本文主要介绍Redis的几种数据淘汰机制。
I、上帝视角
由于Redis是内存型数据库，其允许用户设置最大使用内存大小为maxmemory，在内存有限的情况下，为减少内存紧张的情况，当内存数据集大小上升至一定值时，就会实施数据淘汰机制。
Redis提供了以下几种数据淘汰策略：
1、 volatile-lru：从设置过期的数据集中淘汰最少使用的数据；
2、volatile-ttl：从设置过期的数据集中淘汰即将过期的数据（离过期时间最近）；
3、volatile-random：从设置过期的数据集中随机选取数据淘汰；
4、allkeys-lru：从所有 数据集中选取使用最少的数据；
5、allkeys-random：从所有数据集中任意选取数据淘汰；
6、no-envicition：不进行淘汰；
II、LRU数据淘汰
1、redisServer中保存了lru计数器server.lrulock，会定时更新，这是根据server.unixtime计算出来的：
// redisServer 保存了lru 计数器
/*src/redis.h/redisServer*/
struct redisServer {
...
unsigned lruclock:22; /* Clock incrementing every minute, for LRU */
...
};
2、LRU数据淘汰机制使这样的：从数据集中随机挑选几个键值对，取出其中lru最大的键值对淘汰。
III、TTL数据淘汰
1、TTL淘汰机制使从过期时间redisDB.expires表中随机挑选几个键值对，取出其中ttl最大的键值对淘汰。
IV、淘汰发生
1、Redis服务器没执行一个命令，都会检测内存，判断是否需要进行数据淘汰：
// 执行命令
/*src/redis.cprocessCommand*/
int processCommand(redisClient *c) {
......
// 内存超额
/* Handle the maxmemory directive.
**
First we try to free some memory if possible (if there are volatile
* keys in the dataset). If there are not the only thing we can do
* is returning an error. */
if (server.maxmemory) {
int retval = freeMemoryIfNeeded();
if ((c->cmd->flags & REDIS_CMD_DENYOOM) && retval == REDIS_ERR) {
flagTransaction(c);
addReply(c, shared.oomerr);
return REDIS_OK;
}
}
......
}
2、这其中主要调用了freeMemoryIfNeeded函数，它完成了完整的数据淘汰机制：
int freeMemoryIfNeeded(void) {
size_t mem_used, mem_tofree, mem_freed;
int slaves = listLength(server.slaves);
/* Remove the size of slaves output buffers and AOF buffer from the
* count of used memory. */
// 计算出 Redis 目前占用的内存总数，但有两个方面的内存不会计算在内：
// 1）从服务器的输出缓冲区的内存
// 2）AOF 缓冲区的内存
mem_used = zmalloc_used_memory();
if (slaves) {
listIter li;
listNode *ln;
listRewind(server.slaves,&li);
while((ln = listNext(&li))) {
redisClient *slave = listNodeValue(ln);
unsigned long obuf_bytes = getClientOutputBufferMemoryUsage(slave);
if (obuf_bytes > mem_used)
mem_used = 0;
else
mem_used -= obuf_bytes;
}
}
if (server.aof_state != REDIS_AOF_OFF) {
mem_used -= sdslen(server.aof_buf);
mem_used -= aofRewriteBufferSize();
}
/* Check if we are over the memory limit. */
// 如果目前使用的内存大小比设置的 maxmemory 要小，那么无须执行进一步操作
if (mem_used <= server.maxmemory) return REDIS_OK;
// 如果占用内存比 maxmemory 要大，但是 maxmemory 策略为不淘汰，那么直接返回
if (server.maxmemory_policy == REDIS_MAXMEMORY_NO_EVICTION)
return REDIS_ERR; /* We need to free memory, but policy forbids. */
/* Compute how much memory we need to free. */
// 计算需要释放多少字节的内存
mem_tofree = mem_used - server.maxmemory;
// 初始化已释放内存的字节数为 0
mem_freed = 0;
// 根据 maxmemory 策略，
// 遍历字典，释放内存并记录被释放内存的字节数
while (mem_freed < mem_tofree) {
int j, k, keys_freed = 0;
// 遍历所有字典
for (j = 0; j < server.dbnum; j++) {
long bestval = 0; /* just to prevent warning */
sds bestkey = NULL;
dictEntry *de;
redisDb *db = server.db+j;
dict *dict;
if (server.maxmemory_policy == REDIS_MAXMEMORY_ALLKEYS_LRU ||
server.maxmemory_policy == REDIS_MAXMEMORY_ALLKEYS_RANDOM)
{
// 如果策略是 allkeys-lru 或者 allkeys-random
// 那么淘汰的目标为所有数据库键
dict = server.db[j].dict;
} else {
// 如果策略是 volatile-lru 、 volatile-random 或者 volatile-ttl
// 那么淘汰的目标为带过期时间的数据库键
dict = server.db[j].expires;
}
// 跳过空字典
if (dictSize(dict) == 0) continue;
