Size default thread pools from ORT_*_NUM_THREADS environment variables

[Oscar Friedman (OscarFree)]

What

When a thread pool's size is not set programmatically (thread_pool_size <= 0), size it from ORT_INTRA_OP_NUM_THREADS / ORT_INTER_OP_NUM_THREADS before falling back to the machine-sized default. Explicit SessionOptions / global threading options are completely unaffected.

• Strict parsing — a non-integer or negative value fails loudly (ORT_ENFORCE).
• = 0 escape hatch — an explicit 0 opts back into the machine-sized default (useful when a platform sets these but you want host-sized pools).
• Nothing set → current behavior, unchanged.

Implements #29510.

Why

ORT's default intra-op pool sizes to the host's physical cores (GetNumPhysicalCpuCores()). In CPU-limited containers (Kubernetes, Modal, Cloud Run, …) the cgroup CPU reservation is invisible to core detection, so a container reserved N cores on a much larger host gets an N′-sized pool sized to the host — oversubscribing the reservation and degrading tail latency badly under concurrency, with no environment-level way to bound ORT today. The only current fix is passing SessionOptions at every InferenceSession call site, which is unreachable when sessions are created inside third-party libraries.

Scope note (re: Scott McKay (@skottmckay)'s feedback on #29510)

The original proposal also honored OMP_NUM_THREADS as an intra-op fallback. Per the discussion on #29510, this PR is deliberately scoped to ORT-specific variables only — that keeps the semantics under ORT's control and avoids the "why not OPENBLAS_NUM_THREADS / MKL_NUM_THREADS next" ambiguity. An OMP_NUM_THREADS fallback can be a follow-up if there's demand.

Tests

Adds 4 ThreadPoolTest cases (all pin the env vars via ScopedEnvironmentVariables so ambient values can't leak): intra + inter sizing from the ORT vars, explicit size wins over env, the 0 escape hatch, and invalid-value-throws. Comparisons are made against a reference pool of the intended explicit size because DegreeOfParallelism scales by a granularity factor on hybrid CPUs.

I was not able to build/run the ORT test suite locally on this machine, so I'm relying on CI to validate the build + ThreadPoolTest.

[azure-pipelines]

Azure Pipelines:
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[Oscar Friedman (OscarFree)]

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[Oscar Friedman (OscarFree)]

Scott McKay (@skottmckay) the ci failure is unrelated, could you re-run please?

[Scott McKay (skottmckay)]

It's related. The code change is bumping the minimal Android build binary size past the current max. It's fine to add 10K to that in .github\workflows\android.yml

        echo "Binary size threshold in bytes: 1440768"
        BINARY_SIZE_THRESHOLD_ARGS="--threshold_size_in_bytes 1440768"

You'll also need to sync with the latest main to pickup some workflow config changes. Without that a number of CIs will fail.

[Oscar Friedman (OscarFree)]

ah, thanks for the tip!

[Oscar Friedman (OscarFree)]

Scott McKay (@skottmckay) pushed the threshold bump + rebase you suggested — android size check is green now (the other build reds are the current main breakage, not this PR). ready for another look when you get a sec 🙏

[Oscar Friedman (OscarFree)]

Scott McKay (@skottmckay) could you kick off CI again?