[code-reflection] Integrated: 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.

This pull request has now been integrated.

Changeset: 0380011c
Author:    Maurizio Cimadamore <mcimadamore at openjdk.org>
URL:       https://git.openjdk.org/babylon/commit/0380011c784d9b40fe9c5de08109cc753f0cad90
Stats:     84 lines in 4 files changed: 25 ins; 7 del; 52 mod

Model lifetimes of onnx session-related objects more explicitly

Reviewed-by: asotona

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PR: https://git.openjdk.org/babylon/pull/332