Redis过期键与内存淘汰策略深入分析讲解

目录

• 一、Redis数据库的组织方式
• 1.1 redisServer结构定义
• 1.2 redisDb 结构定义
• 1.3 redisdb初始化
• 二、过期键
• 2.1 设置键的过期时间
• 2.2 过期键的判定
• 2.3 过期键的删除策略
• 2.3.1 惰性删除的实现
• 2.3.2 定时删除的实现
• 三、Redis内存淘汰策略
• 3.1 Redis针对过期key的淘汰策略
• 3.2 Redis最对所有key的淘汰策略
• 3.3 禁止淘汰策略
• 四、增删改查图解
• 4.1 新增键值对
• 4.2 更新键值对
• 4.3 获取键的值
• 4.4 删除键值对

以下内容是基于Redis 6.2.6 版本整理总结

一、Redis数据库的组织方式

Redis服务器将所有的数据库 都保存在src/server.h/redisServer结构中的db数组中。db数组的每个entry都是src/server.h/redisDb结构，每个redisDb结构代表一个数据库。Redis默认有16个数据库。

1.1 redisServer结构定义

struct redisServer {
    /* General */
    pid_t pid;                  /* Main process pid. */
    pthread_t main_thread_id;         /* Main thread id */
	...
    redisDb *db;   // db数组
    ...
    int db编程客栈num;     // redis db的数量
    ...
};

1.2 redisDb 结构定义

typedef struct redisDb {
    dict *dict;                 /* The keyspace for this DB */ //键空间，保存数据库中所有的键值对
    dict *expires;              /* Timeout of keys with a timeout set */
    dict *blocking_keys;        /* Keys with clients waiting for data (BLPOP)*/
    dict *ready_keys;           /* Blocked keys that received a PUSH */
    dict *watched_keys;         /* WATCHED keys for MULTI/EXEC CAS */
    int id;                     /* Database ID */
    long long avg_ttl;          /* Average TTL, just for stats */
    unsigned long expires_cursor; /* Cursor of the active expire cycle. */
    list *defrag_later;         /* List of key names to attempt to defrag one by one, gradually. */
} redisDb;

各字段含义解释：

• dict保存了数据库中的所有键值对，这个字典也被称为：键空间（key space）。键空间的键就是数据库的键，每个键都是字符串对象；键空间的值就是数据库的值，每个值可以是五种对象中的任意一种对象。
• expires字典保存了数据库中所有键的过期时间，也叫过期字典。过期字典的键是指向键空间中的某个键的指针；值是一个long long类型的Unix毫秒级时间戳。
• blocking_keys使用比较少，redis只有blpop、brpop等命令造成主动阻塞。
• ready_keys和blocking_keys配合使用，比如：一个客户端blpop阻塞等待数据，另一个客户端在push时，会检查blocking_keys中是否存在相应的key，如果有就将该key移动到ready_keys中，阻塞的客户端收到数据。
• watched_keys用来实现WATCH功能，实际线上环境不会使用，影响redis性能。

1.3 redisdb初始化

// src/server.c
void initServer(void) {
    int j;
    // ...
	server.db = zmalloc(sizeof(redisDb)*server.dbnum);
	// ...
	/* Create the Redis databases, and initialize other internal state. */
    for (j = 0; j < server.dbnum; j++) {
        server.db[j].dict = dictCreate(&dbDictType,NULL);
        server.db[j].expires = dictCreate(&dbExpiresDictType,NULL);
        server.db[j].expires_cursor = 0;
        server.db[j].blocking_keys = dictCreate(&keylistDictType,NULL);
        server.db[j].ready_keys = dictCreate(&objectKeyPointerValueDictType,NULL);
        server.db[j].watched_keys = dictCreate(&keylistDictType,NULL);
        server.db[j].id = j;
        server.db[j].avg_ttl = 0;
        server.db[j].defrag_later = listCreate();
        listSetFreeMethod(server.db[j].defrag_later,(void (*)(void*))sdsfree);
    }
   //...
}

