Memory usage with multiple named caches?

[oconnor0]

Hi,

Currently investigating using FusionCache, it's looking good so far but I do have a question.

I'm converting a system with multiple cache instances backed by multiple Redis servers/databases. I haven't decided if I'm going to convert "multiple cache instances" to some variant of "named caches" or not yet. That'll obviously influence whether I keep the multiple Redis servers/databases or not.

As I have been reading the documentation, it seemed like having FusionCache own its MemoryCache made the most sense. Clear didn't sound like it would work properly with a shared MemoryCache (tho perhaps I avoid that by just tagging everything?). If I have multiple MemoryCaches (one per FusionCache), do they coordinate their total memory usage? I don't want to instantiate a whole bunch of FusionCaches and end up spending too much of my memory budget on caching.

Additionally, if there are multiple FusionCaches (with different CacheKeyPrefixes) pointing to a single Redis database, are there performance considerations here vs just using a single FusionCache instance and then prefixing the keys?

[jodydonetti]

Hi @oconnor0

I'm converting a system with multiple cache instances backed by multiple Redis servers/databases. I haven't decided if I'm going to convert "multiple cache instances" to some variant of "named caches" or not yet. That'll obviously influence whether I keep the multiple Redis servers/databases or not.

As I have been reading the documentation, it seemed like having FusionCache own its MemoryCache made the most sense.

In general, I would say yes.

Clear didn't sound like it would work properly with a shared MemoryCache

It works with any combination of L1/L2, dedicated or isolated, because... (see below)

(tho perhaps I avoid that by just tagging everything?).

... the Clear() feature uses tagging underneath 🙂 (plus some extra internal optimizations to make it even more performant).

If I have multiple MemoryCaches (one per FusionCache), do they coordinate their total memory usage?

No, in that case each is an isolated MemoryCache instance.

You can use a single MemoryCache instance shared with multiple FusionCache instances, but watch out for some things:

• in that case you should use Named Caches and a CacheKeyPrefix, to avoid unintentional cache key collisions
• if you set a SizeLimit on the MemoryCache, be aware that any entry without a Size specified will throw an exception
• also be aware that, with SizeLimit, when you try to add a new entry and the limit is reached, instead of removing an existing entry to make room for the new one (eviction policy), the new one will not be added (admission policy) and a background compaction will be triggered, that will make room for future new entries (later)

I don't want to instantiate a whole bunch of FusionCaches and end up spending too much of my memory budget on caching.

You can still set a SizeLimit (with the warnings mentioned above) even when assigning a separate MemoryCache instance to each FusionCache instance.
Of course in that case the SizeLimit will be per-cache and not a globally shared one.

Additionally, if there are multiple FusionCaches (with different CacheKeyPrefixes) pointing to a single Redis database, are there performance considerations here vs just using a single FusionCache instance and then prefixing the keys?

Regarding performance: no, no differences.
Regarding correctness: yes, there are, even if you manually prefix all your cache keys.
If you use the same Redis instance from multiple FusionCache instances and, without a properly configured CacheKeyPrefix at the FusionCache level you'll have issues (or anyway strange behaviors) when using Tagging or Clear, because those features use extra cache entries to work correctly.

If instead you just set a CacheKeyPrefix, all will work naturally.

Hope this helps, let me know.

[oconnor0]

I guess given the support for Named Caches, I'm probably overthinking it with wanting a single MemoryCache across all of them.

Also, to be clear, Named Caches are essentially just different FusionCache instantiations with disjoint CacheKeyPrefixes (if I'm not in a DI project and need to construct them manually). No extra magic there, right?

[jodydonetti]

Also, to be clear, Named Caches are essentially just different FusionCache instantiations with disjoint CacheKeyPrefixes (if I'm not in a DI project and need to construct them manually). No extra magic there, right?

If I got it right then yes, no extra magic.

Named Caches basically are:

• multiple FusionCache instances
• usually with a CacheKeyPrefix specified at the cache level

The part about using CacheKeyPrefix is particularly important because, by defining it for the cache (instead of just adding it manually to each cache key) you are giving FusionCache the information needed for it to add it also to internal cache keys, like the ones used to make Tagging and Clear work correctly.

Hope this helps.