redis可以被作为类似memcached的应用级缓存使用，在内存超过限制时，按照配置的策略，淘汰掉相应的kv，使得内存可以继续留有足够的空间保存新的数据。
redis的conf文件中有对该机制的一份很好的解释：
194 # Don't use more memory than the specified amount of bytes.
195 # When the memory limit is reached Redis will try to remove keys
196 # accordingly to the eviction policy selected (see maxmemmory-policy).
197 #
198 # If Redis can't remove keys according to the policy, or if the policy is
199 # set to 'noeviction', Redis will start to reply with errors to commands
200 # that would use more memory, like SET, LPUSH, and so on, and will continue
201 # to reply to read-only commands like GET.
202 #
203 # This option is usually useful when using Redis as an LRU cache, or to set
204 # an hard memory limit for an instance (using the 'noeviction' policy).
205 #
206 # WARNING: If you have slaves attached to an instance with maxmemory on,
207 # the size of the output buffers needed to feed the slaves are subtracted
208 # from the used memory count, so that network problems / resyncs will
209 # not trigger a loop where keys are evicted, and in turn the output
210 # buffer of slaves is full with DELs of keys evicted triggering the deletion
211 # of more keys, and so forth until the database is completely emptied.
212 #
213 # In short... if you have slaves attached it is suggested that you set a lower
214 # limit for maxmemory so that there is some free RAM on the system for slave
215 # output buffers (but this is not needed if the policy is 'noeviction').
216 #
217 # maxmemory <bytes>
注意，在redis按照master-slave使用时，其maxmeory应设置的比实际物理内存稍小一些，给slave output buffer留有足够的空间。
redis支持如下五种缓存淘汰策略：
219 # MAXMEMORY POLICY: how Redis will select what to remove when maxmemory
220 # is reached? You can select among five behavior:
221 #
222 # volatile-lru -> remove the key with an expire set using an LRU algorithm
223 # allkeys-lru -> remove any key accordingly to the LRU algorithm
224 # volatile-random -> remove a random key with an expire set
225 # allkeys->random -> remove a random key, any key
226 # volatile-ttl -> remove the key with the nearest expire time (minor TTL)
227 # noeviction -> don't expire at all, just return an error on write operations
注释已经解释的很清楚了，不再赘述。
其缓存管理功能，由redis.c文件中的freeMemoryIfNeeded函数实现。如果maxmemory被设置，则在每次进行命令执行之前，该函数均被调用，用以判断是否有足够内存可用，释放内存或返回错误。如果没有找到足够多的内存，程序主逻辑将会阻止设置了REDIS_COM_DENYOOM flag的命令执行，对其返回command not allowed when used memory > 'maxmemory'的错误消息。
具体代码如下：
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. */ 计算占用内存大小时，并不计算slave output buffer和aof buffer，因此maxmemory应该比实际内存小，为这两个buffer留足空间。
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.appendonly) {
mem_used -= sdslen(server.aofbuf);
mem_used -= sdslen(server.bgrewritebuf);
}
/* Check if we are over the memory limit. */
if (mem_used <= server.maxmemory) return REDIS_OK;
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. */
mem_tofree = mem_used - server.maxmemory;
mem_freed = 0;
while (mem_freed < mem_tofree) {
int j, k, keys_freed = 0;
for (j = 0; j < server.dbnum; j++) {
long bestval = 0; /* just to prevent warning */
sds bestkey = NULL;
struct dictEntry *de;
redisDb *db = server.db+j;
dict *dict;
if (server.maxmemory_policy == REDIS_MAXMEMORY_ALLKEYS_LRU ||
server.maxmemory_policy == REDIS_MAXMEMORY_ALLKEYS_RANDOM)
{
dict = server.db[j].dict;
} else {
dict = server.db[j].expires;
}
if (dictSize(dict) == 0) continue;
/* volatile-random and allkeys-random policy */
if (server.maxmemory_policy == REDIS_MAXMEMORY_ALLKEYS_RANDOM ||
server.maxmemory_policy == REDIS_MAXMEMORY_VOLATILE_RANDOM)
{
de = dictGetRandomKey(dict);
bestkey = dictGetEntryKey(de);
}//如果是random delete,则从dict中随机选一个key
/* volatile-lru and allkeys-lru policy */
else if (server.maxmemory_policy == REDIS_MAXMEMORY_ALLKEYS_LRU ||
server.maxmemory_policy == REDIS_MAXMEMORY_VOLATILE_LRU)
{
for (k = 0; k < server.maxmemory_samples; k++) {
sds thiskey;
long thisval;
robj *o;
de = dictGetRandomKey(dict);
thiskey = dictGetEntryKey(de);
/* When policy is volatile-lru we need an additonal lookup
* to locate the real key, as dict is set to db->expires. */
if (server.maxmemory_policy == REDIS_MAXMEMORY_VOLATILE_LRU)
de = dictFind(db->dict, thiskey); //因为dict->expires维护的数据结构里并没有记录该key的最后访问时间
o = dictGetEntryVal(de);
thisval = estimateObjectIdleTime(o);
/* Higher idle time is better candidate for deletion */
if (bestkey == NULL || thisval > bestval) {
bestkey = thiskey;
bestval = thisval;
}
}//为了减少运算量,redis的lru算法和expire淘汰算法一样，都是非最优解，lru算法是在相应的dict中，选择maxmemory_samples(默认设置是3)份key，挑选其中lru的，进行淘汰
}
/* volatile-ttl */
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 = dictGetEntryKey(de);
thisval = (long) dictGetEntryVal(de);
/* Expire sooner (minor expire unix timestamp) is better
* candidate for deletion */
if (bestkey == NULL || thisval < bestval) {
bestkey = thiskey;
bestval = thisval;
}
}//注意ttl实现和上边一样，都是挑选出maxmemory_samples份进行挑选
}
/* Finally remove the selected key. */
if (bestkey) {
long long delta;
robj *keyobj = createStringObject(bestkey,sdslen(bestkey));
propagateExpire(db,keyobj); //将del命令扩散给slaves
/* 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();
dbDelete(db,keyobj);
delta -= (long long) zmalloc_used_memory();
mem_freed += delta;
server.stat_evictedkeys++;
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();
}
}//在所有的db中遍历一遍，然后判断删除的key释放的空间是否足够
if (!keys_freed) return REDIS_ERR; /* nothing to free... */
}
return REDIS_OK;
}
注意，此函数是在执行特定命令之前进行调用的，并且在当前占用内存低于限制后即返回OK。因此可能在后续执行命令后，redis占用的内存就超过了maxmemory的限制。因此,maxmemory是redis执行命令所需保证的最大内存占用，而非redis实际的最大内存占用。（在不考虑slave buffer和aof buffer的前提下）
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