Tag memory consumption and SetSize

[tomwarner13]

We're migrating a huge application which was previously using a hand-rolled L2 cache system that got put together in haste some time in the mid-2010s, to FusionCache. Overall it's been a good experience (I especially appreciate the comprehensize docs!), but there is something we're running into that I want to understand.

To preserve some of the legacy compatibility, we are currently using a LOT of tags. Like, potentially 10-12 tags per every "bucket" of a few dozen entries, to grotesquely oversimplify. We noticed that the application using FusionCache on production servers will consistently chew through all its available memory and start thrashing to disk if not restarted regularly. We attempted to set up a SizeLimit on the MemoryCache object that FusionCache is using, and set a default size of 1 on every FusionCache entry, but this had no effect.

I've been investigating more and I think the issue is our tagging implementation. We do ultimately plan to refactor out most of the tagging logic and consolidate down to far fewer tags, but that is a longer-term effort.

Meanwhile, what I have been seeing is that every time we set a tag, FusionCache adds this as a new entry to the MemoryCache with a Size of 0 (even when the default size for other entries is >0), a long absolute expiration (like 2 weeks in the future), and a priority of NeverRemove. If I understand what I'm seeing correctly, this means that memcache is continuing to fill its capacity up with tag items as the application runs, and even when we attempt to compact items out due to memory pressure, none of these tags will ever be evicted and eventually we have to restart the server and start over.

So I have a few questions around this:

• Do I understand this behavior correctly that every tag which is set on any entry leads to a MemoryCache entry which can never disappear short of a deliberate key-targeted clear or disposal of the whole cache?
• Is there any other best practice or way around this other than "use less tags"? (We're working toward that I promise 🙃)
• Should this be intended behavior? There's always a possibility that the entry disappears from memory cache anyway due to a restart or something so I imagine there needs to be default handling (calling L2 if available, etc) for tags which are missing from memory cache - could we configure something that allows them to not eat memory forever?

[tomwarner13]

.....and now I see there's a TagsDefaultEntryOptions that I can probably leverage to fix at least some of this, looking into whether it fixes our issues.

[tomwarner13]

It does appear that setting TagsDefaultEntryOptions lets us manage tag duration/expiration a lot more effectively, I think there's nothing more to follow up on here.