二、过期键

2.1 设置键的过期时间

redis客户端提供了expire或pexpire命令来设置键的过期时间（Time to live， TTL），在经过指定秒数或者毫秒数后，redis服务器会自动删除生存时间为0的键。ttl命令是以秒为单位返回键的剩余生存时间，pttl命令则是以毫秒为单位。

也可以通过 setex 在设置某个键的同时为其设置过期时间：

如果一个键没有设置过期时间或者设置了过期时间又通过persist命令取消了过期时间，则执行ttl查看键的过期时间返回-1

2.2 过期键的判定

开头我们在学习redisDb 结构的时候说过，过redisDb 中的expires过期字典保存了数据中的所有键的过期时间。要判断一个键是否过期：

• 检查给定键是不是在过期字典中，如果在，则拿到过期时间
• 跟当前unix时间戳比较，如果小于当前unix时间戳则过期，否则还没过期。

2.3 过期键的删除策略

惰性删除：放任过期键不管，但是每次从键空间获取键的时候，都会先检查键是否过期，如果过期了就删除，否则就正常返回。

优点：对CPU友好，对内存不友好，如果有访问的不到键，且已经过期了，则永远不会被删除。

定期删除：每隔一段时间，检查一次数据库，删除里面的过期键。要扫描多少个数据库，以及要删除多少过期键，由算法控制。

Redis服务器采用了上面两种策略的组合使用，很好的平衡了CPU的使用和内存的使用。

2.3.1 惰性删除的实现

惰性删除由expireIfNeeded函数实现，Redis在执行读写命令时都会先调用expireIfNeeded函数对键进行检查。如果已经过期，开发者_数据库expireIfNeeded函数就会删除该键值对；如果没有过期，php则什么都不做。

// db.c
int expireIfNeeded(redisDb *db, robj *key) {
    // 如果没过期，什么都不做，直接返回
    if (!keyIsExpired(db,key)) return 0;
    /* If we are running in the context of a slave, instead of
     * evicting the expired key from the database, we return ASAP:
     * the slave key expiration is controlled by the master that will
     * send us synthesized DEL operations for expired keys.
     *
     * Still we try to return the right information to the caller,
     * that is, 0 if we think the key should be still valid, 1 if
     * we think the key is expired at this time. */
    if (server.masterhost != NULL) return 1;
    /* If clients are paused, we keep the current dataset constant,
     * but return to the client what we believe is the right state. Typically,
     * at the end of the pause we will properly expire the key OR we will
     * have failed over and the new primary will send us the expire. */
    if (checkClientPauseTimeoutAndReturnIfPaused()) return 1;
    /* Delete the key */
    // 删除过期键
    deleteExpiredKeyAndPropagate(db,key);
    return 1;
}
/* Check if the key is expired. */
int keyIsExpired(redisDb *db, robj *key) {
    mstime_t when = getExpire(db,key);
    mstime_t now;
    // 如果该键没有设置过期时间
    if (when < 0) return 0; /* No expire for this key */
    /* Don't expire anything while loading. It will be done later. */
    // server加载过程中，不执行任何过期键删除操作
    if (server.loading) return 0;
    // 获取当前时间now
    /* If we are in the context of a Lua script, we pretend that time is
     * blocked to when the Lua script started. This way a key can expire
     * only the first time it is Accessed and not in the middle of the
     * script execution, making propagation to slaves / AOF consistent.
     * See issue #1525 on github for more information. */
    if (server.lua_caller) {
        now = server.lua_time_snapshot;
    }
    /* If we are in the middle of a command execution, we still want to use
     * a reference time that does not change: in that case we just use the
     * cached time, that we update before each call in the call() function.
     * This way we avoid that commands such as RPOPLPUSH or similar, that
     * may re-open the same key multiple times, can invalidate an already
     * open object in a next call, if the next call will see the key expired,
     * while the first did not. */
    else if (server.fixed_time_expire > 0) {
        now = server.mstime;
    }
    /* For the other cases, we want to use the most fresh time we have. */
    else {
        now = mstime();
    }
    /* The key expired if the current (virtual or real) time is greater
     * than the expire time of the key. */
    // 如果当前时间大于过期时间，则该键过期，返回true
    return now > when;
}
/* Return the expire time of the specified key, or -1 if no expire
 * is associated with this key (i.e. the key is non volatile) */
// 从过期字典中获取key的过期时间
long long getExpire(redisDb *db, robj *key) {
    dictEntry *de;
    /* No expire? return ASAP */
    // dictSize = db对应的ht[0].used+ht[1].used
    // 在过期字典中找不到该key，则直接返回-1
    if (dictSize(db->expires) == 0 ||
       (de = dictFind(db->expires,key->ptr)) == NULL) return -1;
    /* The entry was found in the expire dict, this means it should also
     * be present in the main dict (safety check). */
    serverAssertWithInfo(NULL,key,dictFind(db->dict,key->ptr) != NULL);
    // 如果找到了，返回键的unix时间戳
    return dictGetSignedIntegerVal(de);
}

