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redis系列介绍七-过期策略
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<p>这一篇不再是数据结构介绍了，大致的数据结构基本都介绍了，这一篇主要是查漏补缺，或者说讲一些重要且基本的概念，也可能是经常被忽略的，很多讲 redis 的系列文章可能都会忽略，学习 redis 的时候也会，因为觉得源码学习就是讲主要的数据结构和“算法”学习了就好了。<br>redis 的主要应用就是拿来作为高性能的缓存，那么缓存一般有些啥需要注意的，首先是访问速度，如果取得跟数据库一样快，那就没什么存在的意义，第二个是缓存的字面意思，我只是为了让数据读取快一些，通常大部分的场景这个是需要更新过期的，这里就把我要讲的第一点引出来了（真累，</p>
<h2 id="redis过期策略"><a href="#redis过期策略" class="headerlink" title="redis过期策略"></a>redis过期策略</h2><p>redis 是如何过期缓存的，可以猜测下，最无脑的就是每个设置了过期时间的 key 都设个定时器，过期了就删除，这种显然消耗太大，清理地最及时，还有的就是 redis 正在采用的懒汉清理策略和定期清理<br>懒汉策略就是在使用的时候去检查缓存是否过期，比如 get 操作时，先判断下这个 key 是否已经过期了，如果过期了就删掉，并且返回空，如果没过期则正常返回<br>主要代码是</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br><span class="line">59</span><br><span class="line">60</span><br><span class="line">61</span><br><span class="line">62</span><br><span class="line">63</span><br><span class="line">64</span><br><span class="line">65</span><br><span class="line">66</span><br><span class="line">67</span><br><span class="line">68</span><br><span class="line">69</span><br><span class="line">70</span><br><span class="line">71</span><br><span class="line">72</span><br><span class="line">73</span><br><span class="line">74</span><br><span class="line">75</span><br><span class="line">76</span><br><span class="line">77</span><br><span class="line">78</span><br><span class="line">79</span><br><span class="line">80</span><br><span class="line">81</span><br><span class="line">82</span><br><span class="line">83</span><br><span class="line">84</span><br><span class="line">85</span><br><span class="line">86</span><br><span class="line">87</span><br><span class="line">88</span><br><span class="line">89</span><br><span class="line">90</span><br><span class="line">91</span><br><span class="line">92</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/* This function is called when we are going to perform some operation</span></span><br><span class="line"><span class="comment"> * in a given key, but such key may be already logically expired even if</span></span><br><span class="line"><span class="comment"> * it still exists in the database. The main way this function is called</span></span><br><span class="line"><span class="comment"> * is via lookupKey*() family of functions.</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * The behavior of the function depends on the replication role of the</span></span><br><span class="line"><span class="comment"> * instance, because slave instances do not expire keys, they wait</span></span><br><span class="line"><span class="comment"> * for DELs from the master for consistency matters. However even</span></span><br><span class="line"><span class="comment"> * slaves will try to have a coherent return value for the function,</span></span><br><span class="line"><span class="comment"> * so that read commands executed in the slave side will be able to</span></span><br><span class="line"><span class="comment"> * behave like if the key is expired even if still present (because the</span></span><br><span class="line"><span class="comment"> * master has yet to propagate the DEL).</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * In masters as a side effect of finding a key which is expired, such</span></span><br><span class="line"><span class="comment"> * key will be evicted from the database. Also this may trigger the</span></span><br><span class="line"><span class="comment"> * propagation of a DEL/UNLINK command in AOF / replication stream.</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * The return value of the function is 0 if the key is still valid,</span></span><br><span class="line"><span class="comment"> * otherwise the function returns 1 if the key is expired. */</span></span><br><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">expireIfNeeded</span><span class="params">(redisDb *db, robj *key)</span> </span>&#123;</span><br><span class="line"> <span class="keyword">if</span> (!keyIsExpired(db,key)) <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* If we are running in the context of a slave, instead of</span></span><br><span class="line"><span class="comment"> * evicting the expired key from the database, we return ASAP:</span></span><br><span class="line"><span class="comment"> * the slave key expiration is controlled by the master that will</span></span><br><span class="line"><span class="comment"> * send us synthesized DEL operations for expired keys.