Include buffer/cache memory as free memory on linux

[dustin-decker]

The Linux kernel will typically use almost all free memory for buffering/cache purposes such as the page cache, freeing immediately as needed for applications. This buffer should be considered free memory.

[pbnjay]

I know that BSD + ZFS has similar filesystem caching going on, we could make a similar change there.
I'll have to read about darwin's page types in vm_stat to see what may be possible there (inactive/speculative/purgeable/etc all sound viable)

I had debated this in the initial implementation, but hesitated because the primary use is to allow "dynamic code to minimize thrashing and other contention." I don't have any benchmarks to back up the possible contention from filling available memory with application data in a read-heavy app, which is why I hesitate to include it directly. Do you have any ideas on how to validate that? At the very least I think it should be mentioned in the documentation.

I think a simple solution to both challenges could be to feature gate the inclusion with some sort of explicit opt-in and explanation. e.g. you'd have to call memory.IncludeCaches(enabled bool) or similar first.

[dustin-decker]

In my application, we throttle down some concurrency when there is memory pressure, which we've defined as <2GB free. On a machine with 16GB of memory we had ~11GB of buffer/cache (primarily page cache) but only ~500mb actually free.

This certainly would be a breaking change for the library.

What about introducing a BufferMemoryUsed() function?

[dustin-decker]

Actually, I think it may be best to add a AvailableMemory() function that includes free+buffer.

https://haydenjames.io/free-vs-available-memory-in-linux/#:~:text=The%20difference%20between%20free%20memory,penalty%20of%20using%20swap%20space.

[dustin-decker]

Closing this in favor of #10