A brief comparative study of Redis and Memcached, their pros and cons, some use case scenarios, the memory management model used in both and a note on clustering. Based on research done on the Internet. This presentation will be updated as I get more info on Redis/Memcached

1. REDIS
Is it Memcached with persistence and more data types?
Prabhu Missier
Samvit Software Solutions
Melbourne, Australia

2. What is REDIS
• An in-memory nosql data structure store vs Memcached is a key/value store
• Caching is just one of its options
• Several powerful data-types such as Hashes, Sorted Sets
• Persistence to disk
• In-built cluster support
• By default is not LRU
• Values can be upto 512MB in size vs 1MB for Memcached

3. What is REDIS
• Generally used as a persistent data store with LRU configured when it reaches
maximum capacity
• Can selectively delete items in the cache
• Suitable for Key -> someObject scenarios as opposed to Key -> string Memcached
scenarios
• Can selectively invalidate items in the cache
• REDIS is limited to a single core but now with 3.0 includes out-of-the-box cluster
support

4. REDIS – Memory Management
• Has no fixed Max Memory limit on the cache instance
• By default uses the jemalloc memory allocator which cannot keep up with a
high rate of allocations and purging
• More objects put in, more memory is used
• Certain loads could cause memory leaks at times

5. MEMCACHED- Memory Mgmt
• Allocates one large chunk of memory. The max page size is 1MB
• Uses its own slab-allocator and remember each slab-class has its own LRU
• This means memory chunks get allocated only from their specific class even if there’s a free
chunk in another slab-class
• Internal fragmentation is low as it uses per-slab LRU algorithm
• Suitable where memory usage has to be enforced
• Consistent hashing is used which reduces key misses.
• REDIS doesn’t match up when for this use case – LRU based, non-persistent

6. Slab Allocation
• Frequent initialization and destruction of kernel objects can hamper
performance.
• When an object is destroyed the memory is not released. Instead it gets
added to a list of available nodes
• The memory gets reused when a similar type of object has to be initialized
• Minimizes fragmentation by avoiding frequent allocation/deallocation
• Ref : https://en.wikipedia.org/wiki/Slab_allocation

7. Where does REDIS make sense
• Handles variety of formats : lists, sets, array
• Working with sorted sets in memory. It beats memcached here
• collating page clicks analytics data
• displaying real-time leaderboard info
• High speed data ingest
• Message queuing
• Session Storage

8. REDIS pros
• Works better with a geographically distributed cache, say AWS zones
• Pipelining helps in multiple operations on cached datasets at once
• Stores data in a variety of formats : sets, lists, sorted sets, bitmaps, hashes, geospatial indices. Atomic ops on
these is possible. Eg. Push new elem to list
• Blocking reads – will wait till a write completes
• Backs data to disk
• Partitions data across multiple REDIS instances
• Fine-tune cache contents by choosing from 6 eviction policies
• Persistent by default, so restarts are better. No cache warmup is needed
• Lua scripting

9. REDIS cons
• SENTINEL which controls slave-master promotion is unstable. If it fails
then failover from slave-master fails
• Complex to configure and administer for monitoring, partitioning &
balancing
• Ref: http://www.bigdatalittlegeek.com/blog/2014/3/25/memcached-vs-
redis

10. Where does MEMCACHED make sense
• Database queries, reduce read load on databases
• Page fragments
• REST round trips

11. MEMCACHED pros and cons
• PROS
• Low complexity, easy to configure, simple to cluster since a hashing algo is used by the client to shard
across the cluster
• CONS
• Is just an in-memory key-value store
• Caches sharded by client can’t be distributed across AWS zones
• Adding a member to the pool requires a client reconfigure and re-start
• Eviction policy is LRU and sometimes the object being evicted is simply the same size as the new one
http://www.bogotobogo.com/DevOps/Redis/Redis_vs_Memcached.php
• Cache misses happen on a restart

12. MEMCACHED pros and cons
• MEMCACHED is supposed to perform better when there are concurrent
requests
• Eviction policy is better implemented
• Depends on:
• Type of hardware,Types of data you store
• Amount of get and set, App concurrency
• Do you need data structure storage

13. A word on clustering
Consistent hashing on Memcached is still a more robust model for graceful
failure (at least if you are using it as a transient cache) than the Redis sharding.
As nodes fall the cache keys automatically move to other servers. As long as
you have a single server your cache still operates, albeit poorly. As opposed to
redis where, to my understanding, if you lose master and slave for any
hashgroup your cluster fails.

14. References
• http://www.bigdatalittlegeek.com/blog/2014/3/25/memcached-vs-redis
• http://www.bogotobogo.com/DevOps/Redis/Redis_vs_Memcached.php
• http://blog.andolasoft.com/2014/02/memcached-vs-redis-which-one-to-pick-for-large-web-app.html
• https://www.adayinthelifeof.nl/2011/02/06/memcache-internals/
• http://stackoverflow.com/questions/4188620/redis-and-memcache-or-just-redis?rq=1
• http://stackoverflow.com/questions/7888880/what-is-redis-and-what-do-i-use-it-for?rq=1
• http://stackoverflow.com/questions/10558465/memcached-vs-redis?rq=1
• http://stackoverflow.com/questions/23601622/if-redis-is-already-a-part-of-the-stack-why-is-memcached-
still-used-alongside-r?rq=1
• http://stackoverflow.com/questions/10558465/memcached-vs-redis