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itemprop=description></span><span hidden itemprop=post itemscope itemtype=http://schema.org/CreativeWork><meta content="redis系列介绍八-淘汰策略 | Nicksxs's Blog" itemprop=name><meta content="说完了过期策略再说下淘汰策略,redis 使用的策略是近似的 lru 策略,为什么是近似的呢,先来看下什么是 lru,看下 wiki 的介绍,图中一共有四个槽的存储空间,依次访问顺序是 A B C D E D F, 当第一次访问 D 时刚好占满了坑,并且值是 4,这个值越小代表…" itemprop=description></span><header class=post-header><h1 itemprop="name headline" class=post-title>redis系列介绍八-淘汰策略</h1><div class=post-meta-container><div class=post-meta><span class=post-meta-item><span class=post-meta-item-icon><i class="far fa-calendar"></i> </span><span class=post-meta-item-text>发表于</span> <time itemprop="dateCreated datePublished" title="创建时间:2020-04-18 23:23:41" datetime=2020-04-18T23:23:41+08:00>2020-04-18</time> </span><span class=post-meta-item><span class=post-meta-item-icon><i class="far fa-folder"></i> </span><span class=post-meta-item-text>分类于</span> <span itemprop=about itemscope itemtype=http://schema.org/Thing><a href=/categories/Redis/ itemprop=url rel=index><span itemprop=name>Redis</span></a> </span>, <span itemprop=about itemscope itemtype=http://schema.org/Thing><a href=/categories/Redis/%E6%95%B0%E6%8D%AE%E7%BB%93%E6%9E%84/ itemprop=url rel=index><span itemprop=name>数据结构</span></a> </span>, <span itemprop=about itemscope itemtype=http://schema.org/Thing><a href=/categories/C/ itemprop=url rel=index><span itemprop=name>C</span></a> </span>, <span itemprop=about itemscope itemtype=http://schema.org/Thing><a href=/categories/Redis/%E6%BA%90%E7%A0%81/ itemprop=url rel=index><span itemprop=name>源码</span></a> </span>, <span itemprop=about itemscope itemtype=http://schema.org/Thing><a href=/categories/C/Redis/ itemprop=url rel=index><span itemprop=name>Redis</span></a> </span></span><span class="post-meta-item cloudflare-page-counter" data-increment=true data-path=/2020/04/18/redis%E7%B3%BB%E5%88%97%E4%BB%8B%E7%BB%8D%E5%85%AB/ data-title=redis系列介绍八-淘汰策略 title=阅读次数><span class=post-meta-item-icon><i class="far fa-eye"></i> </span><span class=post-meta-item-text>阅读次数:</span> <span class=cloudflare-page-counter-count></span></span></div><div class=post-description>说完了过期策略再说下淘汰策略,redis 使用的策略是近似的 lru 策略,为什么是近似的呢,先来看下什么是 lru,看下 wiki 的介绍,图中一共有四个槽的存储空间,依次访问顺序是 A B C D E D F, 当第一次访问 D 时刚好占满了坑,并且值是 4,这个值越小代表…</div></div></header><div class=post-body itemprop=articleBody><h3 id=LRU><a class=headerlink href=#LRU title=LRU></a>LRU</h3><p>说完了过期策略再说下淘汰策略,redis 使用的策略是近似的 lru 策略,为什么是近似的呢,先来看下什么是 lru,看下 wiki 的介绍<br><img data-src=https://img.nicksxs.me/uPic/z5cDpo.jpg>,图中一共有四个槽的存储空间,依次访问顺序是 A B C D E D F,<br>当第一次访问 D 时刚好占满了坑,并且值是 4,这个值越小代表越先被淘汰,当 E 进来时,看了下已经存在的四个里 A 是最小的,代表是最早存在并且最早被访问的,那就先淘汰它了,E 占领了 A 的位置,并设置值为 4,然后又访问 D 了,D 已经存在了,不过又被访问到了,得更新值为 5,然后是 F 进来了,这时 B 是最老的且最近未被访问,所以就淘汰它了。以上是一个 lru 的简要说明,但是 redis 没有严格按照这个去执行,理由跟前面过期策略一致,最严格的过期策略应该是每个 key 都有对应的定时器,当超时时马上就能清除,但是问题是这样的cpu 消耗太大,所换来的内存效率不太值得,淘汰策略也是这样,类似于上图,要维护所有 key 的一个有序 lru 值,并且遍历将最小的淘汰,redis 采用的是抽样的形式,最初的实现方式是随机从 dict 抽取 5 个 key,淘汰一个 lru 最小的,这样子勉强能达到淘汰的目的,但是效果不是特别好,后面在 redis 3.0开始,将随机抽取改成了维护一个 pool,pool 的大小默认是 16,每次放入的都是按lru 值有序排列好,每一次放入的必须是 lru小于 pool 中最小的 lru 才允许放入,直到放满,后面再有新的就会将大的踢出。<br>redis 针对这个策略的改进做了一个实验,这里借用下图<br><img data-src=https://img.nicksxs.me/uPic/lEx4Ug.jpg><br>首先背景是这图中的所有点都对应一个 redis 的 key,灰色部分加入后被顺序访问过一遍,然后又加入了绿色部分,那么按照理论的 lru 算法,应该是图左上中,浅灰色部分全都被淘汰,那么对比来看看图右上,左下和右下,左下表示 2.8 版本就是随机抽样 5 个 key,淘汰其中 lru 最小的一个,发现是灰色和浅灰色的都有被淘汰的,右下的 3.0 版本抽样数量不变的情况下,稍好一些,当 3.0 版本的抽样数量调整成 10 后,已经较为接近理论上的 lru 策略了,通过代码来简要分析下<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></pre><td class=code><pre><span class=line><span class=keyword>typedef</span> <span class=class><span class=keyword>struct</span> <span class=title>redisObject</span> {</span></span><br><span class=line> <span class=type>unsigned</span> type:<span class=number>4</span>;</span><br><span class=line> <span class=type>unsigned</span> encoding:<span class=number>4</span>;</span><br><span class=line> <span class=type>unsigned</span> lru:LRU_BITS; <span class=comment>/* LRU time (relative to global lru_clock) or</span></span><br><span class=line><span class=comment> * LFU data (least significant 8 bits frequency</span></span><br><span class=line><span class=comment> * and most significant 16 bits access time). */</span></span><br><span class=line> <span class=type>int</span> refcount;</span><br><span class=line> <span class=type>void</span> *ptr;</span><br><span class=line>} robj;</span><br></pre></table></figure><p>对于 lru 策略来说,lru 字段记录的就是<a href=https://github.com/antirez/redis/blob/unstable/src/server.h#L603 rel=noopener target=_blank><code>redisObj</code></a> 的LRU time,<br>redis 在访问数据时,都会调用<a href=https://github.com/antirez/redis/blob/unstable/src/db.c#L55 rel=noopener target=_blank><code>lookupKey</code></a>方法<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></pre><td class=code><pre><span class=line><span class=comment>/* Low level key lookup API, not actually called directly from commands</span></span><br><span class=line><span class=comment> * implementations that should instead rely on lookupKeyRead(),</span></span><br><span class=line><span class=comment> * lookupKeyWrite() and lookupKeyReadWithFlags(). */</span></span><br><span class=line>robj *<span class="title function_">lookupKey</span><span class=params>(redisDb *db, robj *key, <span class=type>int</span> flags)</span> {</span><br><span class=line> dictEntry *de = dictFind(db->dict,key->ptr);</span><br><span class=line> <span class=keyword>if</span> (de) {</span><br><span class=line> robj *val = dictGetVal(de);</span><br><span class=line></span><br><span class=line> <span class=comment>/* Update the access time for the ageing algorithm.</span></span><br><span class=line><span class=comment> * Don't do it if we have a saving child, as this will trigger</span></span><br><span class=line><span class=comment> * a copy on write madness. */</span></span><br><span class=line> <span class=keyword>if</span> (!hasActiveChildProcess() && !(flags & LOOKUP_NOTOUCH)){</span><br><span class=line> <span class=keyword>if</span> (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {</span><br><span class=line> <span class=comment>// 这个是后面一节的内容</span></span><br><span class=line> updateLFU(val);</span><br><span class=line> } <span class=keyword>else</span> {</span><br><span class=line> <span class=comment>// 对于这个分支,访问时就会去更新 lru 值</span></span><br><span class=line> val->lru = LRU_CLOCK();</span><br><span class=line> }</span><br><span class=line> }</span><br><span class=line> <span class=keyword>return</span> val;</span><br><span class=line> } <span class=keyword>else</span> {</span><br><span class=line> <span class=keyword>return</span> <span class=literal>NULL</span>;</span><br><span class=line> }</span><br><span class=line>}</span><br><span class=line><span class=comment>/* This function is used to obtain the current LRU clock.