Clarifications about Redis and Memcached | Hacker News Reader

Clarifications about Redis and Memcached

antirez.com

191 pointsjuanfatas38 comments

I dropped Memcached in favor of Redis for caching a long time ago, because as far as it matters for my purposes, Redis is a strictly-superior superset of Memcached's functionality, and I have no desire to maintain multiple pieces of software in the stack if one will do the job.

I'm sure there are extreme cases where Memcached is in fact the better tool for the job over Redis for caching workloads. I also expect that 99%+ of people trying to decide between Redis and Memcached will never get into that territory. Redis is so fast that unless you're doing O(n) operations on very large data sets, you're unlikely to notice any substantial differences.

The other thing about caching is that the data is, by its nature, disposable and rebuildable. So even in the extreme minority case where Redis would no longer be sufficient, migration from one KV cache system to another is about as easy as it gets. Pre-optimizing your caching layer stack for Facebook levels of traffic isn't even justifiable from a lock-in standpoint like it might be with other data storage needs.

In the case of your average Sidekiq user, serving cache fragments for a Rails app, memcached vs redis for your caching layer is almost certainly an inconsequential choice WRT the performance of your application, and the choice of Redis reduces your ops and client library overhead. The choice should be pretty clear in those circumstances.

This is a really good blog post. I find it very neutral even when the author happens to be the one who created Redis.

What I really like about both pieces of software is that they are dead simple to set up. If I were to make a decision today for new projects who are just starting to scale I'd probably take a good hard look at redis and see if you don't need any of the extra features. I could see a lot of projects eventually needing those features and leveraging redis. I really like the pub/sub feature in particular. Persistence is always nice if you don't have to pay too much for it.

Does anyone know if amazon has a redis/memcache service? I can never remember what they name things.

https://aws.amazon.com/elasticache/

They do - ElastiCache. And as it happens, you can choose either Redis or Memcached!

I'm curious about the "threaded redis" reference. Back a couple of years ago I built a Collaborative Filtering recommendation system that used Redis for graph storage and relied heavily on sorted sets to compute the recommendation right in Redis.

I really needed some kind of a parallelism and so I hacked together http://thredis.org/ (and then mostly for fun added SQL operations to it by linking it with SQLite).

Since then I've kind of abandoned this project and moved on to other things, but I still think that there is a valid case for some form parallelism in Redis. I had learned some tough lessons while hacking on Thredis such as importance of ordering locks, having retry strategies, and there are still bugs that can cause it to crash AFAIK, but the take away was that it's doable - I was a newbie at it, today I'd probably do a much better job. In my (not so scientific) testing Thredis was only slightly slower than Redis.

I remember thredis very well! But it's different compared to what memcached does and Redis has plans for: memcached just threads the I/O part, not the access to the key space which is serialized via a mutex. However what you had in mind is also in our long term plans... and was addressed in another blog post here: http://antirez.com/news/93

The locks are becoming more fine-grained in memcached [1], so that should be less of a problem now.

It is possible to remove lock contention on the read path [2] if a concurrent hash table is used. This can be done while using an O(1) eviction policy that outperforms LRU [3].

[1] https://github.com/memcached/memcached/pull/97 [2] https://github.com/ben-manes/caffeine/wiki/Design [3] https://github.com/ben-manes/caffeine/wiki/Efficiency

NovaX: thanks for the interesting references. The point is, is it worth for memcached to avoid the global interpreter lock in the hash table with the number of cores currently deployed machines have? I would expect to see very little contention. The concurrent hash table looks a good idea for memcached, for sure to have a mutex per key would be likely an overkill in terms of memory usage. I'll try to read with care the links you provided, thank you.

As you said elsewhere, the network I/O is the primary bottleneck. There are a lot of different hashtable designs (so per-key locks not required), but fine grained locking of the table/LRU is probably enough. Since an in-app cache has a different perf profile, the latter two links summarize my work.

I wrote up a fairly detailed explanation of the Thredis locking strategy, it took me a while to realize that a lot fewer locks than it seems at first is needed: https://github.com/grisha/thredis/blob/c9207a373c3ff0c9fb70a...

