从零开始学习redis源码

 1 int expireIfNeeded(redisDb *db, robj *key) {
 2 
 3     // 取出键的过期时间
 4     mstime_t when = getExpire(db,key);
 5     mstime_t now;
 6 
 7     // 没有过期时间
 8     if (when < 0) return 0; /* No expire for this key */
 9 
10     /* Don't expire anything while loading. It will be done later. */
11     // 如果服务器正在进行载入，那么不进行任何过期检查
12     if (server.loading) return 0;
13 
14     /* If we are in the context of a Lua script, we claim that time is
15      * blocked to when the Lua script started. This way a key can expire
16      * only the first time it is accessed and not in the middle of the
17      * script execution, making propagation to slaves / AOF consistent.
18      * See issue #1525 on Github for more information. */
19     now = server.lua_caller ? server.lua_time_start : mstime();
20 
21     /* If we are running in the context of a slave, return ASAP:
22      * the slave key expiration is controlled by the master that will
23      * send us synthesized DEL operations for expired keys.
24      *
25      * Still we try to return the right information to the caller, 
26      * that is, 0 if we think the key should be still valid, 1 if
27      * we think the key is expired at this time. */
28     // 当服务器运行在 replication 模式时
29     // 附属节点并不主动删除 key
30     // 它只返回一个逻辑上正确的返回值
31     // 真正的删除操作要等待主节点发来删除命令时才执行
32     // 从而保证数据的同步
33     if (server.masterhost != NULL) return now > when;
34 
35     // 运行到这里，表示键带有过期时间，并且服务器为主节点
36 
37     /* Return when this key has not expired */
38     // 如果未过期，返回 0
39     if (now <= when) return 0;
40 
41     /* Delete the key */
42     server.stat_expiredkeys++;
43 
44     // 向 AOF 文件和附属节点传播过期信息
45     propagateExpire(db,key);
46 
47     // 发送事件通知
48     notifyKeyspaceEvent(REDIS_NOTIFY_EXPIRED,
49         "expired",key,db->id);
50 
51     // 将过期键从数据库中删除
52     return dbDelete(db,key);
53 }

  1 void activeExpireCycle(int type) {
  2     /* This function has some global state in order to continue the work
  3      * incrementally across calls. */
  4     // 静态变量，用来累积函数连续执行时的数据
  5     static unsigned int current_db = 0; /* Last DB tested. */
  6     static int timelimit_exit = 0;      /* Time limit hit in previous call? */
  7     static long long last_fast_cycle = 0; /* When last fast cycle ran. */
  8 
  9     unsigned int j, iteration = 0;
 10     // 默认每次处理的数据库数量
 11     unsigned int dbs_per_call = REDIS_DBCRON_DBS_PER_CALL;
 12     // 函数开始的时间
 13     long long start = ustime(), timelimit;
 14 
 15     // 快速模式
 16     if (type == ACTIVE_EXPIRE_CYCLE_FAST) {
 17         /* Don't start a fast cycle if the previous cycle did not exited
 18          * for time limt. Also don't repeat a fast cycle for the same period
 19          * as the fast cycle total duration itself. */
 20         // 如果上次函数没有触发 timelimit_exit ，那么不执行处理
 21         if (!timelimit_exit) return;
 22         // 如果距离上次执行未够一定时间，那么不执行处理
 23         if (start < last_fast_cycle + ACTIVE_EXPIRE_CYCLE_FAST_DURATION*2) return;
 24         // 运行到这里，说明执行快速处理，记录当前时间
 25         last_fast_cycle = start;
 26     }
 27 
 28     /* We usually should test REDIS_DBCRON_DBS_PER_CALL per iteration, with
 29      * two exceptions:
 30      *
 31      * 一般情况下，函数只处理 REDIS_DBCRON_DBS_PER_CALL 个数据库，
 32      * 除非：
 33      *
 34      * 1) Don't test more DBs than we have.
 35      *    当前数据库的数量小于 REDIS_DBCRON_DBS_PER_CALL
 36      * 2) If last time we hit the time limit, we want to scan all DBs
 37      * in this iteration, as there is work to do in some DB and we don't want
 38      * expired keys to use memory for too much time. 