</span></span><br><span class=line><span class=comment> * If the current resolution is lower than the frequency we refresh the</span></span><br><span class=line><span class=comment> * LRU clock (as it should be in production servers) we return the</span></span><br><span class=line><span class=comment> * precomputed value, otherwise we need to resort to a system call. */</span></span><br><span class=line><span class=type>unsigned</span> <span class=type>int</span> <span class="title function_">LRU_CLOCK</span><span class=params>(<span class=type>void</span>)</span> {</span><br><span class=line> <span class=type>unsigned</span> <span class=type>int</span> lruclock;</span><br><span class=line> <span class=keyword>if</span> (<span class=number>1000</span>/server.hz <= LRU_CLOCK_RESOLUTION) {</span><br><span class=line> <span class=comment>// 如果服务器的频率server.hz大于 1 时就是用系统预设的 lruclock</span></span><br><span class=line> lruclock = server.lruclock;</span><br><span class=line> } <span class=keyword>else</span> {</span><br><span class=line> lruclock = getLRUClock();</span><br><span class=line> }</span><br><span class=line> <span class=keyword>return</span> lruclock;</span><br><span class=line>}</span><br><span class=line><span class=comment>/* Return the LRU clock, based on the clock resolution. This is a time</span></span><br><span class=line><span class=comment> * in a reduced-bits format that can be used to set and check the</span></span><br><span class=line><span class=comment> * object->lru field of redisObject structures. */</span></span><br><span class=line><span class=type>unsigned</span> <span class=type>int</span> <span class="title function_">getLRUClock</span><span class=params>(<span class=type>void</span>)</span> {</span><br><span class=line> <span class=keyword>return</span> (mstime()/LRU_CLOCK_RESOLUTION) & LRU_CLOCK_MAX;</span><br><span class=line>}</span><br></pre></table></figure><p>redis 处理命令是在这里<a href=https://github.com/antirez/redis/blob/unstable/src/server.c#L3355 rel=noopener target=_blank><code>processCommand</code></a><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></pre><td class=code><pre><span class=line><span class=comment>/* If this function gets called we already read a whole</span></span><br><span class=line><span class=comment> * command, arguments are in the client argv/argc fields.</span></span><br><span class=line><span class=comment> * processCommand() execute the command or prepare the</span></span><br><span class=line><span class=comment> * server for a bulk read from the client.</span></span><br><span class=line><span class=comment> *</span></span><br><span class=line><span class=comment> * If C_OK is returned the client is still alive and valid and</span></span><br><span class=line><span class=comment> * other operations can be performed by the caller. Otherwise</span></span><br><span class=line><span class=comment> * if C_ERR is returned the client was destroyed (i.e. after QUIT). */</span></span><br><span class=line><span class=type>int</span> <span class="title function_">processCommand</span><span class=params>(client *c)</span> {</span><br><span class=line> moduleCallCommandFilters(c);</span><br><span class=line></span><br><span class=line> </span><br><span class=line></span><br><span class=line> <span class=comment>/* Handle the maxmemory directive.</span></span><br><span class=line><span class=comment> *</span></span><br><span class=line><span class=comment> * Note that we do not want to reclaim memory if we are here re-entering</span></span><br><span class=line><span class=comment> * the event loop since there is a busy Lua script running in timeout</span></span><br><span class=line><span class=comment> * condition, to avoid mixing the propagation of scripts with the</span></span><br><span class=line><span class=comment> * propagation of DELs due to eviction. */</span></span><br><span class=line> <span class=keyword>if</span> (server.maxmemory && !server.lua_timedout) {</span><br><span class=line> <span class=type>int</span> out_of_memory = freeMemoryIfNeededAndSafe() == C_ERR;</span><br><span class=line> <span class=comment>/* freeMemoryIfNeeded may flush slave output buffers. This may result</span></span><br><span class=line><span class=comment> * into a slave, that may be the active client, to be freed. */</span></span><br><span class=line> <span class=keyword>if</span> (server.current_client == <span class=literal>NULL</span>) <span class=keyword>return</span> C_ERR;</span><br><span class=line></span><br><span class=line> <span class=comment>/* It was impossible to free enough memory, and the command the client</span></span><br><span class=line><span class=comment> * is trying to execute is denied during OOM conditions or the client</span></span><br><span class=line><span class=comment> * is in MULTI/EXEC context? Error. */</span></span><br><span class=line> <span class=keyword>if</span> (out_of_memory &&</span><br><span class=line> (c->cmd->flags & CMD_DENYOOM ||</span><br><span class=line> (c->flags & CLIENT_MULTI &&</span><br><span class=line> c->cmd->proc != execCommand &&</span><br><span class=line> c->cmd->proc != discardCommand)))</span><br><span class=line> {</span><br><span class=line> flagTransaction(c);</span><br><span class=line> addReply(c, shared.oomerr);</span><br><span class=line> <span class=keyword>return</span> C_OK;</span><br><span class=line> }</span><br><span class=line> }</span><br><span class=line>}</span><br></pre></table></figure><p>这里只摘了部分,当需要清理内存时就会调用, 然后调用了<a href=https://github.com/antirez/redis/blob/unstable/src/evict.c#L631 rel=noopener target=_blank><code>freeMemoryIfNeededAndSafe</code></a><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><span class=line>93</span><br><span class=line>94</span><br><span class=line>95</span><br><span class=line>96</span><br><span class=line>97</span><br><span class=line>98</span><br><span class=line>99</span><br><span class=line>100</span><br><span class=line>101</span><br><span class=line>102</span><br><span class=line>103</span><br><span class=line>104</span><br><span class=line>105</span><br><span class=line>106</span><br><span class=line>107</span><br><span class=line>108</span><br><span class=line>109</span><br><span class=line>110</span><br><span class=line>111</span><br><span class=line>112</span><br><span class=line>113</span><br><span