Understanding the performance of my FFI-based API

[Alan Paxton]

Hi,

I hope this is an appropriate list for this question.

I have been prototyping an FFI-based version of the RocksDB Java API, which
is currently implemented in JNI. RocksDB is a C++ based key,value-store
with a Java API layered on top. I have done some benchmarking of the FFI
implementation, versus the JNI version and I find it performs consistently
slightly slower than the current API.

I would like to understand if this is to be expected, e.g. does FFI do more
safety checking under the covers when calling a native method ?
Or is the performance likely to improve between the preview in Java 19 and
release in Java 21 ?
If there are resources or suggestions that would help me dig into the
performance I'd be very grateful to be pointed to them.

For the use case I'm measuring, data is transferred in native memory
originally allocated by RocksDB in C++ which I wrap as a MemorySegment; I
do allocate native memory for the request structure.

These are links to the PR and some documentation of the work:

https://github.com/facebook/rocksdb/pull/11095
https://github.com/alanpaxton/rocksdb/blob/eb-1680-panama-ffi/java/JavaFFI.md

Many thanks,
Alan Paxton
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