</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * Still we try to return the right information to the caller,</span></span><br><span class="line"><span class="comment"> * that is, 0 if we think the key should be still valid, 1 if</span></span><br><span class="line"><span class="comment"> * we think the key is expired at this time. */</span></span><br><span class="line"> <span class="keyword">if</span> (server.masterhost != <span class="literal">NULL</span>) <span class="keyword">return</span> <span class="number">1</span>;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* Delete the key */</span></span><br><span class="line"> server.stat_expiredkeys++;</span><br><span class="line"> propagateExpire(db,key,server.lazyfree_lazy_expire);</span><br><span class="line"> notifyKeyspaceEvent(NOTIFY_EXPIRED,</span><br><span class="line"> <span class="string">"expired"</span>,key,db-&gt;id);</span><br><span class="line"> <span class="keyword">return</span> server.lazyfree_lazy_expire ? dbAsyncDelete(db,key) :</span><br><span class="line"> dbSyncDelete(db,key);</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line"><span class="comment">/* Check if the key is expired. */</span></span><br><span class="line"><span class="function"><span class="keyword">int</span> <span class="title">keyIsExpired</span><span class="params">(redisDb *db, robj *key)</span> </span>&#123;</span><br><span class="line"> <span class="keyword">mstime_t</span> when = getExpire(db,key);</span><br><span class="line"> <span class="keyword">mstime_t</span> now;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (when &lt; <span class="number">0</span>) <span class="keyword">return</span> <span class="number">0</span>; <span class="comment">/* No expire for this key */</span></span><br><span class="line"></span><br><span class="line"> <span class="comment">/* Don't expire anything while loading. It will be done later. */</span></span><br><span class="line"> <span class="keyword">if</span> (server.loading) <span class="keyword">return</span> <span class="number">0</span>;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* If we are in the context of a Lua script, we pretend that time is</span></span><br><span class="line"><span class="comment"> * blocked to when the Lua script started. This way a key can expire</span></span><br><span class="line"><span class="comment"> * only the first time it is accessed and not in the middle of the</span></span><br><span class="line"><span class="comment"> * script execution, making propagation to slaves / AOF consistent.</span></span><br><span class="line"><span class="comment"> * See issue #1525 on Github for more information. */</span></span><br><span class="line"> <span class="keyword">if</span> (server.lua_caller) &#123;</span><br><span class="line"> now = server.lua_time_start;</span><br><span class="line"> &#125;</span><br><span class="line"> <span class="comment">/* If we are in the middle of a command execution, we still want to use</span></span><br><span class="line"><span class="comment"> * a reference time that does not change: in that case we just use the</span></span><br><span class="line"><span class="comment"> * cached time, that we update before each call in the call() function.</span></span><br><span class="line"><span class="comment"> * This way we avoid that commands such as RPOPLPUSH or similar, that</span></span><br><span class="line"><span class="comment"> * may re-open the same key multiple times, can invalidate an already</span></span><br><span class="line"><span class="comment"> * open object in a next call, if the next call will see the key expired,</span></span><br><span class="line"><span class="comment"> * while the first did not. */</span></span><br><span class="line"> <span class="keyword">else</span> <span class="keyword">if</span> (server.fixed_time_expire &gt; <span class="number">0</span>) &#123;</span><br><span class="line"> now = server.mstime;</span><br><span class="line"> &#125;</span><br><span class="line"> <span class="comment">/* For the other cases, we want to use the most fresh time we have. */</span></span><br><span class="line"> <span class="keyword">else</span> &#123;</span><br><span class="line"> now = mstime();</span><br><span class="line"> &#125;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* The key expired if the current (virtual or real) time is greater</span></span><br><span class="line"><span class="comment"> * than the expire time of the key. */</span></span><br><span class="line"> <span class="keyword">return</span> now &gt; when;</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">/* Return the expire time of the specified key, or -1 if no expire</span></span><br><span class="line"><span class="comment"> * is associated with this key (i.e. the key is non volatile) */</span></span><br><span class="line"><span class="function"><span class="keyword">long</span> <span class="keyword">long</span> <span class="title">getExpire</span><span class="params">(redisDb *db, robj *key)</span> </span>&#123;</span><br><span class="line"> dictEntry *de;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* No expire? return ASAP */</span></span><br><span class="line"> <span class="keyword">if</span> (dictSize(db-&gt;expires) == <span class="number">0</span> ||</span><br><span class="line"> (de = dictFind(db-&gt;expires,key-&gt;ptr)) == <span class="literal">NULL</span>) <span class="keyword">return</span> <span class="number">-1</span>;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* The entry was found in the expire dict, this means it should also</span></span><br><span class="line"><span class="comment"> * be present in the main dict (safety check). */</span></span><br><span class="line"> serverAssertWithInfo(<span class="literal">NULL</span>,key,dictFind(db-&gt;dict,key-&gt;ptr) != <span class="literal">NULL</span>);</span><br><span class="line"> <span class="keyword">return</span> dictGetSignedIntegerVal(de);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>这里有几点要注意的，第一是当惰性删除时会根据lazyfree_lazy_expire这个参数去判断是执行同步删除还是异步删除，另外一点是对于 slave，是不需要执行的，因为会在 master 过期时向 slave 发送 del 指令。<br>光采用这个策略会有什么问题呢，假如一些key 一直未被访问，那这些 key 就不会过期了，导致一直被占用着内存，所以 redis 采取了懒汉式过期加定期过期策略，定期策略是怎么执行的呢</p>
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class="line">302</span><br><span class="line">303</span><br><span class="line">304</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/* This function handles 'background' operations we are required to do</span></span><br><span class="line"><span class="comment"> * incrementally in Redis databases, such as active key expiring, resizing,</span></span><br><span class="line"><span class="comment"> * rehashing. */</span></span><br><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">databasesCron</span><span class="params">(<span class="keyword">void</span>)</span> </span>&#123;</span><br><span class="line"> <span class="comment">/* Expire keys by random sampling. Not required for slaves</span></span><br><span class="line"><span class="comment"> * as master will synthesize DELs for us. */</span></span><br><span class="line"> <span class="keyword">if</span> (server.active_expire_enabled) &#123;</span><br><span class="line"> <span class="keyword">if</span> (server.masterhost == <span class="literal">NULL</span>) &#123;</span><br><span class="line"> activeExpireCycle(ACTIVE_EXPIRE_CYCLE_SLOW);</span><br><span class="line"> &#125; <span class="keyword">else</span> &#123;</span><br><span class="line"> expireSlaveKeys();</span><br><span class="line"> &#125;</span><br><span class="line"> &#125;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* Defrag keys gradually. */</span></span><br><span class="line"> activeDefragCycle();</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* Perform hash tables rehashing if needed, but only if there are no</span></span><br><span class="line"><span class="comment"> * other processes saving the DB on disk. Otherwise rehashing is bad</span></span><br><span class="line"><span class="comment"> * as will cause a lot of copy-on-write of memory pages. */</span></span><br><span class="line"> <span class="keyword">if</span> (!hasActiveChildProcess()) &#123;</span><br><span class="line"> <span class="comment">/* We use global counters so if we stop the computation at a given</span></span><br><span class="line"><span class="comment"> * DB we'll be able to start from the successive in the next</span></span><br><span class="line"><span class="comment"> * cron loop iteration. */</span></span><br><span class="line"> <span class="keyword">static</span> <span class="keyword">unsigned</span> <span class="keyword">int</span> resize_db = <span class="number">0</span>;</span><br><span class="line"> <span class="keyword">static</span> <span class="keyword">unsigned</span> <span class="keyword">int</span> rehash_db = <span class="number">0</span>;</span><br><span class="line"> <span class="keyword">int</span> dbs_per_call = CRON_DBS_PER_CALL;</span><br><span class="line"> <span class="keyword">int</span> j;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* Don't test more DBs than we have. */</span></span><br><span class="line"> <span class="keyword">if</span> (dbs_per_call &gt; server.dbnum) dbs_per_call = server.dbnum;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* Resize */</span></span><br><span class="line"> <span class="keyword">for</span> (j = <span class="number">0</span>; j &lt; dbs_per_call; j++) &#123;</span><br><span class="line"> tryResizeHashTables(resize_db % server.dbnum);</span><br><span class="line"> resize_db++;</span><br><span class="line"> &#125;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* Rehash */</span></span><br><span class="line"> <span class="keyword">if</span> (server.activerehashing) &#123;</span><br><span class="line"> <span class="keyword">for</span> (j = <span class="number">0</span>; j &lt; dbs_per_call; j++) &#123;</span><br><span class="line"> <span class="keyword">int</span> work_done = incrementallyRehash(rehash_db);</span><br><span class="line"> <span class="keyword">if</span> (work_done) &#123;</span><br><span class="line"> <span class="comment">/* If the function did some work, stop here, we'll do</span></span><br><span class="line"><span class="comment"> * more