class=line>114</span><br><span class=line>115</span><br><span class=line>116</span><br><span class=line>117</span><br><span class=line>118</span><br><span class=line>119</span><br><span class=line>120</span><br><span class=line>121</span><br><span class=line>122</span><br><span class=line>123</span><br><span class=line>124</span><br><span class=line>125</span><br><span class=line>126</span><br><span class=line>127</span><br><span class=line>128</span><br><span class=line>129</span><br><span class=line>130</span><br><span class=line>131</span><br><span class=line>132</span><br><span class=line>133</span><br><span class=line>134</span><br><span class=line>135</span><br><span class=line>136</span><br><span class=line>137</span><br><span class=line>138</span><br><span class=line>139</span><br><span class=line>140</span><br><span class=line>141</span><br><span class=line>142</span><br><span class=line>143</span><br><span class=line>144</span><br><span class=line>145</span><br><span class=line>146</span><br><span class=line>147</span><br><span class=line>148</span><br><span class=line>149</span><br><span class=line>150</span><br><span class=line>151</span><br><span class=line>152</span><br><span class=line>153</span><br><span class=line>154</span><br><span class=line>155</span><br><span class=line>156</span><br><span class=line>157</span><br><span class=line>158</span><br><span class=line>159</span><br><span class=line>160</span><br><span class=line>161</span><br><span class=line>162</span><br><span class=line>163</span><br><span class=line>164</span><br><span class=line>165</span><br><span class=line>166</span><br><span class=line>167</span><br><span class=line>168</span><br><span class=line>169</span><br><span class=line>170</span><br><span class=line>171</span><br><span class=line>172</span><br><span class=line>173</span><br><span class=line>174</span><br><span class=line>175</span><br><span class=line>176</span><br><span class=line>177</span><br><span class=line>178</span><br><span class=line>179</span><br><span class=line>180</span><br><span class=line>181</span><br><span class=line>182</span><br><span class=line>183</span><br><span class=line>184</span><br><span class=line>185</span><br><span class=line>186</span><br><span class=line>187</span><br><span class=line>188</span><br><span class=line>189</span><br><span class=line>190</span><br><span class=line>191</span><br><span class=line>192</span><br><span class=line>193</span><br><span class=line>194</span><br><span class=line>195</span><br><span class=line>196</span><br><span class=line>197</span><br></pre><td class=code><pre><span class=line><span class=comment>/* This is a wrapper for freeMemoryIfNeeded() that only really calls the</span></span><br><span class=line><span class=comment> * function if right now there are the conditions to do so safely:</span></span><br><span class=line><span class=comment> *</span></span><br><span class=line><span class=comment> * - There must be no script in timeout condition.</span></span><br><span class=line><span class=comment> * - Nor we are loading data right now.</span></span><br><span class=line><span class=comment> *</span></span><br><span class=line><span class=comment> */</span></span><br><span class=line><span class=type>int</span> <span class="title function_">freeMemoryIfNeededAndSafe</span><span class=params>(<span class=type>void</span>)</span> {</span><br><span class=line> <span class=keyword>if</span> (server.lua_timedout || server.loading) <span class=keyword>return</span> C_OK;</span><br><span class=line> <span class=keyword>return</span> freeMemoryIfNeeded();</span><br><span class=line>}</span><br><span class=line><span class=comment>/* This function is periodically called to see if there is memory to free</span></span><br><span class=line><span class=comment> * according to the current "maxmemory" settings. In case we are over the</span></span><br><span class=line><span class=comment> * memory limit, the function will try to free some memory to return back</span></span><br><span class=line><span class=comment> * under the limit.</span></span><br><span class=line><span class=comment> *</span></span><br><span class=line><span class=comment> * The function returns C_OK if we are under the memory limit or if we</span></span><br><span class=line><span class=comment> * were over the limit, but the attempt to free memory was successful.</span></span><br><span class=line><span class=comment> * Otehrwise if we are over the memory limit, but not enough memory</span></span><br><span class=line><span class=comment> * was freed to return back under the limit, the function returns C_ERR. */</span></span><br><span class=line><span class=type>int</span> <span class="title function_">freeMemoryIfNeeded</span><span class=params>(<span class=type>void</span>)</span> {</span><br><span class=line> <span class=type>int</span> keys_freed = <span class=number>0</span>;</span><br><span class=line> <span class=comment>/* By default replicas should ignore maxmemory</span></span><br><span class=line><span class=comment> * and just be masters exact copies. */</span></span><br><span class=line> <span class=keyword>if</span> (server.masterhost && server.repl_slave_ignore_maxmemory) <span class=keyword>return</span> C_OK;</span><br><span class=line></span><br><span class=line> <span class=type>size_t</span> mem_reported, mem_tofree, mem_freed;</span><br><span class=line> <span class=type>mstime_t</span> latency, eviction_latency;</span><br><span class=line> <span class=type>long</span> <span class=type>long</span> delta;</span><br><span class=line> <span class=type>int</span> slaves = listLength(server.slaves);</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> C_OK;</span><br><span class=line> <span class=keyword>if</span> (getMaxmemoryState(&mem_reported,<span class=literal>NULL</span>,&mem_tofree,<span class=literal>NULL</span>) == C_OK)</span><br><span class=line> <span class=keyword>return</span> C_OK;</span><br><span class=line></span><br><span class=line> mem_freed = <span class=number>0</span>;</span><br><span class=line></span><br><span class=line> <span class=keyword>if</span> (server.maxmemory_policy == MAXMEMORY_NO_EVICTION)</span><br><span class=line> <span class=keyword>goto</span> cant_free; <span class=comment>/* We need to free memory, but policy forbids. */</span></span><br><span class=line></span><br><span class=line> latencyStartMonitor(latency);</span><br><span class=line> <span class=keyword>while</span> (mem_freed < mem_tofree) {</span><br><span class=line> <span class=type>int</span> j, k, i;</span><br><span class=line> <span