MemoryCache Docs Missing Details

[tomwarner13]

As far as I can tell, the only public-facing documentation provided by Microsoft about Microsoft.Extensions.Caching.Memory is this article. I have several questions that are not covered in this article, and it would be nice to have canonical answers there (or even anywhere!).

When does an expired item get removed?

My best understanding of the implementation is that there are two reasons (other than explicit removal) that an item might get removed from the cache:

• Expiration Scan: there's apparently a MemoryCacheOptions.ExpirationScanFrequency that configures the minimum interval between these scans. The memory cache docs mention that any activity on the cache can "trigger a background scan" though they do not mention this option at all. What does this scan do? Does it remove all expired items, or just some arbitrary portion of them? Is it an expensive operation? Can it block execution of the call to the MemoryCache method which triggered it? Are there any considerations around performance if we set this frequency higher or lower?
• Compact(): Good news: the docs do explain, very briefly, what calling compact actually does. Bad news: they also don't mention whether it's a blocking operation, any performance considerations around calling it, etc.

Also, how does .Compact() interact with a set size? If it is called on a cache with a set size, will it remove based on a percentage of the total size limit, the current size of the cache, or as a percentage of the total number of entries in the cache (disregarding size)? If it is called on a cache without a set size, does it do anything? Does it use the total number of entries in its percentage calculation? Are there any callbacks available to respond if an entry exceeded the size limit?

[tomwarner13]

A) this uhhhh isn't related to AI at all?

B) Because some of these questions are critical to the implementation of the project I'm working on, I've set up a test app to observe cache behavior to be able to answer them. Assuming this issue gets the attention I'd expect from MS, here are some answers:

• Expiration scans happen in the background, where they do not block execution of the operation which triggered them. They appear to remove all expired items rather than a portion. According to the MemoryCacheOptions source code, default value for ExpirationScanFrequency is 1 minute.
• Without a set size, calling Compact() reduces the number of cache entries by the desired percentage from the starting amount. For example, if the cache contains 100 entries and you call Compact(.75) you will have 25 entries remaining.
• With a set size, it appears that Compact() tries to reduce a percentage of the size of the cache, not the total entries, though this is harder to verify. For example, if your size limit is 100 but your entries may have any individual size between 0 and 20 each, calling Compact(.2) will try and bring the Size value down to 80, removing whatever amount of entries lets it do so.

There's also a CompactionPercentage in the MemoryCacheOptions which will, when paired with a sized cache, automatically call Compact() with that percentage when an entry exceeds the remaining size - this seems a lot more useful than many other methods of managing the cache size and it really ought to be called out somewhere!