[code-reflection] RFR: Model lifetimes of onnx session-related objects more explicitly [v2]

[Adam Sotona]

On Mon, 3 Mar 2025 12:21:38 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.
>
> Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Rename local arena variables

Marked as reviewed by asotona (Reviewer).

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PR Review: https://git.openjdk.org/babylon/pull/332#pullrequestreview-2653983689