Redis: Amazon EC2 vs Elasticache

Question

I want to host a Redis Server by myself. I compared EC2 to Elasticache. And I would like to know what the disadvantage of EC2 are.

An EC2 tiny instance costs as much as the ELasticache tiny instance but hast 400 mb of ram more. Why should use Elasticache and not setup an own Redis Server on the ec2 tiny isntance?

Answer 1

tl;dr: Elasticache forces you to use a single instance of redis, which is sub-optimal.

The long version:

I realize this is an old post (2 years at the time of this writing) but I think it's important to note a point I don't see here.

On elasticache your redis deployment is managed by Amazon. This means you're stuck with however they choose to run your redis.

Redis uses a single thread of execution for reads/writes. This ensures consistency w/o locking. It's a major asset in terms of performance not to manage locks and latches. The unfortunate consequence, though, is that if your EC2 has more than 1 vCPU they will go unused. This is the case for all elasticache instances with more than one vCPU.

The default elasticache instance size is cache.r3.large, which has two cores.

In fact, there are a number of instance sizes with multiple vCPUs. Lots of opportunity for this issue to manifest.

It seems Amazon is already aware of this issue, but they seem a bit dismissive of it.

The part that makes this especially relevant to this question is that on your EC2 (since you're managing your own deployment) you're able to implement multi-tenancy. This means you have many instances of the redis process listening on different ports. By choosing which port to read/write to/from in the application based on a hash of the record's key you can leverage all your vCPUs.

As a side note; an redis elasticache deployment on a multi-core machine should always under perform compared to memcached elasticache deployment on the instance size. With multi-tenancy redis tends to be the winner.

Update:

Amazon now provides separate metrics for your redis instance CPU, EngineCPUUtilization. You no longer need to compute your CPU with the shoddy multiplication, but multi-tenancy is still not implemented.