at the next cron loop. */</span></span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line"> &#125; <span class="keyword">else</span> &#123;</span><br><span class="line"> <span class="comment">/* If this db didn't need rehash, we'll try the next one. */</span></span><br><span class="line"> rehash_db++;</span><br><span class="line"> rehash_db %= server.dbnum;</span><br><span class="line"> &#125;</span><br><span class="line"> &#125;</span><br><span class="line"> &#125;</span><br><span class="line"> &#125;</span><br><span class="line">&#125;</span><br><span class="line"><span class="comment">/* Try to expire a few timed out keys. The algorithm used is adaptive and</span></span><br><span class="line"><span class="comment"> * will use few CPU cycles if there are few expiring keys, otherwise</span></span><br><span class="line"><span class="comment"> * it will get more aggressive to avoid that too much memory is used by</span></span><br><span class="line"><span class="comment"> * keys that can be removed from the keyspace.</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * Every expire cycle tests multiple databases: the next call will start</span></span><br><span class="line"><span class="comment"> * again from the next db, with the exception of exists for time limit: in that</span></span><br><span class="line"><span class="comment"> * case we restart again from the last database we were processing. Anyway</span></span><br><span class="line"><span class="comment"> * no more than CRON_DBS_PER_CALL databases are tested at every iteration.</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * The function can perform more or less work, depending on the "type"</span></span><br><span class="line"><span class="comment"> * argument. It can execute a "fast cycle" or a "slow cycle". The slow</span></span><br><span class="line"><span class="comment"> * cycle is the main way we collect expired cycles: this happens with</span></span><br><span class="line"><span class="comment"> * the "server.hz" frequency (usually 10 hertz).</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * However the slow cycle can exit for timeout, since it used too much time.</span></span><br><span class="line"><span class="comment"> * For this reason the function is also invoked to perform a fast cycle</span></span><br><span class="line"><span class="comment"> * at every event loop cycle, in the beforeSleep() function. The fast cycle</span></span><br><span class="line"><span class="comment"> * will try to perform less work, but will do it much more often.</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * The following are the details of the two expire cycles and their stop</span></span><br><span class="line"><span class="comment"> * conditions:</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * If type is ACTIVE_EXPIRE_CYCLE_FAST the function will try to run a</span></span><br><span class="line"><span class="comment"> * "fast" expire cycle that takes no longer than EXPIRE_FAST_CYCLE_DURATION</span></span><br><span class="line"><span class="comment"> * microseconds, and is not repeated again before the same amount of time.</span></span><br><span class="line"><span class="comment"> * The cycle will also refuse to run at all if the latest slow cycle did not</span></span><br><span class="line"><span class="comment"> * terminate because of a time limit condition.</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * If type is ACTIVE_EXPIRE_CYCLE_SLOW, that normal expire cycle is</span></span><br><span class="line"><span class="comment"> * executed, where the time limit is a percentage of the REDIS_HZ period</span></span><br><span class="line"><span class="comment"> * as specified by the ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC define. In the</span></span><br><span class="line"><span class="comment"> * fast cycle, the check of every database is interrupted once the number</span></span><br><span class="line"><span class="comment"> * of already expired keys in the database is estimated to be lower than</span></span><br><span class="line"><span class="comment"> * a given percentage, in order to avoid doing too much work to gain too</span></span><br><span class="line"><span class="comment"> * little memory.</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * The configured expire "effort" will modify the baseline parameters in</span></span><br><span class="line"><span class="comment"> * order to do more work in both the fast and slow expire cycles.