class=type>static</span> <span class=type>unsigned</span> <span class=type>int</span> next_db = <span class=number>0</span>;</span><br><span class=line> sds bestkey = <span class=literal>NULL</span>;</span><br><span class=line> <span class=type>int</span> bestdbid;</span><br><span class=line> redisDb *db;</span><br><span class=line> dict *dict;</span><br><span class=line> dictEntry *de;</span><br><span class=line></span><br><span class=line> <span class=keyword>if</span> (server.maxmemory_policy & (MAXMEMORY_FLAG_LRU|MAXMEMORY_FLAG_LFU) ||</span><br><span class=line> server.maxmemory_policy == MAXMEMORY_VOLATILE_TTL)</span><br><span class=line> {</span><br><span class=line> <span class=class><span class=keyword>struct</span> <span class=title>evictionPoolEntry</span> *<span class=title>pool</span> =</span> EvictionPoolLRU;</span><br><span class=line></span><br><span class=line> <span class=keyword>while</span>(bestkey == <span class=literal>NULL</span>) {</span><br><span class=line> <span class=type>unsigned</span> <span class=type>long</span> total_keys = <span class=number>0</span>, keys;</span><br><span class=line></span><br><span class=line> <span class=comment>/* We don't want to make local-db choices when expiring keys,</span></span><br><span class=line><span class=comment> * so to start populate the eviction pool sampling keys from</span></span><br><span class=line><span class=comment> * every DB. */</span></span><br><span class=line> <span class=keyword>for</span> (i = <span class=number>0</span>; i < server.dbnum; i++) {</span><br><span class=line> db = server.db+i;</span><br><span class=line> dict = (server.maxmemory_policy & MAXMEMORY_FLAG_ALLKEYS) ?</span><br><span class=line> db->dict : db->expires;</span><br><span class=line> <span class=keyword>if</span> ((keys = dictSize(dict)) != <span class=number>0</span>) {</span><br><span class=line> evictionPoolPopulate(i, dict, db->dict, pool);</span><br><span class=line> total_keys += keys;</span><br><span class=line> }</span><br><span class=line> }</span><br><span class=line> <span class=keyword>if</span> (!total_keys) <span class=keyword>break</span>; <span class=comment>/* No keys to evict. */</span></span><br><span class=line></span><br><span class=line> <span class=comment>/* Go backward from best to worst element to evict. */</span></span><br><span class=line> <span class=keyword>for</span> (k = EVPOOL_SIZE<span class=number>-1</span>; k >= <span class=number>0</span>; k--) {</span><br><span class=line> <span class=keyword>if</span> (pool[k].key == <span class=literal>NULL</span>) <span class=keyword>continue</span>;</span><br><span class=line> bestdbid = pool[k].dbid;</span><br><span class=line></span><br><span class=line> <span class=keyword>if</span> (server.maxmemory_policy & MAXMEMORY_FLAG_ALLKEYS) {</span><br><span class=line> de = dictFind(server.db[pool[k].dbid].dict,</span><br><span class=line> pool[k].key);</span><br><span class=line> } <span class=keyword>else</span> {</span><br><span class=line> de = dictFind(server.db[pool[k].dbid].expires,</span><br><span class=line> pool[k].key);</span><br><span class=line> }</span><br><span class=line></span><br><span class=line> <span class=comment>/* Remove the entry from the pool. */</span></span><br><span class=line> <span class=keyword>if</span> (pool[k].key != pool[k].cached)</span><br><span class=line> sdsfree(pool[k].key);</span><br><span class=line> pool[k].key = <span class=literal>NULL</span>;</span><br><span class=line> pool[k].idle = <span class=number>0</span>;</span><br><span class=line></span><br><span class=line> <span class=comment>/* If the key exists, is our pick. Otherwise it is</span></span><br><span class=line><span class=comment> * a ghost and we need to try the next element. */</span></span><br><span class=line> <span class=keyword>if</span> (de) {</span><br><span class=line> bestkey = dictGetKey(de);</span><br><span class=line> <span class=keyword>break</span>;</span><br><span class=line> } <span class=keyword>else</span> {</span><br><span class=line> <span class=comment>/* Ghost... Iterate again. */</span></span><br><span class=line> }</span><br><span class=line> }</span><br><span class=line> }</span><br><span class=line> }</span><br><span class=line></span><br><span class=line> <span class=comment>/* volatile-random and allkeys-random policy */</span></span><br><span class=line> <span class=keyword>else</span> <span class=keyword>if</span> (server.maxmemory_policy == MAXMEMORY_ALLKEYS_RANDOM ||</span><br><span class=line> server.maxmemory_policy == MAXMEMORY_VOLATILE_RANDOM)</span><br><span class=line> {</span><br><span class=line> <span class=comment>/* When evicting a random key, we try to evict a key for</span></span><br><span class=line><span class=comment> * each DB, so we use the static 'next_db' variable to</span></span><br><span class=line><span class=comment> * incrementally visit all DBs. */</span></span><br><span class=line> <span class=keyword>for</span> (i = <span class=number>0</span>; i < server.dbnum; i++) {</span><br><span class=line> j = (++next_db) % server.dbnum;</span><br><span class=line> db = server.db+j;</span><br><span class=line> dict = (server.maxmemory_policy == MAXMEMORY_ALLKEYS_RANDOM) ?</span><br><span class=line> db->dict : db->expires;</span><br><span class=line> <span class=keyword>if</span> (dictSize(dict) != <span class=number>0</span>) {</span><br><span class=line> de = dictGetRandomKey(dict);</span><br><span class=line> bestkey = dictGetKey(de);</span><br><span class=line> bestdbid = j;</span><br><span class=line> <span class=keyword>break</span>;</span><br><span class=line> }</span><br><span class=line> }</span><br><span class=line> }</span><br><span class=line></span><br><span class=line> <span class=comment>/* Finally remove the selected key. */</span></span><br><span class=line> <span class=keyword>if</span> (bestkey) {</span><br><span class=line> db = server.db+bestdbid;</span><br><span class=line> robj *keyobj = createStringObject(bestkey,sdslen(bestkey));</span><br><span class=line> propagateExpire(db,keyobj,server.lazyfree_lazy_eviction);</span><br><span class=line> <span class=comment>/* We compute the amount of memory freed by db*Delete() alone.</span></span><br><span class=line><span class=comment> * It is possible that actually the memory needed to propagate</span></span><br><span class=line><span class=comment> * the DEL in AOF and replication link is greater than the one</span></span><br><span class=line><span class=comment> * we are freeing removing the key, but we can't account for</span></span><br><span class=line><span class=comment> * that otherwise we would never exit the loop.