/* volatile-random and allkeys-random policy */
// 如果使用的是随机策略，那么从目标字典中随机选出键
if (server.maxmemory_policy == REDIS_MAXMEMORY_ALLKEYS_RANDOM ||
server.maxmemory_policy == REDIS_MAXMEMORY_VOLATILE_RANDOM)
{
de = dictGetRandomKey(dict);
bestkey = dictGetKey(de);
}
/* volatile-lru and allkeys-lru policy */
// 如果使用的是 LRU 策略，
// 那么从一集 sample 键中选出 IDLE 时间最长的那个键
else if (server.maxmemory_policy == REDIS_MAXMEMORY_ALLKEYS_LRU ||
server.maxmemory_policy == REDIS_MAXMEMORY_VOLATILE_LRU)
{
struct evictionPoolEntry *pool = db->eviction_pool;
while(bestkey == NULL) {
//随机取一集键值对
evictionPoolPopulate(dict, db->dict, db->eviction_pool);
/* Go backward from best to worst element to evict. */
for (k = REDIS_EVICTION_POOL_SIZE-1; k >= 0; k--) {
if (pool[k].key == NULL) continue;
de = dictFind(dict,pool[k].key);
/* Remove the entry from the pool. */
sdsfree(pool[k].key);
/* Shift all elements on its right to left. */
memmove(pool+k,pool+k+1,
sizeof(pool[0])*(REDIS_EVICTION_POOL_SIZE-k-1));
/* Clear the element on the right which is empty
* since we shifted one position to the left. */
pool[REDIS_EVICTION_POOL_SIZE-1].key = NULL;
pool[REDIS_EVICTION_POOL_SIZE-1].idle = 0;
/* If the key exists, is our pick. Otherwise it is
* a ghost and we need to try the next element. */
if (de) {
bestkey = dictGetKey(de);
break;
} else {
/* Ghost... */
continue;
}
}
}
}
/* volatile-ttl */
// 策略为 volatile-ttl ，从一集 sample 键中选出过期时间距离当前时间最接近的键
else if (server.maxmemory_policy == REDIS_MAXMEMORY_VOLATILE_TTL) {
for (k = 0; k < server.maxmemory_samples; k++) {
sds thiskey;
long thisval;
de = dictGetRandomKey(dict);
thiskey = dictGetKey(de);
thisval = (long) dictGetVal(de);
/* Expire sooner (minor expire unix timestamp) is better
* candidate for deletion */
if (bestkey == NULL || thisval < bestval) {
bestkey = thiskey;
bestval = thisval;
}
}
}
/* Finally remove the selected key. */
// 删除被选中的键
if (bestkey) {
long long delta;
robj *keyobj = createStringObject(bestkey,sdslen(bestkey));
propagateExpire(db,keyobj);
/* We compute the amount of memory freed by dbDelete() alone.
* It is possible that actually the memory needed to propagate
* the DEL in AOF and replication link is greater than the one
* we are freeing removing the key, but we can't account for
* that otherwise we would never exit the loop.
*
* AOF and Output buffer memory will be freed eventually so
* we only care about memory used by the key space. */
// 计算删除键所释放的内存数量
delta = (long long) zmalloc_used_memory();
dbDelete(db,keyobj);
delta -= (long long) zmalloc_used_memory();
mem_freed += delta;
// 对淘汰键的计数器增一
server.stat_evictedkeys++;
notifyKeyspaceEvent(REDIS_NOTIFY_EVICTED, "evicted",
keyobj, db->id);
decrRefCount(keyobj);
keys_freed++;
/* When the memory to free starts to be big enough, we may
* start spending so much time here that is impossible to
* deliver data to the slaves fast enough, so we force the
* transmission here inside the loop. */
if (slaves) flushSlavesOutputBuffers();
}
}
if (!keys_freed) return REDIS_ERR; /* nothing to free... */
}
return REDIS_OK;
}
【参考】
[1] 《Redis设计与实现》
[2] 《Redis源码日志》