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Redis过期键与内存淘汰策略深入分析讲解

开发者 https://www.devze.com 2022-12-08 12:43 出处:网络 作者: 基层搬砖的Panda
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、R
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
                    ...
                };
                

                Redis过期键与内存淘汰策略深入分析讲解

                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。

                Redis过期键与内存淘汰策略深入分析讲解

                也可以通过 setex 在设置某个键的同时为其设置过期时间:

                Redis过期键与内存淘汰策略深入分析讲解

                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

                Redis过期键与内存淘汰策略深入分析讲解

                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]>

                Redis过期键与内存淘汰策略深入分析讲解

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

                OK

                127.0.0.1:6379[1]>

                Redis过期键与内存淘汰策略深入分析讲解

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

                (integer) 3

                127.0.0.1:6379[1]> 

                Redis过期键与内存淘汰策略深入分析讲解

                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]>

                Redis过期键与内存淘汰策略深入分析讲解

                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]>

                Redis过期键与内存淘汰策略深入分析讲解

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