pytest runner overrides --dist=worksteal

Type: Performance Issue

Behaviour

pytest runner in the left panel seems to do something like dist=load despite my settings.json, resulting in periodic bottlenecking on slow tests.

Steps to reproduce:

Running my large suite with uv run pytest tests -n auto --dist worksteal saturates CPU and gives long ..... lines undifferentiated by filenames. But execting the runner with settings.json

{
    "python.testing.pytestArgs": [
        "tests",
        "-n", "auto",
        "--dist", "worksteal"
    ],
    "python.testing.unittestEnabled": false,
    "python.testing.pytestEnabled": true
}

produces a long list of FILENAME ..... [PCT%] lines, and repeated bursts of full core saturation interspersed by long stretches of single-core usage in a single suite run.

Diagnostic data

Details

Excerpt from the TEST RESULTS panel:

Running pytest with args: ['-p', 'vscode_pytest', '-n', 'auto', '--dist', 'worksteal', '--rootdir=/home/tbertalan/lake/Projects/ModelCalibration-issue-118', (long list of specific path/file.py::class entries snipped here)
============================= test session starts ==============================
platform linux -- Python 3.12.13, pytest-9.0.3, pluggy-1.6.0
rootdir: /home/tbertalan/lake/Projects/ModelCalibration-issue-118
configfile: pyproject.toml
plugins: cov-7.1.0, xdist-3.8.0, typeguard-2.13.3, jaxtyping-0.3.11, anyio-4.13.0
collected 446 items

tests/test_anp_loader.py ........................................        [  8%]
tests/test_anp_registry.py ..............................                [ 15%]
tests/test_behavior.py .......                                           [ 17%]

vs a (non-vscode) terminal run

tbertalan@swiftie:~/lake/Projects/ModelCalibration-issue-118$ uv run pytest -n auto -p no:slow-first
      Built modelcalibration @ file:///home/tbertalan/lake/Projects/ModelCalibration-issue-118
Uninstalled 1 package in 0.81ms
Installed 1 package in 3ms
============================================================================================== test session starts ==============================================================================================
platform linux -- Python 3.12.13, pytest-9.0.3, pluggy-1.6.0
rootdir: /home/tbertalan/lake/Projects/ModelCalibration-issue-118
configfile: pyproject.toml
testpaths: tests
plugins: cov-7.1.0, xdist-3.8.0, typeguard-2.13.3, jaxtyping-0.3.11, anyio-4.13.0
4 workers [446 items]
...............................................................................................................................................................x.xxx............................x........ [ 45%]
.................x..x.....................................................................Xss............................................................................................................ [ 90%]
............................................

Extension version: 2026.4.0
VS Code version: Code 1.129.1 (8a7abeba6e03ea3af87bfbce9a1b7e48fed567b8, 2026-07-17T15:35:09Z)
OS version: Windows_NT x64 10.0.26200
Modes:
Remote OS version: Linux x64 6.14.0-37-generic

Python version (& distribution if applicable, e.g. Anaconda): 3.13.3.final.0
Type of virtual environment used (e.g. conda, venv, virtualenv, etc.): System
Value of the python.languageServer setting: Default

User Settings


languageServer: "Pylance"

testing
• pytestArgs: "<placeholder>"
• pytestEnabled: true

Installed Extensions

Extension Name	Extension Id	Version
autopep8	ms-	2026.4.0
claude-code	Ant	2.1.215
copilot-chat	Git	0.57.0
debugpy	ms-	2026.6.0
git-graph	mhu	1.30.0
git-worktrees	Git	2.16.0
js-debug	ms-	1.117.0
jsts-chat-features	Typ	0.0.4
jupyter	ms-	2025.9.1
jupyter-renderers	ms-	1.3.0
markdown-all-in-one	yzh	3.6.3
markdown-mermaid	bie	1.32.1
mypy-type-checker	ms-	2026.6.0
pth-viewer	pth	0.0.12
pylint	ms-	2026.6.0
python	ms-	2026.4.0
rainbow-csv	mec	3.24.1
vscode-js-profile-table	ms-	1.0.10
vscode-jupyter-cell-tags	ms-	0.1.9
vscode-jupyter-slideshow	ms-	0.1.6
vscode-markdownlint	Dav	0.61.2
vscode-pull-request-github	Git	0.158.0
vscode-pylance	ms-	2026.2.1
vscode-python-envs	ms-	1.36.0
webm-preview	gso	0.1.2

System Info

Item	Value
CPUs	AMD Ryzen 5 7530U with Radeon Graphics (12 x 1996)
GPU Status	2d_canvas: enabled GPU0: VENDOR= 0x1002, DEVICE=0x15e7 [AMD Radeon (TM) Graphics], DRIVER_VENDOR=AMD, DRIVER_VERSION=31.0.21923.13006 ACTIVE GPU1: VENDOR= 0x1414, DEVICE=0x008c [Microsoft Basic Render Driver], DRIVER_VERSION=10.0.26100.8875 Machine model name: Machine model version: direct_rendering_display_compositor: disabled_off_ok gpu_compositing: enabled multiple_raster_threads: enabled_on opengl: enabled_on rasterization: enabled raw_draw: disabled_off_ok skia_graphite: disabled_off trees_in_viz: disabled_off video_decode: enabled video_encode: enabled webgl: enabled webgpu: enabled webnn: disabled_off
Load (avg)	undefined
Memory (System)	14.83GB (0.10GB free)
Process Argv	--crash-reporter-id 3819bd9c-9640-441a-b637-f0405bc1724f
Screen Reader	no
VM	0%