I tried both and i prefer Redis over Memcached except for very simple use cases. It is true that for a pure caching system Memcached is easier to setup, easier to scale and faster. But by just a small margin. On the opposite side, Redis provide a lot of primitive very fast and implemented in C that allow for more complex use cases. For instance, in one of the project I was following, the ability to merge two or more sorted set was pivotal in implementing a more efficient warm-up scheme. I could get the same result with Memcached and a software layer, but it would be less efficient in term of latency and bandwidth. Given these huge advantages, my choice is almost always Redis because I prefer to manage one system instead of two.

If data access is equally distributed across keys, then running multiple Redis instances (with sharding) is perfect, since ultimately you want each key to be serviced by a single core, for optimal performance in a system with caches and NUMAs.

However, if data access is read-heavy and not equally distributed across keys, then having shared memory (aka multithreading) is quite essential since you want all cores to operate on the same data, and shared memory is much better than sending the data across especially if the heavily accessed keys vary quickly over time.

I'm not sure if memcached handles this well (it requires a lightweight rwlock/RCU/MVCC mechanism, for instance), but a shared-nothing system like Redis cannot provide good performance in all cases.

Good point, but I could add that case 1 is vanishingly unlikely to happen in a real system. You always have hot keys, i.e. a Zipfian distribution. That's almost tautological for a cache -- by using a small amount of space you can handle most of the reads.

So, not knowing much about Redis, I would conclude based on this blog post that memcached has a pretty big advantage as a cache in real systems (multithreading).

I think this is not the case, I already replied to the parent comment.

In a networked server getting data from memory, the time required to access the data itself is negligible, so the real performance in mostly a factor of how much I/O a thread can sustain (Redis with heavy pipelining will handle at least 500k ops/sec in the same hardware it handles 100k ops/sec without pipelining). There is still the case of the load to be very biased towards, like, 5% of keys. But that 5% of keys are very very likely to be distributed across all processes.

However this is not true when you have a case with, like, 2/3 super hot keys that are requested a lot more than any other. But in this case what allows scalability is replication with many read-replicas.

I think once you implement threaded i/o, requests for hot keys will hit in the cpu cache and you'll become NIC limited. At that point, read replicas is the best solution rather than shared memory since contention will move to NIC and adding more cpus won't help.

Edit: Salvatore, you should also look at the Seastar/ScyllaDB design (if you haven't yet) - that architecture would work well for redis as well. And if user has access to DPDK (or other kernel bypass enabled NICs, like Solarflare), their performance will go up even further.

The group behind Arrakis did some testing by bypassing the kernel through NIC's and the results were pretty amazing. They also made modifications to memcached and got similarly great results FWIW.

https://www.usenix.org/system/files/conference/osdi14/osdi14...

Already exists for memcached!

https://github.com/scylladb/seastar/wiki/Memcached-Benchmark

I'm glad antirez spoke up and clarified. I do think Redis is perfectly acceptable as a cache if you make a few config tweaks and run a second instance.

Many of the developers I work with are unfamiliar with Redis. Maybe they've heard of it and they install it to use Sidekiq and that's it. From the perspective of a newcomer, memcached is "safer" to run because it'll be memory limited automatically with literally zero config necessary. Redis does require the non-default LRU and memory tuning to be a safe cache.

It sounds like my performance concerns are primarily a thing of the past. Glad to hear it and I look forward to the threaded I/O coming after lazyfree.

For a post that attempts to compare Redis to memcached for caching, it's amazing how few actual numbers appear in the post.

"Actual numbers" for caching services only have meaning in the context of a particular application.

Yes, but as-is the post is pure hand-waving. At least some measurement could confirm the theories being stated. Did someone actually try taking one use case where they had memcached and replace it with Redis? What actually happened?

Unless it was some very limiting case, I doubt there was much of a difference -- for simple operations e.g. REDIS (GET/SET) the IPC/network stack will contribute more the to the runtime than the difference between the choice of caching programs.

Ok then. Build an application heavily dependent on caching. Restart memcached in the middle a production workload. How long does it take to get out from under the dog pile?

EDIT: This started out a bit flippant. Wanted to make the point that antirez is not just handwaving.

For example one of the points without elaboration was 'there are “pure caching” use cases where persistence and replication are important'. Sometimes caching warming isn't feasible i.e. needing to restart the cache in the middle of a production workload. Using entirely volatile cache one can find extended downtime from the dog pile of requests waiting for the cache to warm up. Persistence can be an attractive form of insurance against this scenario.

