[code-reflection] RFR: Model lifetimes of onnx session-related objects more explicitly

[Maurizio Cimadamore]

On Fri, 28 Feb 2025 12:42:24 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> The class representing an onnx session is auto closeable. But, in the current code, a session is closed immediately after its `run` method is called. This is problematic because a session returns some ORTValues (tensors) which also need to be freed, but that cannot be freed immediately after calling `run` (as they need to be used by clients).
> 
> To address this problem, I tweaked the session code to accept an external arena. All the allocation of session-related data structures now happens using that external arena. This means that the client can now be in charge of managing the lifetime of a session (see changes to MNIST demo).
> 
> To test, I tweaked the MNIST code to do 10K iterations on each button pressed. Predictably, a single button pressed resulted in over 3g of memory being leaked. With these changes the memory arrives at ~400K (there is still some minor leak, but not sure worth pushing more).
> 
> If the changes to the demo are not deemed good, I can withdraw this PR -- I mostly wanted to capture the result of my exploration somewhere.

cr-examples/onnx/src/test/java/oracle/code/onnx/MNISTDemo.java line 149:

> 147:                     var imageData = new float[IMAGE_SIZE * IMAGE_SIZE];
> 148:                     scaledImage.getData().getSamples(0, 0, IMAGE_SIZE, IMAGE_SIZE, 0, imageData);
> 149:                     var imageTensor = Tensor.ofShape(new long[]{1, 1, IMAGE_SIZE, IMAGE_SIZE}, imageData);

Would be nice if we could move this inside the TWR, so that we can associate the tensor memory with the arena session (and dispose everything in one go)

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PR Review Comment: https://git.openjdk.org/babylon/pull/332#discussion_r1975538107