Implementing a get() using FFI
We are building and accessing function(s)
(1) Still working out why the data is not coming back, as it has been read into the PinnableSlice and copied to the rocksdb_output_slice
(2) Make classes with VarHandles as members for repeated, efficient access to slices..

- why do we need to pass memory addresses ?
- lots of refactor/cleanup yet to do..

Pull out MethodHandles, MemoryLayouts and VarHandles used by the RocksDB FFI so that the core of the methods in FFIDB (get(), etc) are compact and readable.

Make the first test check the returned value of a key originally written by the old API.

Change FFI API into a byte[]-based one from String, consistent with the current API (String was just for initial testing)
Make reset() of pinnable slice public (it shoud be, and jmh benchmarks need it to be)
Build configurations need updated to build RocksDB JAR in java 19 and enable preview features. Same for jmh test jar, and invocations

This should mean that allocation is done with a default Cleaner linked to the session which should release the allocated segments via GC. We seem to be getting away with it in jmh GetBenchmark.java

JMH benchmarks are sufficiently good that we have implemented a paranoid test confirming that multiple get()s are all really working and fetching back from a database.

Start commenting the important methods and classes.

implement it.

Investigate whether mutiple calls over FFI are a problem; we have to do 2 calls per op when using pinnable slices, 2nd to release the slice.
So we implement a copying get (getOutputSlice) that obviously doesn’t need to hold a pinnable slice back in Java; It may be more efficient for small value sizes.

Describes what we have done and contains some results.

These are still part of exploring why FFI implementations are slightly slower than JNI.

Summary at current state, where we are still at a loss as to wny performance is not as good as with JNI. We have tested some hypotheses, but none is able to account for the difference.

Allocate a single segment that contains both input and output slice structs

Avoid copying and recycling for a potentially fairer benchmarking.

Since we are not using it as heavily;
and because our recycling allocation is not happy when we hold something long-lived.

Allocate one separately at the start
reset() it when needed
dispose at the end

Try performance again

Remove the custom allocator that we don’t use any more; we have reduced allocation use to a mimmum instead.
Type things a little better where we pass segment addresses to method invocation.
Polish error/exception messages.
Name parameters consistently.
Make GetParams auto-closeable.

Type checking is tighter.
Does it help performance ? It can’t hurt.

On M1 Max, maybe 1% performance gain with 64k value size. There’s something there, if not much.

We can just fill the key segment with the bytes we need.

Make all VarHandle(s) conform to exact invocation.

This reverts commit a1f158f.

Reflect the fact performance is now very nearly equal in the writeup, acknowledge help from Maurizio at Oracle. (panama-dev).

Analyse performance cost of the extra FFI call to release a pinnable slice versus the extra copy into a foreign memory segment allocated on the Java side. 1k-4k break even.

First part of replacement is to not do anything with the replaced SliceBuffer, just check the CI run..