</span></span><br><span class="line"><span class="comment"> */</span></span><br><span class="line"></span><br><span class="line"><span class="meta">#<span class="meta-keyword">define</span> ACTIVE_EXPIRE_CYCLE_KEYS_PER_LOOP 20 <span class="comment">/* Keys for each DB loop. */</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">define</span> ACTIVE_EXPIRE_CYCLE_FAST_DURATION 1000 <span class="comment">/* Microseconds. */</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">define</span> ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC 25 <span class="comment">/* Max % of CPU to use. */</span></span></span><br><span class="line"><span class="meta">#<span class="meta-keyword">define</span> ACTIVE_EXPIRE_CYCLE_ACCEPTABLE_STALE 10 <span class="comment">/* % of stale keys after which</span></span></span><br><span class="line"><span class="meta"><span class="comment"> we do extra efforts. */</span></span></span><br><span class="line"><span class="function"><span class="keyword">void</span> <span class="title">activeExpireCycle</span><span class="params">(<span class="keyword">int</span> type)</span> </span>&#123;</span><br><span class="line"> <span class="comment">/* Adjust the running parameters according to the configured expire</span></span><br><span class="line"><span class="comment"> * effort. The default effort is 1, and the maximum configurable effort</span></span><br><span class="line"><span class="comment"> * is 10. */</span></span><br><span class="line"> <span class="keyword">unsigned</span> <span class="keyword">long</span></span><br><span class="line"> effort = server.active_expire_effort<span class="number">-1</span>, <span class="comment">/* Rescale from 0 to 9. */</span></span><br><span class="line"> config_keys_per_loop = ACTIVE_EXPIRE_CYCLE_KEYS_PER_LOOP +</span><br><span class="line"> ACTIVE_EXPIRE_CYCLE_KEYS_PER_LOOP/<span class="number">4</span>*effort,</span><br><span class="line"> config_cycle_fast_duration = ACTIVE_EXPIRE_CYCLE_FAST_DURATION +</span><br><span class="line"> ACTIVE_EXPIRE_CYCLE_FAST_DURATION/<span class="number">4</span>*effort,</span><br><span class="line"> config_cycle_slow_time_perc = ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC +</span><br><span class="line"> <span class="number">2</span>*effort,</span><br><span class="line"> config_cycle_acceptable_stale = ACTIVE_EXPIRE_CYCLE_ACCEPTABLE_STALE-</span><br><span class="line"> effort;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* This function has some global state in order to continue the work</span></span><br><span class="line"><span class="comment"> * incrementally across calls. */</span></span><br><span class="line"> <span class="keyword">static</span> <span class="keyword">unsigned</span> <span class="keyword">int</span> current_db = <span class="number">0</span>; <span class="comment">/* Last DB tested. */</span></span><br><span class="line"> <span class="keyword">static</span> <span class="keyword">int</span> timelimit_exit = <span class="number">0</span>; <span class="comment">/* Time limit hit in previous call? */</span></span><br><span class="line"> <span class="keyword">static</span> <span class="keyword">long</span> <span class="keyword">long</span> last_fast_cycle = <span class="number">0</span>; <span class="comment">/* When last fast cycle ran. */</span></span><br><span class="line"></span><br><span class="line"> <span class="keyword">int</span> j, iteration = <span class="number">0</span>;</span><br><span class="line"> <span class="keyword">int</span> dbs_per_call = CRON_DBS_PER_CALL;</span><br><span class="line"> <span class="keyword">long</span> <span class="keyword">long</span> start = ustime(), timelimit, elapsed;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* When clients are paused the dataset should be static not just from the</span></span><br><span class="line"><span class="comment"> * POV of clients not being able to write, but also from the POV of</span></span><br><span class="line"><span class="comment"> * expires and evictions of keys not being performed. */</span></span><br><span class="line"> <span class="keyword">if</span> (clientsArePaused()) <span class="keyword">return</span>;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (type == ACTIVE_EXPIRE_CYCLE_FAST) &#123;</span><br><span class="line"> <span class="comment">/* Don't start a fast cycle if the previous cycle did not exit</span></span><br><span class="line"><span class="comment"> * for time limit, unless the percentage of estimated stale keys is</span></span><br><span class="line"><span class="comment"> * too high. Also never repeat a fast cycle for the same period</span></span><br><span class="line"><span class="comment"> * as the fast cycle total duration itself. */</span></span><br><span class="line"> <span class="keyword">if</span> (!timelimit_exit &amp;&amp;</span><br><span class="line"> server.stat_expired_stale_perc &lt; config_cycle_acceptable_stale)</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (start &lt; last_fast_cycle + (<span class="keyword">long</span> <span class="keyword">long</span>)config_cycle_fast_duration*<span class="number">2</span>)</span><br><span class="line"> <span class="keyword">return</span>;</span><br><span class="line"></span><br><span class="line"> last_fast_cycle = start;</span><br><span class="line"> &#125;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* We usually should test CRON_DBS_PER_CALL per iteration, with</span></span><br><span class="line"><span class="comment"> * two exceptions:</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * 1) Don't test more DBs than we have.