2.3.2 定时删除的实现

惰性删除由src/db.c/activeExpireCycle函数实现.

#define ACTIVE_EXPIRE_CYCLE_KEYS_PER_LOOP 20 /* Keys for each DB loop. */ // 每个数据库默认检查20个key
#define ACTIVE_EXPIRE_CYCLE_FAST_DURATION 1000 /* Microseconds. */        // 每个数据库默认检查20个key
#define ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC 25 /* Max % of CPU to use. */  // CPU最大使用率25%
#define ACTIVE_EXPIRE_CYCLE_ACCEPTABLE_STALE 10 /* % of stale keys after which
                                                   we do extra efforts. */
void activeExpireCycle(int type) {
    /* Adjust the running parameters according to the configured expire
     * effort. The default effort is 1, and the maximum configurable effort
     * is 10. */
    unsigned long
    effort = server.active_expire_effort-1, /* Rescale from 0 to 9. */
    config_keys_per_loop = ACTIVE_EXPIRE_CYCLE_KEYS_PER_LOOP +
                           ACTIVE_EXPIRE_CYCLE_KEYS_PER_LOOP/4*effort,
    config_cycle_fast_duration = ACTIVE_EXPIRE_CYCLE_FAST_DURATION +
                                 ACTIVE_EXPIRE_CYCLE_FAST_DURATION/4*effort,
    config_cycle_slow_time_perc = ACTIVE_EXPIRE_CYCLE_SLOW_TIME_PERC +
                                  2*effort,
    config_cycle_acceptable_stale = ACTIVE_EXPIRE_CYCLE_ACCEPTABLE_STALE-
                                    effort;
    /* This function has some global state in order to continue the work
     * incrementally across calls. */
    static unsigned int current_db = 0; /* Next DB to test. */
    static int timelimit_exit = 0;      /* Time limit hit in previous call? */
    static long long last_fast_cycle = 0; /* When last fast cycle ran. */
    int j, iteration = 0;
    int dbs_per_call = CRON_DBS_PER_CALL;  // 每次默认检查16个数据库
    long long start = ustime(), timelimit, elapsed;
    /* When clients are paused the dataset should be static not just from the
     * POV of clients not being able to write, but also from the POV of
     * expires and evictions of keys not being performed. */
    if (checkClientPauseTimeoutAndReturnIfPaused()) return;
    if (type == ACTIVE_EXPIRE_CYCLE_FAST) {
        /* Don't start a fast cycle if the previous cycle did not exit
         * for time limit, unless the percentage of estimated stale keys is
         * too high. Also never repeat a fast cycle for the same period
         * as the fast cycle total duration itself. */
        if (!timelimit_exit &&
            server.stat_expired_stale_perc < config_cycle_acceptable_stale)
            return;
        if (start < last_fast_cycle + (long long)config_cycle_fast_duration*2)
            return;
        last_fast_cycle = start;
    }
    /* We usually should test CRON_DBS_PER_CALL per iteration, with
     * two exceptions:
     *
     * 1) Don't test more DBs than we have.
     * 2) If last time we hit the time limit, we want to scan all DBs
     * in this iteration, as there is work to do in some DB and we don't want
     * expired keys to use memory for too much time. */
    if (dbs_per_call > server.dbnum || timelimit_exit)
        dbs_per_call = server.dbnum;
    /* We can use at max 'config_cycle_slow_time_perc' percentage of CPU
     * time per iteration. Since this function gets called with a frequency of
     * server.hz times per second, the following is the max amount of
     * microseconds we can spend in this function. */
    timelimit = config_cycle_slow_time_perc*1000000/server.hz/100;
    timelimit_exit = 0;
    if (timelimit <= 0) timelimit = 1;
    if (type == ACTIVE_EXPIRE_CYCLE_FAST)
        timelimit = config_cycle_fast_duration; /* in microseconds. */
    /* Accumulate some global stats as we expire keys, to have some idea
     * about the number of keys that are already logically expired, but still
     * existing inside the database. */
    long total_sampled = 0;
    long total_expired = 0;
    // 遍历各个数据库
    for (j = 0; j < dbs_per_call && timelimit_exit == 0; j++) {
        /* Expired and checked in a single loop. */
        unsigne编程客栈d long expired, sampled;
        // 获取当前要处理的数据库
        redisDb *db = server.db+(current_db % server.dbnum);
        /* Increment the DB now so we are sure if we run out of time
         * in the current DB we'll restart from the next. This allows to
         * distribute the time evenly across DBs. */
        current_db++;
        /* Continue to expire if at the end of the cycle there are still
         * a big percentage of keys to expire, compared to the number of keys
         * we scanned. The percentage, stored in config_cycle_acceptable_stale
         * is not fixed, but depends on the Redis configured "expire effort". */
        do {
            unsigned long num, slots;
            long long now, ttl_sum;
            int ttl_samples;
            iteration++;
            /* If there is nothing to expire try next DB ASAP. */