Exactly, basically we can provide numbers only for GET/SET/DEL or similar workloads. Since Redis has more capabilities that may speedup dramatically certain use cases the only good comparison would be per-use-case. However to provide the basic operations benchmarks for different data sizes can give at least an idea about the "HTML fragments caching" use case and other similar workloads.

Hello, it's a shame I was not able to provide numbers. I did in the past, and went to the point of porting redis-benchmark to memcached in order to test it in a very similar fashion, but now the code is deprecated and I've no easy way to test it again. From what I saw back then, the difference "per core" is not going to be big either side. I remember Redis was faster in my tests per single core, but after that both Redis and Memcached got many optimizations, so now it can be the other way around. But I doubt to spot huge differences. Yet... it's worth to try and maybe Redis Labs memtier_benchmark may be helpful here: https://redislabs.com/blog/memtier_benchmark-a-high-throughp.... Soon or later I'll try again, this may be very useful to spot inefficiencies in both sides.

I took the liberty of doing a very naive test on my laptop's VM - the results are at https://gist.github.com/itamarhaber/2ed09513ec6f6ae10071.

  Memcached text: 43585 OPS @ 4.58 msec 
  Memcached binary: 40242 OPS @ 4.96 msec
  Redis: 39727 OPS @ 5.03 msec
  Redis, pipeline 4: 131312 OPS @ 6.09 msec

Some minor points

> Redis in its special case of memcached replacement, may be addressed executing multiple processes

This is always less optimal to a single process. Simpler is better.

> Redis is very very observable

This has a cost...trashing your phenomenal read and write performance. Running a concurrent redis, just for monitoring, is a hack that I have used to continue leveraging the observability.

I agree that simpler is often better, but running multiple cache processes may be a requirement for either memcached or redis if you want to get consistent latency from them. I have observed a significant improvement in performance (at large scale) when binding memcached to a single NUMA zone.

NUMA lets you address non-local memory, but memcached and redis don't utilize libnuma so they don't know whether the memory is local or not. The entire system's memory is available as one contiguous blob, but some of it is a lot slower than the other (depending on which CPU core you're running on). To get around this, on most servers you'll need to run two processes (and two different ports) and bind them to appropriate CPU core sets and memory.

Thanks for the info, I think similar results were reproduced with Redis too.

I would guess that Mike Perham's point of view is based on how Sidekiq uses Redis for persisting jobs. If you don't configure Redis properly and you use Redis as a job queue as well as a cache then you risk the problem of your job queue entries getting evicted depending on your Redis config/version of Redis you are using. That type of thing happening frequently would be very annoying for someone that made a job queue that uses Redis for persistence of jobs.

Caching web content can grow and grow until you run out of memory. If you aren't using Redis for data that can be easily regenerated, then go ahead and use Redis as a cache. But if you are, I think it will give you operational peace of mind to segregate where you store your background jobs and where you cache content.

That makes zero sense. Why would you run them out of the same instance / with the same configuration? Mike is already running a second process for caching; he'd simply replace it with a different instance of Redis.

You're right. It doesn't make sense. Does that prevent tons of people from doing it? Nope.

Because users don't read docs. They don't understand these tools. They don't catch on that two completely different use cases require two completely different configurations and two completely different instances of Redis.

I cache with redis:

https://github.com/seamusabshere/lock_and_cache

(and lock too, with Redis Redlock)

Last time I looked Redis would balk if it ran out of memory whilst memcached just pushed out the oldest caches.

I guess that's not a problem anymore..?

By changing the eviction policy and using the maxmemory directive, you can effectively reproduce memcached's behavior in redis.

http://redis.io/topics/lru-cache

Both are relatively small projects and really there is no much stuff to compare.

I'm slowly working on my own memcached clone and plan to add persistence there eventually. Not so sure about replication, it might be too hard. In principle, after I'm done with getting a better version of memcached, both feature-wise and performance-wise, if time permits, I can also add the redis protocol support.

Therefore, it will be possible to have both in a single package and don't worry finding which one is better.

This is my project, if anyone is interested: https://github.com/ademakov/MainMemory

[Update: my statement was only about cache-related functionality, admittedly redis supports very interesting data structures, persistence, replication. But as just a very fast in-memory cache, there is nothing particularly advanced in either case. On the other hand there are projects like RAMCloud, Seastar that I find inspiring when I work on my own project.]

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