Official document

Using Redis as an LRU cache

Eviction policies

There are total 6 evicition policies so far:

• noeviction
• allkeys-lru
• allkeys-random
• volatile-lru
• volatile-random
• volatile-ttl

allkeys for all keys, while volatile means among keys that have an expire set.
lru means evict keys by trying to remove the less recently used (LRU) keys first, random means evict keys randomly, ttl means try to evict keys with a shorter time to live (TTL) first.

The range of the keys to be evicted:

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;
}

Getting a paricular key by using random:

/* 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 = dictGetKey(de);
}

Getting a particular key by using LRU:

/* volatile-lru and allkeys-lru policy */
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;
            }
        }
    }
}

Getting a particular key by using TTL:

/* 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 = 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;
        }
    }
}