            // 如果当前数据库过期字典为空，跳过这个数据库
            if ((num = dictSize(db->expires)) == 0) {
                db->avg_ttl = 0;
                break;
            }
            slots = dictSlots(db->expires);
            now = mstime();
            /* When there are less than 1% filled slots, sampling the key
             * space is expensive, so stop here waiting for better times...
             * The dictionary will be resized asap. */
            if (slots > DICT_HT_INITIAL_SIZE &&
                (num*100/slots < 1)) break;
            /* The main collection cycle. Sample random keys among keys
             * with an expire set, checking for expired ones. */
            expired = 0;
            sampled = 0;
            ttl_sum = 0;
            ttl_samples = 0;
            if (num > config_keys_per_loop)
                num = config_keys_per_loop;
            /* Here we access the low level representation of the hash table
             * for speed concerns: this makes this code coupled with dict.c,
             * but it hardly changed in ten years.
             *
             * Note that certain places of the hash table may be empty,
             * so we want also a stop condition about the number of
     android        * buckets that we scanned. However scanning for free buckets
             * is very fast: we are in the cache line scanning a sequential
             * array of NULL pointers, so we can scan a lot more buckets
             * than keys in the same time. */
            long max_buckets = num*20;
            long checked_buckets = 0;
            while (sampled < num && checked_buckets < max_buckets) {
                for (int table = 0; table < 2; table++) {
                    if (table == 1 && !dictIsRehashing(db->expires)) break;
                    unsigned long idx = db->expires_cursor;
                    idx &= db->expires->ht[table].sizemask;
                    dictEntry *de = db->expires->ht[table].table[idx];
                    long long ttl;
                    /* Scan the current bucket of the current table. */
                    checked_buckets++;
                    while(de) {
                        /* Get the next entry now since this entry may get
                         * deleted. */
                        dictEntry *e = de;
                        de = de->next;
                        ttl = dictGetSignedIntegerVal(e)-now;
                        if (activeExpireCycleTryExpire(db,e,now)) expired++;
                        if (ttl > 0) {
                            /* We want the average TTL of keys yet
                             * not expired. */
                            ttl_sum += ttl;
                            ttl_samples++;
                        }
                        sampled++;
                    }
                }
                db->expires_cursor++;
            }
            total_expired += expired;
            total_sampled += sampled;
            /* Update the average TTL stats for this database. */
            if (ttl_samples) {
                long long avg_ttl = ttl_sum/ttl_samples;
                /* Do a simple running average with a few samples.
                 * We just use the current estimate with a weight of 2%
                 * and the previous estimate with a weight of 98%. */
                if (db->avg_ttl == 0) db->avg_ttl = avg_ttl;
                db->avg_ttl = (db->avg_ttl/50)*49 + (avg_ttl/50);
            }
            /* We can't block forever here even if there are many keys to
             * expire. So after a given amount of milliseconds return to the
             * caller waiting for the other active expire cycle. */
            if ((iteration & 0xf) == 0) { /* check once every 16 iterations. */
                elapsed = ustime()-start;
                if (elapsed > timelimit) {
                    timelimit_exit = 1;
                    server.stat_expired_time_cap_reached_count++;
                    break;
                }
            }
            /* We don't repeat the cycle for the current database if there are
             * an acceptable amount of stale keys (logically expired but yet
             * not reclaimed). */
        } while (sampled == 0 ||
                 (expired*100/sampled) > config_cycle_acceptable_stale);
    }
    elapsed = ustime()-start;
    server.stat_expire_cycle_time_used += elapsed;
    latencyAddSampleIfNeeded("expire-cycle",elapsed/1000);
    /* Update our estimate of keys existing but yet to be expired.
     * Running average with this sample accounting for 5%. */
    double current_perc;
    if (total_sampled) {
        current_perc = (double)total_expired/total_sampled;
    } else
        current_perc = 0;
    server.stat_expired_stale_perc = (current_perc*0.05)+
                                     (server.stat_expired_stale_perc*0.95);
}

