/* A redis object, that is a type able to hold a string / list / set */
/* The actual Redis Object */
#define REDIS_LRU_BITS 24
#define REDIS_LRU_CLOCK_MAX ((1<<REDIS_LRU_BITS)-1) /* Max value of obj->lru */
#define REDIS_LRU_CLOCK_RESOLUTION 1000 /* LRU clock resolution in ms */
typedef struct redisObject {
unsigned type:4;
unsigned encoding:4;
unsigned lru:REDIS_LRU_BITS; /* lru time (relative to server.lruclock) */
int refcount;
void *ptr;
} robj;
/* Redis database representation. There are multiple databases identified
* by integers from 0 (the default database) up to the max configured
* database. The database number is the 'id' field in the structure. */
typedef struct redisDb {
dict *dict; // 保存所有 key
dict *expires; // 设置了过期时间的 key 及其过期时间
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 */
struct evictionPoolEntry *eviction_pool; /* Eviction pool of keys */
int id; /* Database ID */
long long avg_ttl; /* Average TTL, just for stats */
} redisDb;
int freeMemoryIfNeeded(void) {
if (mem_used <= server.maxmemory) return REDIS_OK; // 如果已经使用的没有达到上限，则直接返回ok
if (server.maxmemory_policy == REDIS_MAXMEMORY_NO_EVICTION) // NO EVICTION 策略，直接报错返回
return REDIS_ERR;
mem_tofree = mem_used - server.maxmemory; // 计算需要释放多少内存
mem_freed = 0;
latencyStartMonitor(latency);
while (mem_freed < mem_tofree) { // 选择最合适的key释放内存直到达到了需要的内存为止
int j, k, keys_freed = 0;
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;
// 判断淘汰策略是否与过期key有关,即从哪个哈希表找相应的key
if (server.maxmemory_policy == REDIS_MAXMEMORY_ALLKEYS_LRU || server.maxmemory_policy == REDIS_MAXMEMORY_ALLKEYS_RANDOM) {
dict = server.db[j].dict; // ALLKEYS 类的策略查 dict
} else {
dict = server.db[j].expires; // VOTILE 类的策略查 expires
}
if (dictSize(dict) == 0) continue;
// RANDOM 类策略，就随机选 key 出来删除
if (server.maxmemory_policy == REDIS_MAXMEMORY_ALLKEYS_RANDOM || server.maxmemory_policy == REDIS_MAXMEMORY_VOLATILE_RANDOM) {
de = dictGetRandomKey(dict);
bestkey = dictGetKey(de);
}
// LRU 类的策略
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) {
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;
}
}
}
}
// TTL 类的策略
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;
}
}
}
// 最后删除选择的key
/* Finally remove the selected key. */
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. */
delta = (long long) zmalloc_used_memory();
latencyStartMonitor(eviction_latency);
dbDelete(db,keyobj);
latencyEndMonitor(eviction_latency);
latencyAddSampleIfNeeded("eviction-del",eviction_latency);
latencyRemoveNestedEvent(latency,eviction_latency);
delta -= (long long) zmalloc_used_memory();
mem_freed += delta;
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) {
latencyEndMonitor(latency);
latencyAddSampleIfNeeded("eviction-cycle",latency);
return REDIS_ERR; /* nothing to free... */
}
}
latencyEndMonitor(latency);
latencyAddSampleIfNeeded("eviction-cycle",latency);
return REDIS_OK;
}