</span></span><br><span class="line"><span class="comment"> * 2) If last time we hit the time limit, we want to scan all DBs</span></span><br><span class="line"><span class="comment"> * in this iteration, as there is work to do in some DB and we don't want</span></span><br><span class="line"><span class="comment"> * expired keys to use memory for too much time. */</span></span><br><span class="line"> <span class="keyword">if</span> (dbs_per_call &gt; server.dbnum || timelimit_exit)</span><br><span class="line"> dbs_per_call = server.dbnum;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* We can use at max 'config_cycle_slow_time_perc' percentage of CPU</span></span><br><span class="line"><span class="comment"> * time per iteration. Since this function gets called with a frequency of</span></span><br><span class="line"><span class="comment"> * server.hz times per second, the following is the max amount of</span></span><br><span class="line"><span class="comment"> * microseconds we can spend in this function. */</span></span><br><span class="line"> timelimit = config_cycle_slow_time_perc*<span class="number">1000000</span>/server.hz/<span class="number">100</span>;</span><br><span class="line"> timelimit_exit = <span class="number">0</span>;</span><br><span class="line"> <span class="keyword">if</span> (timelimit &lt;= <span class="number">0</span>) timelimit = <span class="number">1</span>;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (type == ACTIVE_EXPIRE_CYCLE_FAST)</span><br><span class="line"> timelimit = config_cycle_fast_duration; <span class="comment">/* in microseconds. */</span></span><br><span class="line"></span><br><span class="line"> <span class="comment">/* Accumulate some global stats as we expire keys, to have some idea</span></span><br><span class="line"><span class="comment"> * about the number of keys that are already logically expired, but still</span></span><br><span class="line"><span class="comment"> * existing inside the database. */</span></span><br><span class="line"> <span class="keyword">long</span> total_sampled = <span class="number">0</span>;</span><br><span class="line"> <span class="keyword">long</span> total_expired = <span class="number">0</span>;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">for</span> (j = <span class="number">0</span>; j &lt; dbs_per_call &amp;&amp; timelimit_exit == <span class="number">0</span>; j++) &#123;</span><br><span class="line"> <span class="comment">/* Expired and checked in a single loop. */</span></span><br><span class="line"> <span class="keyword">unsigned</span> <span class="keyword">long</span> expired, sampled;</span><br><span class="line"></span><br><span class="line"> redisDb *db = server.db+(current_db % server.dbnum);</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* Increment the DB now so we are sure if we run out of time</span></span><br><span class="line"><span class="comment"> * in the current DB we'll restart from the next. This allows to</span></span><br><span class="line"><span class="comment"> * distribute the time evenly across DBs. */</span></span><br><span class="line"> current_db++;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* Continue to expire if at the end of the cycle more than 25%</span></span><br><span class="line"><span class="comment"> * of the keys were expired. */</span></span><br><span class="line"> <span class="keyword">do</span> &#123;</span><br><span class="line"> <span class="keyword">unsigned</span> <span class="keyword">long</span> num, slots;</span><br><span class="line"> <span class="keyword">long</span> <span class="keyword">long</span> now, ttl_sum;</span><br><span class="line"> <span class="keyword">int</span> ttl_samples;</span><br><span class="line"> iteration++;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* If there is nothing to expire try next DB ASAP. */</span></span><br><span class="line"> <span class="keyword">if</span> ((num = dictSize(db-&gt;expires)) == <span class="number">0</span>) &#123;</span><br><span class="line"> db-&gt;avg_ttl = <span class="number">0</span>;</span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line"> &#125;</span><br><span class="line"> slots = dictSlots(db-&gt;expires);</span><br><span class="line"> now = mstime();</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* When there are less than 1% filled slots, sampling the key</span></span><br><span class="line"><span class="comment"> * space is expensive, so stop here waiting for better times...