server的单点或多点同时故障。模型参数检查点通过定期向磁盘上保存一份存储在parameter server内存中的模型数据的完整镜像，来保证训练过程可以从中间状态重新启动。在一个不可中断并缺少备份的训练任务中，可以通过阶段性的保存每个parameter server的数据快照（snapshot）到 <strong><em>分布式存储服务</em></strong> 达到容灾的目的，比如每隔10分钟最新的快照，并删除更早的快照。在出现单点故障时，只需要恢复这台节点，或者将这台节点迁移到另一个节点并启动即可恢复训练任务。</p>
<p><img src="src/checkpointing.png" width="500"/></p>
<div class="section" id="">
<span id="id1"></span><h2>快照保存的设计如下：<a class="headerlink" href="#" title="永久链接至标题">¶</a></h2>
<p>说明：</p>
<ul class="simple">
<li>parameter server在集群中启动后，自动挂载分布式存储目录，并把快照保存到这个目录下。</li>
<li><strong><em>注：每个parameter server的检查点各自独立保存，暂时不考虑多个parameter server同步的保存一个特定时间点的全局检查点，因为这样做也没法保证消除随机性。</em></strong></li>
</ul>
<p>检查点保存程序流程：</p>
<ol class="simple">
<li>如果满足条件&#8221;每隔10分钟&#8221;时，parameter server会获取parameters内存的<code class="docutils literal"><span class="pre">read_lock</span></code>，启动一个新的线程开始保存检查点。如果已经正在执行保存检查点的线程，则忽略。由于对parameters的更新需要获取parameters内存的<code class="docutils literal"><span class="pre">write_lock</span></code>，所以在写入快照的过程中，parameter server会暂停参数更新并等待。</li>
<li>parameter server生成一个UUID，向指定的目录中一个新的文件（文件名为此UUID）写入快照数据。在快照写入完成后，计算这个文件的MD5 sum。然后在etcd的<code class="docutils literal"><span class="pre">/checkpoints/[pserver_id]</span></code>中写入json内容：<code class="docutils literal"><span class="pre">{&quot;uuid&quot;:</span> <span class="pre">[UUID],</span> <span class="pre">&quot;md5&quot;,</span> <span class="pre">&quot;MD5</span> <span class="pre">sum&quot;,</span> <span class="pre">&quot;timestamp&quot;:</span> <span class="pre">xxxx}</span></code>。</li>
<li>删除磁盘目录中不是当前uuid的快照文件。</li>
<li>释放对paramters内存的锁定，停止保存检查点的线程。</li>
</ol>
<p>这里需要用户额外注意，在您的实际环境中，训练任务的运行可能会占满trainer和parameter server之间的网络带宽，如果parameter server此时还需要通过网络访问分布式存储以保存快照，可能会造成网络拥塞，而出现阶段性的运行停滞。</p>
</div>
<div class="section" id="">
<span id="id2"></span><h2>从快照恢复<a class="headerlink" href="#" title="永久链接至标题">¶</a></h2>
<p>在parameter server第一次启动或任意时间parameter server故障后被Kubernetes重新启动，则需要回滚到上一个检查点：</p>
<ol class="simple">
<li>从etcd中读取节点：<code class="docutils literal"><span class="pre">/checkpoints/[pserver_id]</span></code>获取最新的检查点的文件uuid</li>
<li>从磁盘文件中加载uuid文件名的检查点快照文件，并加载其中的参数</li>
<li>如果上面两步出现错误，则使用启动参数定义的初始化方法初始化参数</li>
<li>开始提供服务</li>
</ol>
</div>
</div>
<div class="section" id="todo-list">
<span id="todo-list"></span><h1>TODO List<a class="headerlink" href="#todo-list" title="永久链接至标题">¶</a></h1>
<div class="section" id="todo">
<span id="todo"></span><h2>推测执行/加速执行（TODO）<a class="headerlink" href="#todo" title="永久链接至标题">¶</a></h2>
<p>在异构集群中，如果存在某些trainer执行速度过慢会影响整体集群的速度（如图中Trainer 1），此时master将负责启动一个新的Trainer（Accelerate Trainer 2），使用同样的训练数据block。哪个trainer先完成block的训练，则把另一个慢速的kill掉。</p>
</div>
<div class="section" id="">
<span id="id3"></span><h2>动态扩容/缩容<a class="headerlink" href="#" title="永久链接至标题">¶</a></h2>
<p>目前只考虑动态扩容trainer数量，可以减小系统复杂性。</p>
</div>
</div>
<div class="section" id="">
<span id="id4"></span><h1>术语<a class="headerlink" href="#" title="永久链接至标题">¶</a></h1>
<ul class="simple">
<li>model: 指深度学习训练之后得到的所有参数，使用这个神经网络可以完成对新数据的预测</li>
<li>parameters: 神经网络中的参数，包括权重w和偏置b。一个神经网络的模型由大量的参数组成</li>
<li>shard: 分片，通常指将一个整体拆分成多份的其中的一份。</li>
<li>model shard: 将一个神经网络参数拆分成多份，每个shard分别存储在其中一台parameter server之上</li>
<li>parameter block: 多个parameter block构成一个model shard</li>
<li>单点故障: 任意时刻只可能同时有一台服务器故障。由于集群中同时存在两台机器故障的概率极低（（平均故障率*平均故障修复时间）^2）只对特殊在线系统考虑两台以上同时故障的容灾。</li>
</ul>
</div>


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