</span></span><br><span class=line><span class=comment> *</span></span><br><span class=line><span class=comment> * AOF and Output buffer memory will be freed eventually so</span></span><br><span class=line><span class=comment> * we only care about memory used by the key space. */</span></span><br><span class=line> delta = (<span class=type>long</span> <span class=type>long</span>) zmalloc_used_memory();</span><br><span class=line> latencyStartMonitor(eviction_latency);</span><br><span class=line> <span class=keyword>if</span> (server.lazyfree_lazy_eviction)</span><br><span class=line> dbAsyncDelete(db,keyobj);</span><br><span class=line> <span class=keyword>else</span></span><br><span class=line> dbSyncDelete(db,keyobj);</span><br><span class=line> latencyEndMonitor(eviction_latency);</span><br><span class=line> latencyAddSampleIfNeeded(<span class=string>"eviction-del"</span>,eviction_latency);</span><br><span class=line> latencyRemoveNestedEvent(latency,eviction_latency);</span><br><span class=line> delta -= (<span class=type>long</span> <span class=type>long</span>) zmalloc_used_memory();</span><br><span class=line> mem_freed += delta;</span><br><span class=line> server.stat_evictedkeys++;</span><br><span class=line> notifyKeyspaceEvent(NOTIFY_EVICTED, <span class=string>"evicted"</span>,</span><br><span class=line> keyobj, db->id);</span><br><span class=line> decrRefCount(keyobj);</span><br><span class=line> keys_freed++;</span><br><span class=line></span><br><span class=line> <span class=comment>/* When the memory to free starts to be big enough, we may</span></span><br><span class=line><span class=comment> * start spending so much time here that is impossible to</span></span><br><span class=line><span class=comment> * deliver data to the slaves fast enough, so we force the</span></span><br><span class=line><span class=comment> * transmission here inside the loop. */</span></span><br><span class=line> <span class=keyword>if</span> (slaves) flushSlavesOutputBuffers();</span><br><span class=line></span><br><span class=line> <span class=comment>/* Normally our stop condition is the ability to release</span></span><br><span class=line><span class=comment> * a fixed, pre-computed amount of memory. However when we</span></span><br><span class=line><span class=comment> * are deleting objects in another thread, it's better to</span></span><br><span class=line><span class=comment> * check, from time to time, if we already reached our target</span></span><br><span class=line><span class=comment> * memory, since the "mem_freed" amount is computed only</span></span><br><span class=line><span class=comment> * across the dbAsyncDelete() call, while the thread can</span></span><br><span class=line><span class=comment> * release the memory all the time. */</span></span><br><span class=line> <span class=keyword>if</span> (server.lazyfree_lazy_eviction && !(keys_freed % <span class=number>16</span>)) {</span><br><span class=line> <span class=keyword>if</span> (getMaxmemoryState(<span class=literal>NULL</span>,<span class=literal>NULL</span>,<span class=literal>NULL</span>,<span class=literal>NULL</span>) == C_OK) {</span><br><span class=line> <span class=comment>/* Let's satisfy our stop condition. */</span></span><br><span class=line> mem_freed = mem_tofree;</span><br><span class=line> }</span><br><span class=line> }</span><br><span class=line> } <span class=keyword>else</span> {</span><br><span class=line> latencyEndMonitor(latency);</span><br><span class=line> latencyAddSampleIfNeeded(<span class=string>"eviction-cycle"</span>,latency);</span><br><span class=line> <span class=keyword>goto</span> cant_free; <span class=comment>/* nothing to free... */</span></span><br><span class=line> }</span><br><span class=line> }</span><br><span class=line> latencyEndMonitor(latency);</span><br><span class=line> latencyAddSampleIfNeeded(<span class=string>"eviction-cycle"</span>,latency);</span><br><span class=line> <span class=keyword>return</span> C_OK;</span><br><span class=line></span><br><span class=line>cant_free:</span><br><span class=line> <span class=comment>/* We are here if we are not able to reclaim memory. There is only one</span></span><br><span class=line><span class=comment> * last thing we can try: check if the lazyfree thread has jobs in queue</span></span><br><span class=line><span class=comment> * and wait... */</span></span><br><span class=line> <span class=keyword>while</span>(bioPendingJobsOfType(BIO_LAZY_FREE)) {</span><br><span class=line> <span class=keyword>if</span> (((mem_reported - zmalloc_used_memory()) + mem_freed) >= mem_tofree)</span><br><span class=line> <span class=keyword>break</span>;</span><br><span class=line> usleep(<span class=number>1000</span>);</span><br><span class=line> }</span><br><span class=line> <span class=keyword>return</span> C_ERR;</span><br><span class=line>}</span><br></pre></table></figure><p>这里就是根据具体策略去淘汰 key,首先是要往 pool 更新 key,更新key 的方法是<a href=https://github.com/antirez/redis/blob/unstable/src/evict.c#L162 rel=noopener target=_blank><code>evictionPoolPopulate</code></a><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><span class=line>93</span><br><span class=line>94</span><br><span class=line>95</span><br><span class=line>96</span><br></pre><td class=code><pre><span class=line><span class=type>void</span> <span class="title function_">evictionPoolPopulate</span><span class=params>(<span class=type>int</span> dbid, dict *sampledict, dict *keydict, <span class=keyword>struct</span> evictionPoolEntry *pool)</span> {</span><br><span class=line> <span class=type>int</span> j, k, count;</span><br><span class=line> dictEntry *samples[server.maxmemory_samples];</span><br><span class=line></span><br><span class=line> count = dictGetSomeKeys(sampledict,samples,server.maxmemory_samples);</span><br><span class=line> <span class=keyword>for</span> (j = <span class=number>0</span>; j < count; j++) {</span><br><span class=line> <span class=type>unsigned</span> <span class=type>long</span> <span class=type>long</span> idle;</span><br><span class=line> sds key;</span><br><span class=line> robj *o;</span><br><span class=line> dictEntry *de;</span><br><span class=line></span><br><span class=line> de = samples[j];</span><br><span class=line> key = dictGetKey(de);</span><br><span class=line></span><br><span class=line> <span class=comment>/* If the dictionary we are sampling from is not the main</span></span><br><span class=line><span class=comment> * dictionary (but the expires one) we need to lookup the key</span></span><br><span class=line><span