 39      *     如果上次处理遇到了时间上限，那么这次需要对所有数据库进行扫描，
 40      *     这可以避免过多的过期键占用空间
 41      */
 42     if (dbs_per_call > server.dbnum || timelimit_exit)
 43         dbs_per_call = server.dbnum;
 44 
 45     /* We can use at max ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC percentage of CPU time
 46      * per iteration. Since this function gets called with a frequency of
 47      * server.hz times per second, the following is the max amount of
 48      * microseconds we can spend in this function. */
 49     // 函数处理的微秒时间上限
 50     // ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC 默认为 25 ，也即是 25 % 的 CPU 时间
 51     timelimit = 1000000*ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC/server.hz/100;
 52     timelimit_exit = 0;
 53     if (timelimit <= 0) timelimit = 1;
 54 
 55     // 如果是运行在快速模式之下
 56     // 那么最多只能运行 FAST_DURATION 微秒 
 57     // 默认值为 1000 （微秒）
 58     if (type == ACTIVE_EXPIRE_CYCLE_FAST)
 59         timelimit = ACTIVE_EXPIRE_CYCLE_FAST_DURATION; /* in microseconds. */
 60 
 61     // 遍历数据库
 62     for (j = 0; j < dbs_per_call; j++) {
 63         int expired;
 64         // 指向要处理的数据库
 65         redisDb *db = server.db+(current_db % server.dbnum);
 66 
 67         /* Increment the DB now so we are sure if we run out of time
 68          * in the current DB we'll restart from the next. This allows to
 69          * distribute the time evenly across DBs. */
 70         // 为 DB 计数器加一，如果进入 do 循环之后因为超时而跳出
 71         // 那么下次会直接从下个 DB 开始处理
 72         current_db++;
 73 
 74         /* Continue to expire if at the end of the cycle more than 25%
 75          * of the keys were expired. */
 76         do {
 77             unsigned long num, slots;
 78             long long now, ttl_sum;
 79             int ttl_samples;
 80 
 81             /* If there is nothing to expire try next DB ASAP. */
 82             // 获取数据库中带过期时间的键的数量
 83             // 如果该数量为 0 ，直接跳过这个数据库
 84             if ((num = dictSize(db->expires)) == 0) {
 85                 db->avg_ttl = 0;
 86                 break;
 87             }
 88             // 获取数据库中键值对的数量
 89             slots = dictSlots(db->expires);
 90             // 当前时间
 91             now = mstime();
 92 
 93             /* When there are less than 1% filled slots getting random
 94              * keys is expensive, so stop here waiting for better times...
 95              * The dictionary will be resized asap. */
 96             // 这个数据库的使用率低于 1% ，扫描起来太费力了（大部分都会 MISS）
 97             // 跳过，等待字典收缩程序运行
 98             if (num && slots > DICT_HT_INITIAL_SIZE &&
 99                 (num*100/slots < 1)) break;
100 
101             /* The main collection cycle. Sample random keys among keys
102              * with an expire set, checking for expired ones. 
103              *
104              * 样本计数器
105              */
106             // 已处理过期键计数器
107             expired = 0;
108             // 键的总 TTL 计数器
109             ttl_sum = 0;
110             // 总共处理的键计数器
111             ttl_samples = 0;
112 
113             // 每次最多只能检查 LOOKUPS_PER_LOOP 个键
114             if (num > ACTIVE_EXPIRE_CYCLE_LOOKUPS_PER_LOOP)
115                 num = ACTIVE_EXPIRE_CYCLE_LOOKUPS_PER_LOOP;
116 
117             // 开始遍历数据库
118             while (num--) {
119                 dictEntry *de;
120                 long long ttl;
121 
122                 // 从 expires 中随机取出一个带过期时间的键
123                 if ((de = dictGetRandomKey(db->expires)) == NULL) break;
124                 // 计算 TTL
125                 ttl = dictGetSignedIntegerVal(de)-now;
126                 // 如果键已经过期，那么删除它，并将 expired 计数器增一
127                 if (activeExpireCycleTryExpire(db,de,now)) expired++;
128                 if (ttl < 0) ttl = 0;
129                 // 累积键的 TTL
130                 ttl_sum += ttl;
131                 // 累积处理键的个数
132                 ttl_samples++;
133             }
134 
135             /* Update the average TTL stats for this database. */
136             // 为这个数据库更新平均 TTL 统计数据
137             if (ttl_samples) {
138                 // 计算当前平均值
139                 long long avg_ttl = ttl_sum/ttl_samples;
140                 
141                 // 如果这是第一次设置数据库平均 TTL ，那么进行初始化
142                 if (db->avg_ttl == 0) db->avg_ttl = avg_ttl;
143                 /* Smooth the value averaging with the previous one. */
144                 // 取数据库的上次平均 TTL 和今次平均 TTL 的平均值
145                 db->avg_ttl = (db->avg_ttl+avg_ttl)/2;
146             }
147 
148             /* We can't block forever here even if there are many keys to
149              * expire. So after a given amount of milliseconds return to the
150              * caller waiting for the other active expire cycle. */
151             // 我们不能用太长时间处理过期键，
152             // 所以这个函数执行一定时间之后就要返回
153 
154             // 更新遍历次数
155             iteration++;
156 
157             // 每遍历 16 次执行一次
158             if ((iteration & 0xf) == 0 && /* check once every 16 iterations. */
159                 (ustime()-start) > timelimit)
160             {
161                 // 如果遍历次数正好是 16 的倍数
162                 // 并且遍历的时间超过了 timelimit
163                 // 那么断开 timelimit_exit
164                 timelimit_exit = 1;
165             }
166 
167             // 已经超时了，返回
168             if (timelimit_exit) return;
169 
170             /* We don't repeat the cycle if there are less than 25% of keys
171              * found expired in the current DB. */
172             // 如果已删除的过期键占当前总数据库带过期时间的键数量的 25 %
173             // 那么不再遍历
174         } while (expired > ACTIVE_EXPIRE_CYCLE_LOOKUPS_PER_LOOP/4);
175     }
176 }