performance - Redis 10x more memory usage than data

Question

I am trying to store a wordlist in redis. The performance is great.

My approach is of making a set called "words" and adding each new word via 'sadd'.

When adding a file thats 15.9 MB and contains about a million words, the redis-server process consumes 160 MB of ram. How come I am using 10x the memory, is there any better way of approaching this problem?

Answer 1

Well this is expected of any efficient data storage: the words have to be indexed in memory in a dynamic data structure of cells linked by pointers. Size of the structure metadata, pointers and memory allocator internal fragmentation is the reason why the data take much more memory than a corresponding flat file.

A Redis set is implemented as a hash table. This includes:

• an array of pointers growing geometrically (powers of two)
• a second array may be required when incremental rehashing is active
• single-linked list cells representing the entries in the hash table (3 pointers, 24 bytes per entry)
• Redis object wrappers (one per value) (16 bytes per entry)
• actual data themselves (each of them prefixed by 8 bytes for size and capacity)

All the above sizes are given for the 64 bits implementation. Accounting for the memory allocator overhead, it results in Redis taking at least 64 bytes per set item (on top of the data) for a recent version of Redis using the jemalloc allocator (>= 2.4)

Redis provides memory optimizations for some data types, but they do not cover sets of strings. If you really need to optimize memory consumption of sets, there are tricks you can use though. I would not do this for just 160 MB of RAM, but should you have larger data, here is what you can do.

If you do not need the union, intersection, difference capabilities of sets, then you may store your words in hash objects. The benefit is hash objects can be optimized automatically by Redis using zipmap if they are small enough. The zipmap mechanism has been replaced by ziplist in Redis >= 2.6, but the idea is the same: using a serialized data structure which can fit in the CPU caches to get both performance and a compact memory footprint.

To guarantee the hash objects are small enough, the data could be distributed according to some hashing mechanism. Assuming you need to store 1M items, adding a word could be implemented in the following way:

• hash it modulo 10000 (done on client side)
• HMSET words:[hashnum] [word] 1

Instead of storing:

words => set{ hi, hello, greetings, howdy, bonjour, salut, ... }

you can store:

words:H1 => map{ hi:1, greetings:1, bonjour:1, ... }
words:H2 => map{ hello:1, howdy:1, salut:1, ... }
...

To retrieve or check the existence of a word, it is the same (hash it and use HGET or HEXISTS).

With this strategy, significant memory saving can be done provided the modulo of the hash is chosen according to the zipmap configuration (or ziplist for Redis >= 2.6):

# Hashes are encoded in a special way (much more memory efficient) when they
# have at max a given number of elements, and the biggest element does not
# exceed a given threshold. You can configure this limits with the following
# configuration directives.
hash-max-zipmap-entries 512
hash-max-zipmap-value 64

Beware: the name of these parameters have changed with Redis >= 2.6.

Here, modulo 10000 for 1M items means 100 items per hash objects, which will guarantee that all of them are stored as zipmaps/ziplists.