class=comment> * again in the key dictionary to obtain the value object. */</span></span><br><span class=line> <span class=keyword>if</span> (server.maxmemory_policy != MAXMEMORY_VOLATILE_TTL) {</span><br><span class=line> <span class=keyword>if</span> (sampledict != keydict) de = dictFind(keydict, key);</span><br><span class=line> o = dictGetVal(de);</span><br><span class=line> }</span><br><span class=line></span><br><span class=line> <span class=comment>/* Calculate the idle time according to the policy. This is called</span></span><br><span class=line><span class=comment> * idle just because the code initially handled LRU, but is in fact</span></span><br><span class=line><span class=comment> * just a score where an higher score means better candidate. */</span></span><br><span class=line> <span class=keyword>if</span> (server.maxmemory_policy & MAXMEMORY_FLAG_LRU) {</span><br><span class=line> idle = estimateObjectIdleTime(o);</span><br><span class=line> } <span class=keyword>else</span> <span class=keyword>if</span> (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {</span><br><span class=line> <span class=comment>/* When we use an LRU policy, we sort the keys by idle time</span></span><br><span class=line><span class=comment> * so that we expire keys starting from greater idle time.</span></span><br><span class=line><span class=comment> * However when the policy is an LFU one, we have a frequency</span></span><br><span class=line><span class=comment> * estimation, and we want to evict keys with lower frequency</span></span><br><span class=line><span class=comment> * first. So inside the pool we put objects using the inverted</span></span><br><span class=line><span class=comment> * frequency subtracting the actual frequency to the maximum</span></span><br><span class=line><span class=comment> * frequency of 255. */</span></span><br><span class=line> idle = <span class=number>255</span>-LFUDecrAndReturn(o);</span><br><span class=line> } <span class=keyword>else</span> <span class=keyword>if</span> (server.maxmemory_policy == MAXMEMORY_VOLATILE_TTL) {</span><br><span class=line> <span class=comment>/* In this case the sooner the expire the better. */</span></span><br><span class=line> idle = ULLONG_MAX - (<span class=type>long</span>)dictGetVal(de);</span><br><span class=line> } <span class=keyword>else</span> {</span><br><span class=line> serverPanic(<span class=string>"Unknown eviction policy in evictionPoolPopulate()"</span>);</span><br><span class=line> }</span><br><span class=line></span><br><span class=line> <span class=comment>/* Insert the element inside the pool.</span></span><br><span class=line><span class=comment> * First, find the first empty bucket or the first populated</span></span><br><span class=line><span class=comment> * bucket that has an idle time smaller than our idle time. */</span></span><br><span class=line> k = <span class=number>0</span>;</span><br><span class=line> <span class=keyword>while</span> (k < EVPOOL_SIZE &&</span><br><span class=line> pool[k].key &&</span><br><span class=line> pool[k].idle < idle) k++;</span><br><span class=line> <span class=keyword>if</span> (k == <span class=number>0</span> && pool[EVPOOL_SIZE<span class=number>-1</span>].key != <span class=literal>NULL</span>) {</span><br><span class=line> <span class=comment>/* Can't insert if the element is < the worst element we have</span></span><br><span class=line><span class=comment> * and there are no empty buckets. */</span></span><br><span class=line> <span class=keyword>continue</span>;</span><br><span class=line> } <span class=keyword>else</span> <span class=keyword>if</span> (k < EVPOOL_SIZE && pool[k].key == <span class=literal>NULL</span>) {</span><br><span class=line> <span class=comment>/* Inserting into empty position. No setup needed before insert. */</span></span><br><span class=line> } <span class=keyword>else</span> {</span><br><span class=line> <span class=comment>/* Inserting in the middle. Now k points to the first element</span></span><br><span class=line><span class=comment> * greater than the element to insert. */</span></span><br><span class=line> <span class=keyword>if</span> (pool[EVPOOL_SIZE<span class=number>-1</span>].key == <span class=literal>NULL</span>) {</span><br><span class=line> <span class=comment>/* Free space on the right? Insert at k shifting</span></span><br><span class=line><span class=comment> * all the elements from k to end to the right. */</span></span><br><span class=line></span><br><span class=line> <span class=comment>/* Save SDS before overwriting. */</span></span><br><span class=line> sds cached = pool[EVPOOL_SIZE<span class=number>-1</span>].cached;</span><br><span class=line> memmove(pool+k+<span class=number>1</span>,pool+k,</span><br><span class=line> <span class=keyword>sizeof</span>(pool[<span class=number>0</span>])*(EVPOOL_SIZE-k<span class=number>-1</span>));</span><br><span class=line> pool[k].cached = cached;</span><br><span class=line> } <span class=keyword>else</span> {</span><br><span class=line> <span class=comment>/* No free space on right? Insert at k-1 */</span></span><br><span class=line> k--;</span><br><span class=line> <span class=comment>/* Shift all elements on the left of k (included) to the</span></span><br><span class=line><span class=comment> * left, so we discard the element with smaller idle time. */</span></span><br><span class=line> sds cached = pool[<span class=number>0</span>].cached; <span class=comment>/* Save SDS before overwriting. */</span></span><br><span class=line> <span class=keyword>if</span> (pool[<span class=number>0</span>].key != pool[<span class=number>0</span>].cached) sdsfree(pool[<span class=number>0</span>].key);</span><br><span class=line> memmove(pool,pool+<span class=number>1</span>,<span class=keyword>sizeof</span>(pool[<span class=number>0</span>])*k);</span><br><span class=line> pool[k].cached = cached;</span><br><span class=line> }</span><br><span class=line> }</span><br><span class=line></span><br><span class=line> <span class=comment>/* Try to reuse the cached SDS string allocated in the pool entry,</span></span><br><span class=line><span class=comment> * because allocating and deallocating this object is costly</span></span><br><span class=line><span class=comment> * (according to the profiler, not my fantasy. Remember:</span></span><br><span class=line><span class=comment> * premature optimizbla bla bla bla. */</span></span><br><span class=line> <span class=type>int</span> klen = sdslen(key);</span><br><span class=line> <span class=keyword>if</span> (klen > EVPOOL_CACHED_SDS_SIZE) {</span><br><span class=line> pool[k].key = sdsdup(key);</span><br><span class=line> } <span class=keyword>else</span> {</span><br><span class=line> <span class=built_in>memcpy</span>(pool[k].cached,key,klen+<span class=number>1</span>);</span><br><span class=line> sdssetlen(pool[k].cached,klen);</span><br><span class=line> pool[k].key = pool[k].cached;</span><br><span class=line> }</span><br><span class=line> pool[k].idle = idle;</span><br><span class=line> pool[k].dbid = dbid;</span><br><span class=line> }</span><br><span class=line>}</span><br></pre></table></figure><p><code>Redis</code>随机选择<code>maxmemory_samples</code>数量的key,然后计算这些<code>key</code>的空闲时间<code>idle time</code>,当满足条件时(比pool中的某些键的空闲时间还大)就可以进<code>pool</code>。