According to this article http://emboss.github.io/blog/2012/12/14/breaking-murmur-hash-flooding-dos-reloaded/, arbitrary collisions can be produced for Murmurhash2 algorithm and, more importantly, Redis performs no encoding on inserted keys.
The picture below shows the collisions.
And these pictures will show you how Redis and server will act while inserting malicious keys.
The hashtable will act like a linked list if massive key-value pairs inserted into Redis with collisional strings as the keys when access these records.
Redis links the collisions with linked list so far,the practical solution could be rehash(eg.use time33 hash).If the item still get collided with existed item(s),then just link them. As far as I am concerned, it is difficult to construct collisions for two different hash functions at the same time.