三、Redis内存淘汰策略

Redis为什么要有内存淘汰策略？因为Redis是内存数据库，不能无限大，达到阈值时需要淘汰部分内存的数据，来存储新的数据。

redis内存配置参数：maxmemory，一般设置为系统内存的一半（经验值），比如你的系统运行内存有哦96G，就设置为48G。

3.1 Redis针对过期key的淘汰策略

看你的业务是否使用了 expire 过期时间，如果使用了，则：

• volatile-lru （Least Recently Used的缩写，即最近最少使用）
• volatile-lfu（east frequently used的缩写，即最少次数使用）
• volatile-ttl（time to live的缩写，最近要过期的）
• volatile-random （随机淘汰）

3.2 Redis最对所有key的淘汰策略

• alllkeys-lru
• allkeys-lfu
• allkeys-random

3.3 禁止淘汰策略

redis还有一种淘汰策略，就是禁止淘汰，这种策略，当redis使用的内存达到设定的最大值时，后续的写进redis的操作会失败。

四、增删改查图解

4.1 新增键值对

举例：我们在一个空的redis数据库中执行分别执行以下命令：

127.0.0.1:6379[1]> keys *

(empty array)  // 表示此时数据库中没有任何数据

127.0.0.1:6379[1]&gwww.devze.comt; set msg "hello world"

OK

127.0.0.1:6379[1]>

 127.0.0.1:6379[1]> hmset student name panda age 20 addr beijing

OK

127.0.0.1:6379[1]>

127.0.0.1:6379[1]> rpush teacher Darren Mark King

(integer) 3

127.0.0.1:6379[1]> 

4.2 更新键值对

127.0.0.1:6379[1]> set msg "redis"

OK

127.0.0.1:6379[1]> get msg

"redis"

127.0.0.1:6379[1]> hset student sex male

(integer) 1

127.0.0.1:6379[1]>

4.3 获取键的值

127.0.0.1:6379[1]> get msg

"redis"

127.0.0.1:6379[1]> hmget student name age addr sex

1) "panda"

2) "20"

3) "beijing"

4) "male"

127.0.0.1:6379[1]>

4.4 删除键值对

127.0.0.1:6379[1]> keys *

1) "msg"

2) "student"

3) "teacher"

127.0.0.1:6379[1]> del student

(integer) 1

127.0.0.1:6379[1]> keys *

1) "msg"

2) "teacher"

127.0.0.1:6379[1]>

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