<code>pool</code>更新之后,就淘汰<code>pool</code>中空闲时间最大的键。<p><a href=https://github.com/antirez/redis/blob/unstable/src/evict.c#L90 rel=noopener target=_blank><code>estimateObjectIdleTime</code></a>用来计算Redis对象的空闲时间:<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></pre><td class=code><pre><span class=line><span class=comment>/* Given an object returns the min number of milliseconds the object was never</span></span><br><span class=line><span class=comment> * requested, using an approximated LRU algorithm. */</span></span><br><span class=line><span class=type>unsigned</span> <span class=type>long</span> <span class=type>long</span> <span class="title function_">estimateObjectIdleTime</span><span class=params>(robj *o)</span> {</span><br><span class=line> <span class=type>unsigned</span> <span class=type>long</span> <span class=type>long</span> lruclock = LRU_CLOCK();</span><br><span class=line> <span class=keyword>if</span> (lruclock >= o->lru) {</span><br><span class=line> <span class=keyword>return</span> (lruclock - o->lru) * LRU_CLOCK_RESOLUTION;</span><br><span class=line> } <span class=keyword>else</span> {</span><br><span class=line> <span class=keyword>return</span> (lruclock + (LRU_CLOCK_MAX - o->lru)) *</span><br><span class=line> LRU_CLOCK_RESOLUTION;</span><br><span class=line> }</span><br><span class=line>}</span><br></pre></table></figure><p>空闲时间第一种是 lurclock 大于对象的 lru,那么就是减一下乘以精度,因为 lruclock 有可能是已经预生成的,所以会可能走下面这个<h3 id=LFU><a class=headerlink href=#LFU title=LFU></a>LFU</h3><p>上面介绍了LRU 的算法,但是考虑一种场景<figure class="highlight plaintext"><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></pre><td class=code><pre><span class=line>~~~~~A~~~~~A~~~~~A~~~~A~~~~~A~~~~~A~~|</span><br><span class=line>~~B~~B~~B~~B~~B~~B~~B~~B~~B~~B~~B~~B~|</span><br><span class=line>~~~~~~~~~~C~~~~~~~~~C~~~~~~~~~C~~~~~~|</span><br><span class=line>~~~~~D~~~~~~~~~~D~~~~~~~~~D~~~~~~~~~D|</span><br></pre></table></figure><p>可以发现,当采用 lru 的淘汰策略的时候,D 是最新的,会被认为是最值得保留的,但是事实上还不如 A 跟 B,然后 antirez 大神就想到了LFU (Least Frequently Used) 这个算法, 显然对于上面的四个 key 的访问频率,保留优先级应该是 B > A > C = D<br>那要怎么来实现这个 LFU 算法呢,其实像LRU,理想的情况就是维护个链表,把最新访问的放到头上去,但是这个会影响访问速度,注意到前面代码的应该可以看到,redisObject 的 lru 字段其实是两用的,当策略是 LFU 时,这个字段就另作他用了,它的 24 位长度被分成两部分<figure class="highlight plaintext"><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></pre><td class=code><pre><span class=line> 16 bits 8 bits</span><br><span class=line>+----------------+--------+</span><br><span class=line>+ Last decr time | LOG_C |</span><br><span class=line>+----------------+--------+</span><br></pre></table></figure><p>前16位字段是最后一次递减时间,因此Redis知道 上一次计数器递减,后8位是 计数器 counter。<br>LFU 的主体策略就是当这个 key 被访问的次数越多频率越高他就越容易被保留下来,并且是最近被访问的频率越高。这其实有两个事情要做,一个是在访问的时候增加计数值,在一定长时间不访问时进行衰减,所以这里用了两个值,前 16 位记录上一次衰减的时间,后 8 位记录具体的计数值。<br>Redis4.0之后为maxmemory_policy淘汰策略添加了两个LFU模式:<p><code>volatile-lfu</code>:对有过期时间的key采用LFU淘汰策略<br><code>allkeys-lfu</code>:对全部key采用LFU淘汰策略<br>还有2个配置可以调整LFU算法:<figure class="highlight plaintext"><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></pre><td class=code><pre><span class=line>lfu-log-factor 10</span><br><span class=line>lfu-decay-time 1</span><br><span class=line>``` </span><br><span class=line>`lfu-log-factor` 可以调整计数器counter的增长速度,lfu-log-factor越大,counter增长的越慢。</span><br><span class=line></span><br><span class=line>`lfu-decay-time`是一个以分钟为单位的数值,可以调整counter的减少速度</span><br><span class=line>这里有个问题是 8 位大小够计么,访问一次加 1 的话的确不够,不过大神就是大神,才不会这么简单的加一。往下看代码</span><br><span class=line>```C</span><br><span class=line>/* Low level key lookup API, not actually called directly from commands</span><br><span class=line> * implementations that should instead rely on lookupKeyRead(),</span><br><span class=line> * lookupKeyWrite() and lookupKeyReadWithFlags(). */</span><br><span class=line>robj *lookupKey(redisDb *db, robj *key, int flags) {</span><br><span class=line> dictEntry *de = dictFind(db->dict,key->ptr);</span><br><span class=line> if (de) {</span><br><span class=line> robj *val = dictGetVal(de);</span><br><span class=line></span><br><span class=line> /* Update the access time for the ageing algorithm.</span><br><span class=line> * Don't do it if we have a saving child, as this will trigger</span><br><span class=line> * a copy on write madness. */</span><br><span class=line> if (!hasActiveChildProcess() && !(flags & LOOKUP_NOTOUCH)){</span><br><span class=line> if (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {</span><br><span class=line> // 当淘汰策略是 LFU 时,就会调用这个updateLFU</span><br><span class=line> updateLFU(val);</span><br><span class=line> } else {</span><br><span class=line> val->lru = LRU_CLOCK();</span><br><span class=line> }</span><br><span class=line> }</span><br><span class=line> return val;</span><br><span class=line> } else {</span><br><span class=line> return NULL;</span><br><span class=line> }</span><br><span class=line>}</span><br></pre></table></figure><p><a href=https://github.com/antirez/redis/blob/unstable/src/db.c#L46 rel=noopener target=_blank><code>updateLFU</code></a> 这个其实个入口,调用了两个重要的方法<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 class=code><pre><span class=line><span class=comment>/* Update LFU when an object is accessed.</span></span><br><span class=line><span class=comment> * Firstly, decrement the counter if the decrement time is reached.</span></span><br><span class=line><span class=comment> * Then logarithmically increment the counter, and update the access time. */</span></span><br><span class=line><span class=type>void</span> <span class="title function_">updateLFU</span><span class=params>(robj *val)</span> {</span><br><span class=line> <span class=type>unsigned</span> <span class=type>long</span> counter = LFUDecrAndReturn(val);</span><br><span class=line> counter = LFULogIncr(counter);</span><br><span class=line> val->lru = (LFUGetTimeInMinutes()<<<span class=number>8</span>) | counter;</span><br><span class=line>}</span><br></pre></table></figure><p>首先来看看<a href=https://github.com/antirez/redis/blob/unstable/src/evict.c#L335 rel=noopener target=_blank><code>LFUDecrAndReturn</code></a>,这个方法的作用是根据上一次衰减时间和系统配置的 <code>lfu-decay-time</code> 参数来确定需要将 counter 减去多少<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></pre><td class=code><pre><span class=line><span class=comment>/* If the object decrement time is reached decrement the LFU counter but</span></span><br><span class=line><span class=comment> * do not update LFU fields of the object, we update the access time</span></span><br><span class=line><span class=comment> * and counter in an explicit way when the object is really accessed.