</span></span><br><span class="line"><span class="comment"> * The dictionary will be resized asap. */</span></span><br><span class="line"> <span class="keyword">if</span> (num &amp;&amp; slots &gt; DICT_HT_INITIAL_SIZE &amp;&amp;</span><br><span class="line"> (num*<span class="number">100</span>/slots &lt; <span class="number">1</span>)) <span class="keyword">break</span>;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* The main collection cycle. Sample random keys among keys</span></span><br><span class="line"><span class="comment"> * with an expire set, checking for expired ones. */</span></span><br><span class="line"> expired = <span class="number">0</span>;</span><br><span class="line"> sampled = <span class="number">0</span>;</span><br><span class="line"> ttl_sum = <span class="number">0</span>;</span><br><span class="line"> ttl_samples = <span class="number">0</span>;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">if</span> (num &gt; config_keys_per_loop)</span><br><span class="line"> num = config_keys_per_loop;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* Here we access the low level representation of the hash table</span></span><br><span class="line"><span class="comment"> * for speed concerns: this makes this code coupled with dict.c,</span></span><br><span class="line"><span class="comment"> * but it hardly changed in ten years.</span></span><br><span class="line"><span class="comment"> *</span></span><br><span class="line"><span class="comment"> * Note that certain places of the hash table may be empty,</span></span><br><span class="line"><span class="comment"> * so we want also a stop condition about the number of</span></span><br><span class="line"><span class="comment"> * buckets that we scanned. However scanning for free buckets</span></span><br><span class="line"><span class="comment"> * is very fast: we are in the cache line scanning a sequential</span></span><br><span class="line"><span class="comment"> * array of NULL pointers, so we can scan a lot more buckets</span></span><br><span class="line"><span class="comment"> * than keys in the same time. */</span></span><br><span class="line"> <span class="keyword">long</span> max_buckets = num*<span class="number">20</span>;</span><br><span class="line"> <span class="keyword">long</span> checked_buckets = <span class="number">0</span>;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">while</span> (sampled &lt; num &amp;&amp; checked_buckets &lt; max_buckets) &#123;</span><br><span class="line"> <span class="keyword">for</span> (<span class="keyword">int</span> table = <span class="number">0</span>; table &lt; <span class="number">2</span>; table++) &#123;</span><br><span class="line"> <span class="keyword">if</span> (table == <span class="number">1</span> &amp;&amp; !dictIsRehashing(db-&gt;expires)) <span class="keyword">break</span>;</span><br><span class="line"></span><br><span class="line"> <span class="keyword">unsigned</span> <span class="keyword">long</span> idx = db-&gt;expires_cursor;</span><br><span class="line"> idx &amp;= db-&gt;expires-&gt;ht[table].sizemask;</span><br><span class="line"> dictEntry *de = db-&gt;expires-&gt;ht[table].table[idx];</span><br><span class="line"> <span class="keyword">long</span> <span class="keyword">long</span> ttl;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* Scan the current bucket of the current table. */</span></span><br><span class="line"> checked_buckets++;</span><br><span class="line"> <span class="keyword">while</span>(de) &#123;</span><br><span class="line"> <span class="comment">/* Get the next entry now since this entry may get</span></span><br><span class="line"><span class="comment"> * deleted. */</span></span><br><span class="line"> dictEntry *e = de;</span><br><span class="line"> de = de-&gt;next;</span><br><span class="line"></span><br><span class="line"> ttl = dictGetSignedIntegerVal(e)-now;</span><br><span class="line"> <span class="keyword">if</span> (activeExpireCycleTryExpire(db,e,now)) expired++;</span><br><span class="line"> <span class="keyword">if</span> (ttl &gt; <span class="number">0</span>) &#123;</span><br><span class="line"> <span class="comment">/* We want the average TTL of keys yet</span></span><br><span class="line"><span class="comment"> * not expired. */</span></span><br><span class="line"> ttl_sum += ttl;</span><br><span class="line"> ttl_samples++;</span><br><span class="line"> &#125;</span><br><span class="line"> sampled++;</span><br><span class="line"> &#125;</span><br><span class="line"> &#125;</span><br><span class="line"> db-&gt;expires_cursor++;</span><br><span class="line"> &#125;</span><br><span class="line"> total_expired += expired;</span><br><span class="line"> total_sampled += sampled;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* Update the average TTL stats for this database. */</span></span><br><span class="line"> <span class="keyword">if</span> (ttl_samples) &#123;</span><br><span class="line"> <span class="keyword">long</span> <span class="keyword">long</span> avg_ttl = ttl_sum/ttl_samples;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* Do a simple running average with a few samples.