Ship the thread pool as its own library so a process has one pool

[shoumikhin]

The thread pool spreads CPU work across cores, and there is meant to
be exactly one per process. Today it lives in a static library, so every component that links it
gets its own private copy.

The wheel ships two copies right now:

_portable_lib.so        has its own thread pool
training extension      has its own thread pool
  -> one process, two pools, each sized for the whole machine

Two pools each sizing themselves to the core count means the process can run roughly twice as
many worker threads as it has cores. They then compete for the same cores, which is slower than
one pool would be, and a setting applied to one pool has no effect on the other.

This change ships the pool as its own shared library that everything links:

executorch/lib/libexecutorch_threadpool.so.1     the one pool
  _portable_lib.so       -> links it
  training extension     -> links it
  your C++ app           -> links it

How you use it

If you just want the default behaviour, nothing changes. If you want to control the pool from
C++:

find_package(executorch REQUIRED)
target_link_libraries(my_app PRIVATE executorch::runtime executorch::threadpool)

Tested

Built the wheel on Linux x86_64 and aarch64 and installed it into a clean environment, then
confirmed by symbol inspection that exactly one shipped library defines the pool accessor,
where before two did. Also linked a C++ application against the component and ran it, and ran
the existing Python tests to confirm behaviour is unchanged for current users.

[shoumikhin]

Stack from ghstack (oldest at bottom):

• Check the shipped device code covers the GPUs a wheel claims #21567
• Publish CUDA wheels so a GPU user does not build from source #21569
• Ship the CUDA delegate as its own library #21531
• Ship the XNNPACK delegate as its own library #21526
• Ship the merged CPU kernels as their own library #21524
• -> Ship the thread pool as its own library so a process has one pool #21522
• Ship a linkable shared runtime and make the Python bindings use it #21514

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/21522

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 13 New Failures, 260 Pending, 14 Unrelated Failures, 16 Unclassified Failures

As of commit add5026 with merge base ad3a71f ():

NEW FAILURES - The following jobs have failed:

• nightly / link-check / lint-urls (gh)
• nightly / link-check / lint-xrefs (gh)
• pull / test-arm-backend-public-api-backward-compatibility / linux-job (gh)
  RuntimeError: Command docker exec -t 349512d0dcb2e267215c1cf1f243a716e7bbe043d57c67329c47693d399f17da /exec failed with exit code 1
• pull / test-models-linux (emformer_join, xnnpack-quantization-delegation, linux.4xlarge.memory) / linux-job (gh)
  RuntimeError: Command docker exec -t e6a2fdb4980cceaeb2fdb23ad42918c9b2900241cb87fd24af1c400b3f0285b5 /exec failed with exit code 1
• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t 71990c217b520ffbd0f70b798cad33d78c54364fad01b75b5f68c9d772d239b9 /exec failed with exit code 1
• pull / test-phi-3-mini-runner-linux / linux-job (gh)
  AttributeError: '_OpNamespace' 'llama' object has no attribute 'sdpa_with_kv_cache'
• trunk / test-arm-backend-ethos-u (test_ootb_tests_ethos_u) / linux-job (gh)
  RuntimeError: Command docker exec -t 197dc05eeff6eb0cc90938bf1bc934e88acb448237e99c39a77c4977dd5edaa4 /exec failed with exit code 1
• trunk / test-arm-backend-ethos-u (test_ootb_tests_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t d8c6021a573788d146c2e539794e7bb2de912234f0661dbb9f9f5329019ecd0c /exec failed with exit code 1
• trunk / test-huggingface-transformers-xnnpack (gemma3-1b|xnnpack|--quantize) / linux-job (gh)
  RuntimeError: Command docker exec -t 2511e7f24838066f06c01220d1e80ef598dc7935ee434872d61a4383604602d7 /exec failed with exit code 1
• trunk / test-huggingface-transformers-xnnpack (llama3.2-1b|xnnpack|--quantize) / linux-job (gh)
  RuntimeError: Command docker exec -t b08b03f612656c4b07db4b2686a7489241fd8b6bad00151886a397c39298a997 /exec failed with exit code 1
• trunk / test-huggingface-transformers-xnnpack (qwen3-0.6b|xnnpack|--quantize) / linux-job (gh)
  RuntimeError: Command docker exec -t e6431e6be94a48ef2a87eccb4cec46b3714aa76960be3b4393562e5045e74bee /exec failed with exit code 1
• trunk / test-huggingface-transformers-xnnpack (smollm2-135m|xnnpack|--quantize) / linux-job (gh)
  RuntimeError: Command docker exec -t e43440a52dfeef55f3731f943685ba555d82fbcf5a61147582f386ec3bdecd5a /exec failed with exit code 1
• trunk / test-huggingface-transformers-xnnpack (smollm3-3b|xnnpack|--quantize) / linux-job (gh)
  RuntimeError: Command docker exec -t 6d7c189f403ca12063d88fc9c6a9e2bb9a82e87ec1c4eb757dadf61c4bbe7c89 /exec failed with exit code 1

UNCLASSIFIED FAILURES - DrCI could not classify the following jobs because the workflow did not run on the merge base. The failures may be pre-existing on trunk or introduced by this PR:

• Build macOS Wheels / pytorch/executorch / build-wheel-py3_10-cpu (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  Library not loaded: @rpath/libtorch_python.dylib
• Build macOS Wheels / pytorch/executorch / build-wheel-py3_11-cpu (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  Library not loaded: @rpath/libtorch_python.dylib
• Build macOS Wheels / pytorch/executorch / build-wheel-py3_12-cpu (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  Library not loaded: @rpath/libtorch_python.dylib
• Build macOS Wheels / pytorch/executorch / build-wheel-py3_13-cpu (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  Library not loaded: @rpath/libtorch_python.dylib
• Build macOS Wheels / pytorch/executorch / build-wheel-py3_14-cpu (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  pyarrow
• Build macOS Wheels / pytorch/executorch / upload / upload-wheel-py3_10-cpu (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  Unable to download artifact(s): Artifact not found
• Build macOS Wheels / pytorch/executorch / upload / upload-wheel-py3_11-cpu (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  API rate limit exceeded
• Build macOS Wheels / pytorch/executorch / upload / upload-wheel-py3_12-cpu (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  Unable to download artifact(s): Artifact not found
• Build macOS Wheels / pytorch/executorch / upload / upload-wheel-py3_13-cpu (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  API rate limit exceeded
• Build macOS Wheels / pytorch/executorch / upload / upload-wheel-py3_14-cpu (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  Unable to download artifact(s): Artifact not found
• Test Vulkan Backend (specialized runners) / build-vulkan-windows-msvc / windows-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
• trunk / test-models-windows (mv3, portable) / windows-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  ##[error]API rate limit exceeded for installation. If you reach out to GitHub Support for help, please include the request ID C2B0:36EB96:223B3E:72EC9E:6A72698F and timestamp 2026-08-04 22:37:03 UTC. For more on scraping GitHub and how it may affect your rights, please review our Terms of Service (https://docs.github.com/en/site-policy/github-terms/github-terms-of-service) - https://docs.github.com/en/rest/using-the-rest-api/getting-started-with-the-rest-api#rate-limiting
• trunk / test-models-windows (mv3, xnnpack-q8) / windows-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  API rate limit exceeded
• trunk / test-models-windows (resnet50, portable) / windows-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  ##[error]API rate limit exceeded for installation. If you reach out to GitHub Support for help, please include the request ID C31B:14A000:232E3D:7602DC:6A7269B4 and timestamp 2026-08-04 22:37:40 UTC. For more on scraping GitHub and how it may affect your rights, please review our Terms of Service (https://docs.github.com/en/site-policy/github-terms/github-terms-of-service) - https://docs.github.com/en/rest/using-the-rest-api/getting-started-with-the-rest-api#rate-limiting
• trunk / test-models-windows (vit, portable) / windows-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  ##[error]API rate limit exceeded for installation. If you reach out to GitHub Support for help, please include the request ID C327:6CF9E:20904D5:6CD83CA:6A726995 and timestamp 2026-08-04 22:37:09 UTC. For more on scraping GitHub and how it may affect your rights, please review our Terms of Service (https://docs.github.com/en/site-policy/github-terms/github-terms-of-service) - https://docs.github.com/en/rest/using-the-rest-api/getting-started-with-the-rest-api#rate-limiting
• Windows MSVC Build / build-windows-msvc / windows-job (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)
  ##[error]API rate limit exceeded for installation. If you reach out to GitHub Support for help, please include the request ID C31D:236910:20CF454:6D0CAB7:6A7269F7 and timestamp 2026-08-04 22:38:47 UTC. For more on scraping GitHub and how it may affect your rights, please review our Terms of Service (https://docs.github.com/en/site-policy/github-terms/github-terms-of-service) - https://docs.github.com/en/rest/using-the-rest-api/getting-started-with-the-rest-api#rate-limiting

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / test-llama-runner-linux (fp32, xnnpack+custom+qe, linux.arm64.2xlarge, executorch-ubuntu-22.04-gc... / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-llama-runner-linux (fp32, xnnpack+quantize_kv, linux.arm64.2xlarge, executorch-ubuntu-22.04-... / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-lora-linux / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-lora-multimethod-linux / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-models-linux (ic4, xnnpack-quantization-delegation, linux.4xlarge.memory) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• trunk / test-huggingface-transformers-xnnpack (qwen3-1.7b|xnnpack|--quantize) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• trunk / test-models-windows (emformer_transcribe, portable) / windows-job (gh) (detected as infra flaky with no log or failing log classifier)
• trunk / test-models-windows (emformer_transcribe, xnnpack-q8) / windows-job (gh) (detected as infra flaky with no log or failing log classifier)
• trunk / test-models-windows (mobilebert, portable) / windows-job (gh) (detected as infra flaky with no log or failing log classifier)
• trunk / test-models-windows (mobilebert, xnnpack-q8) / windows-job (gh) (detected as infra flaky with no log or failing log classifier)
• trunk / test-models-windows (resnet50, xnnpack-q8) / windows-job (gh) (detected as infra flaky with no log or failing log classifier)
• trunk / test-models-windows (vit, xnnpack-q8) / windows-job (gh) (detected as infra flaky with no log or failing log classifier)

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• Test QNN Windows MSVC build / build-qnn-windows-msvc / windows-job (gh) (trunk failure)
• trunk / test-arm-backend-ethos-u (test_deit_e2e_ethos_u) / linux-job (gh) (trunk failure)

This comment was automatically generated by Dr. CI and updates every 15 minutes.