</span></span><br><span class=line><span class=comment> * And we will times halve the counter according to the times of</span></span><br><span class=line><span class=comment> * elapsed time than server.lfu_decay_time.</span></span><br><span class=line><span class=comment> * Return the object frequency counter.</span></span><br><span class=line><span class=comment> *</span></span><br><span class=line><span class=comment> * This function is used in order to scan the dataset for the best object</span></span><br><span class=line><span class=comment> * to fit: as we check for the candidate, we incrementally decrement the</span></span><br><span class=line><span class=comment> * counter of the scanned objects if needed. */</span></span><br><span class=line><span class=type>unsigned</span> <span class=type>long</span> <span class="title function_">LFUDecrAndReturn</span><span class=params>(robj *o)</span> {</span><br><span class=line> <span class=comment>// 右移 8 位,拿到上次衰减时间</span></span><br><span class=line> <span class=type>unsigned</span> <span class=type>long</span> ldt = o->lru >> <span class=number>8</span>;</span><br><span class=line> <span class=comment>// 对 255 做与操作,拿到 counter 值</span></span><br><span class=line> <span class=type>unsigned</span> <span class=type>long</span> counter = o->lru & <span class=number>255</span>;</span><br><span class=line> <span class=comment>// 根据lfu_decay_time来算出过了多少个衰减周期</span></span><br><span class=line> <span class=type>unsigned</span> <span class=type>long</span> num_periods = server.lfu_decay_time ? LFUTimeElapsed(ldt) / server.lfu_decay_time : <span class=number>0</span>;</span><br><span class=line> <span class=keyword>if</span> (num_periods)</span><br><span class=line> counter = (num_periods > counter) ? <span class=number>0</span> : counter - num_periods;</span><br><span class=line> <span class=keyword>return</span> counter;</span><br><span class=line>}</span><br></pre></table></figure><p>然后是加,调用了<a href=https://github.com/antirez/redis/blob/unstable/src/evict.c#L315 rel=noopener target=_blank><code>LFULogIncr</code></a><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></pre><td class=code><pre><span class=line><span class=comment>/* Logarithmically increment a counter. The greater is the current counter value</span></span><br><span class=line><span class=comment> * the less likely is that it gets really implemented. Saturate it at 255. */</span></span><br><span class=line><span class=type>uint8_t</span> <span class="title function_">LFULogIncr</span><span class=params>(<span class=type>uint8_t</span> counter)</span> {</span><br><span class=line> <span class=comment>// 最大值就是 255,到顶了就不加了</span></span><br><span class=line> <span class=keyword>if</span> (counter == <span class=number>255</span>) <span class=keyword>return</span> <span class=number>255</span>;</span><br><span class=line> <span class=comment>// 生成个随机小数</span></span><br><span class=line> <span class=type>double</span> r = (<span class=type>double</span>)rand()/RAND_MAX;</span><br><span class=line> <span class=comment>// 减去个基础值,LFU_INIT_VAL = 5,防止刚进来就被逐出</span></span><br><span class=line> <span class=type>double</span> baseval = counter - LFU_INIT_VAL;</span><br><span class=line> <span class=comment>// 如果是小于 0,</span></span><br><span class=line> <span class=keyword>if</span> (baseval < <span class=number>0</span>) baseval = <span class=number>0</span>;</span><br><span class=line> <span class=comment>// 如果 baseval 是 0,那么 p 就是 1了,后面 counter 直接加一,如果不是的话,得看系统参数lfu_log_factor,这个越大,除出来的 p 越小,那么 counter++的可能性也越小,这样子就把前面的疑问给解决了,不是直接+1 的</span></span><br><span class=line> <span class=type>double</span> p = <span class=number>1.0</span>/(baseval*server.lfu_log_factor+<span class=number>1</span>);</span><br><span class=line> <span class=keyword>if</span> (r < p) counter++;</span><br><span class=line> <span class=keyword>return</span> counter;</span><br><span class=line>}</span><br></pre></table></figure><p>大概的变化速度可以参考<figure class="highlight plaintext"><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></pre><td class=code><pre><span class=line>+--------+------------+------------+------------+------------+------------+</span><br><span class=line>| factor | 100 hits | 1000 hits | 100K hits | 1M hits | 10M hits |</span><br><span class=line>+--------+------------+------------+------------+------------+------------+</span><br><span class=line>| 0 | 104 | 255 | 255 | 255 | 255 |</span><br><span class=line>+--------+------------+------------+------------+------------+------------+</span><br><span class=line>| 1 | 18 | 49 | 255 | 255 | 255 |</span><br><span class=line>+--------+------------+------------+------------+------------+------------+</span><br><span class=line>| 10 | 10 | 18 | 142 | 255 | 255 |</span><br><span class=line>+--------+------------+------------+------------+------------+------------+</span><br><span class=line>| 100 | 8 | 11 | 49 | 143 | 255 |</span><br><span class=line>+--------+------------+------------+------------+------------+------------+</span><br></pre></table></figure><p>简而言之就是 lfu_log_factor 越大变化的越慢<h3 id=总结><a class=headerlink href=#总结 title=总结></a>总结</h3><p>总结一下,redis 实现了近似的 lru 淘汰策略,通过增加了淘汰 key 的池子(pool),并且增大每次抽样的 key 的数量来将淘汰效果更进一步地接近于 lru,这是 lru 策略,但是对于前面举的一个例子,其实 lru 并不能保证 key 的淘汰就如我们预期,所以在后期又引入了 lfu 的策略,lfu的策略比较巧妙,复用了 redis 对象的 lru 字段,并且使用了factor 参数来控制计数器递增的速度,防止 8 位的计数器太早溢出。</div><footer class=post-footer><div class="affiliate-creative affiliate-creative--post-end" data-affiliate-id=tencent-cloud-general><a rel="sponsored noopener" aria-label=腾讯云产品优惠 href=https://cloud.tencent.com/act/cps/redirect?redirect=2446&cps_key=2e33a1eedb380997d03618b4a5ba6ca7&from=console target=_blank><img alt=腾讯云服务器、数据库和云产品优惠 decoding=async height=419 loading=lazy src=/uploads/affiliate/tencent-cloud-general.webp width=690> </a><span class=affiliate-disclosure>推广</span></div><div class=reward-container><div>请我一杯咖啡吧!</div><button>赞赏</button><div 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