</span></span><br><span class="line"><span class="comment"> * We just use the current estimate with a weight of 2%</span></span><br><span class="line"><span class="comment"> * and the previous estimate with a weight of 98%. */</span></span><br><span class="line"> <span class="keyword">if</span> (db-&gt;avg_ttl == <span class="number">0</span>) db-&gt;avg_ttl = avg_ttl;</span><br><span class="line"> db-&gt;avg_ttl = (db-&gt;avg_ttl/<span class="number">50</span>)*<span class="number">49</span> + (avg_ttl/<span class="number">50</span>);</span><br><span class="line"> &#125;</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* We can't block forever here even if there are many keys to</span></span><br><span class="line"><span class="comment"> * expire. So after a given amount of milliseconds return to the</span></span><br><span class="line"><span class="comment"> * caller waiting for the other active expire cycle. */</span></span><br><span class="line"> <span class="keyword">if</span> ((iteration &amp; <span class="number">0xf</span>) == <span class="number">0</span>) &#123; <span class="comment">/* check once every 16 iterations. */</span></span><br><span class="line"> elapsed = ustime()-start;</span><br><span class="line"> <span class="keyword">if</span> (elapsed &gt; timelimit) &#123;</span><br><span class="line"> timelimit_exit = <span class="number">1</span>;</span><br><span class="line"> server.stat_expired_time_cap_reached_count++;</span><br><span class="line"> <span class="keyword">break</span>;</span><br><span class="line"> &#125;</span><br><span class="line"> &#125;</span><br><span class="line"> <span class="comment">/* We don't repeat the cycle for the current database if there are</span></span><br><span class="line"><span class="comment"> * an acceptable amount of stale keys (logically expired but yet</span></span><br><span class="line"><span class="comment"> * not reclained). */</span></span><br><span class="line"> &#125; <span class="keyword">while</span> ((expired*<span class="number">100</span>/sampled) &gt; config_cycle_acceptable_stale);</span><br><span class="line"> &#125;</span><br><span class="line"></span><br><span class="line"> elapsed = ustime()-start;</span><br><span class="line"> server.stat_expire_cycle_time_used += elapsed;</span><br><span class="line"> latencyAddSampleIfNeeded(<span class="string">"expire-cycle"</span>,elapsed/<span class="number">1000</span>);</span><br><span class="line"></span><br><span class="line"> <span class="comment">/* Update our estimate of keys existing but yet to be expired.</span></span><br><span class="line"><span class="comment"> * Running average with this sample accounting for 5%. */</span></span><br><span class="line"> <span class="keyword">double</span> current_perc;</span><br><span class="line"> <span class="keyword">if</span> (total_sampled) &#123;</span><br><span class="line"> current_perc = (<span class="keyword">double</span>)total_expired/total_sampled;</span><br><span class="line"> &#125; <span class="keyword">else</span></span><br><span class="line"> current_perc = <span class="number">0</span>;</span><br><span class="line"> server.stat_expired_stale_perc = (current_perc*<span class="number">0.05</span>)+</span><br><span class="line"> (server.stat_expired_stale_perc*<span class="number">0.95</span>);</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>
<p>执行定期清除分成两种类型，快和慢，分别由<code>beforeSleep</code>和<code>databasesCron</code>调用，快版有两个限制，一个是执行时长由ACTIVE_EXPIRE_CYCLE_FAST_DURATION限制，另一个是执行间隔是 2 倍的ACTIVE_EXPIRE_CYCLE_FAST_DURATION，另外这还可以由配置的server.active_expire_effort参数来控制，默认是 1，最大是 10</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">onfig_cycle_fast_duration = ACTIVE_EXPIRE_CYCLE_FAST_DURATION +</span><br><span class="line"> ACTIVE_EXPIRE_CYCLE_FAST_DURATION/<span class="number">4</span>*effort</span><br></pre></td></tr></table></figure>
<p>然后会从一定数量的 db 中找出一定数量的带过期时间的 key（保存在 expires中），这里的数量是由</p>
<figure class="highlight c"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">config_keys_per_loop = ACTIVE_EXPIRE_CYCLE_KEYS_PER_LOOP +</span><br><span class="line"> ACTIVE_EXPIRE_CYCLE_KEYS_PER_LOOP/<span class="number">4</span>*effort</span><br><span class="line">``` </span><br><span class="line">控制，慢速的执行时长是</span><br><span class="line">```C</span><br><span class="line">config_cycle_slow_time_perc = ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC +</span><br><span class="line"> <span class="number">2</span>*effort</span><br><span class="line">timelimit = config_cycle_slow_time_perc*<span class="number">1000000</span>/server.hz/<span class="number">100</span>;</span><br></pre></td></tr></table></figure>
<p>这里还有一个额外的退出条件，如果当前数据库的抽样结果已经达到我们所允许的过期 key 百分比，则下次不再处理当前 db，继续处理下个 db</p>
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