redis 缓存策略
配置项：
maxmemory <bytes>
maxmemory-policy noeviction
触发时机：每次执行命令(processCommand)的时候会检测
while 循环条件是 (mem_freed < mem_tofree)，每次选择一个 bestkey 进行删除。
1. 确定 dict
如果是 MAXMEMORY_ALLKEYS_LRU, MAXMEMORY_ALLKEYS_RANDOM
使用 dict 键空间
否则使用 expires 空间
2. 确定 bestkey
如果是 MAXMEMORY_ALLKEYS_RANDOM 或 MAXMEMORY_VOLATILE_RANDOM
随机选取一个 key 作为 bestkey
如果是 MAXMEMORY_ALLKEYS_LRU 或 MAXMEMORY_VOLATILE_LRU
随机取 maxmemory_samples 个键，选一个 idletime 最长的键作为 bestkey
如果是 MAXMEMORY_VOLATILE_TTL
随机取 maxmemory_samples 个键，选一个过期时间最小的键作为 bestkey
int freeMemoryIfNeeded(void) {
size_t mem_used, mem_tofree, mem_freed;
int slaves = listLength(server.slaves);
mstime_t latency, eviction_latency;
/* Remove the size of slaves output buffers and AOF buffer from the
* count of used memory. */
mem_used = zmalloc_used_memory();
if (slaves) {
listIter li;
listNode *ln;
listRewind(server.slaves,&li);
while((ln = listNext(&li))) {
client *slave = listNodeValue(ln);
unsigned long obuf_bytes = getClientOutputBufferMemoryUsage(slave);
if (obuf_bytes > mem_used)
mem_used = ;
else
mem_used -= obuf_bytes;
}
}
if (server.aof_state != AOF_OFF) {
mem_used -= sdslen(server.aof_buf);
mem_used -= aofRewriteBufferSize();
}
/* Check if we are over the memory limit. */
if (mem_used <= server.maxmemory) return C_OK;
if (server.maxmemory_policy == MAXMEMORY_NO_EVICTION)
return C_ERR; /* We need to free memory, but policy forbids. */
/* Compute how much memory we need to free. */
mem_tofree = mem_used - server.maxmemory;
mem_freed = ;
latencyStartMonitor(latency);
while (mem_freed < mem_tofree) {
int j, k, keys_freed = ;
for (j = ; j < server.dbnum; j++) {
long bestval = ; /* just to prevent warning */
sds bestkey = NULL;
dictEntry *de;
redisDb *db = server.db+j;
dict *dict;
if (server.maxmemory_policy == MAXMEMORY_ALLKEYS_LRU ||
server.maxmemory_policy == MAXMEMORY_ALLKEYS_RANDOM)
{
dict = server.db[j].dict;
} else {
dict = server.db[j].expires;
}
if (dictSize(dict) == ) continue;
/* volatile-random and allkeys-random policy */
if (server.maxmemory_policy == MAXMEMORY_ALLKEYS_RANDOM ||
server.maxmemory_policy == MAXMEMORY_VOLATILE_RANDOM)
{
de = dictGetRandomKey(dict);
bestkey = dictGetKey(de);
}
/* volatile-lru and allkeys-lru policy */
else if (server.maxmemory_policy == MAXMEMORY_ALLKEYS_LRU ||
server.maxmemory_policy == 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 = MAXMEMORY_EVICTION_POOL_SIZE-; k >= ; 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+,
sizeof(pool[])*(MAXMEMORY_EVICTION_POOL_SIZE-k-));
/* Clear the element on the right which is empty
* since we shifted one position to the left. */
pool[MAXMEMORY_EVICTION_POOL_SIZE-].key = NULL;
pool[MAXMEMORY_EVICTION_POOL_SIZE-].idle = ;
/* 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 */
else if (server.maxmemory_policy == MAXMEMORY_VOLATILE_TTL) {
for (k = ; 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. */
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(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 C_ERR; /* nothing to free... */
}
}
latencyEndMonitor(latency);
latencyAddSampleIfNeeded("eviction-cycle",latency);
return C_OK;
}