Item	Value
Remote	SSH: swiftie
OS	Linux x64 6.14.0-37-generic
CPUs	Intel(R) Core(TM) i7-6700K CPU @ 4.00GHz (8 x 4200)
Memory (System)	30.28GB (18.76GB free)
VM	0%

Process Info

CPU %	Mem MB	   PID	Process
    1	   154	  5568	code
    1	   599	  4812	window [2] (settings.json - ModelCalibration-issue-118 [SSH: swiftie] - Visual Studio Code)
    0	   295	  6100	window [1] (tasks.json - Journal - Visual Studio Code)
    0	    24	  8216	   crashpad-handler
    0	    42	 17384	   utility-network-service
    0	   476	 18044	extension-host [2]
    0	     8	 28028	     C:\WINDOWS\system32\wsl.exe -d Ubuntu -e /bin/sh -c "cd '/mnt/c/Users/Tom Bertalan/AppData/Local/Programs/Microsoft VS Code' && /bin/sh"
    0	    13	 10976	       C:\WINDOWS\system32\wsl.exe -d Ubuntu -e /bin/sh -c "cd '/mnt/c/Users/Tom Bertalan/AppData/Local/Programs/Microsoft VS Code' && /bin/sh"
    0	    12	  8400	          --distro-id {0eceac0a-0de7-44db-ab7e-a953bbca12d9} --vm-id {97cfd741-f0f7-4580-b40a-526ebdaf3204} --handle 728 --event 752 --parent 756
    0	    10	 27972	           C:\WINDOWS\system32\conhost.exe 0x4
    0	     8	 15480	       C:\WINDOWS\system32\conhost.exe 0x4
    0	   102	 23492	pty-host
    0	    87	  2248	     C:\WINDOWS\System32\WindowsPowerShell\v1.0\powershell.exe -noexit -command "try { . \"c:\Users\Tom Bertalan\AppData\Local\Programs\Microsoft VS Code\8a7abeba6e\resources\app\out\vs\workbench\contrib\terminal\common\scripts\shellIntegration.ps1\" } catch {}"
    0	    12	 16552	     "c:\Users\Tom Bertalan\AppData\Local\Programs\Microsoft VS Code\8a7abeba6e\resources\app\node_modules.asar.unpacked\node-pty\build\Release\conpty\OpenConsole.exe" --headless --inheritcursor --width 80 --height 30 --signal 0x55c --server 0x54c
    0	    12	 26416	     "c:\Users\Tom Bertalan\AppData\Local\Programs\Microsoft VS Code\8a7abeba6e\resources\app\node_modules.asar.unpacked\node-pty\build\Release\conpty\OpenConsole.exe" --headless --width 80 --height 30 --signal 0x738 --server 0x730
    0	    91	 26528	     C:\WINDOWS\System32\WindowsPowerShell\v1.0\powershell.exe -noexit -command "try { . \"c:\Users\Tom Bertalan\AppData\Local\Programs\Microsoft VS Code\8a7abeba6e\resources\app\out\vs\workbench\contrib\terminal\common\scripts\shellIntegration.ps1\" } catch {}"
    0	     8	 31012	     C:\WINDOWS\System32\cmd.exe /c (type "C:\Users\TOMBER~1\AppData\Local\Temp\vscode-linux-multi-line-command-swiftie-908041203.sh" | "C:\WINDOWS\System32\OpenSSH\ssh.exe" -T -D 57373 swiftie sh) & exit /b 0
    0	    14	 30288	       "C:\WINDOWS\System32\OpenSSH\ssh.exe"  -T -D 57373 swiftie sh
    0	    12	 34920	     "c:\Users\Tom Bertalan\AppData\Local\Programs\Microsoft VS Code\8a7abeba6e\resources\app\node_modules.asar.unpacked\node-pty\build\Release\conpty\OpenConsole.exe" --headless --inheritcursor --width 131 --height 30 --signal 0x62c --server 0x624
    1	   138	 24056	   gpu-process
    0	   359	 28444	extension-host [1]
    0	    66	 10424	     "C:\Users\Tom Bertalan\AppData\Local\Programs\Microsoft VS Code\Code.exe" "c:\Users\Tom Bertalan\AppData\Local\Programs\Microsoft VS Code\8a7abeba6e\resources\app\extensions\json-language-features\server\dist\node\jsonServerMain" --node-ipc --clientProcessId=28444
    0	   388	 24684	     electron-nodejs (bundle.js)
    0	    60	 30452	     electron-nodejs (server.js)
    0	    66	 34296	     "C:\Users\Tom Bertalan\AppData\Roaming\uv\python\cpython-3.14.4-windows-x86_64-none\python.exe" "c:\Users\Tom Bertalan\.vscode\extensions\ms-python.pylint-2026.6.0\bundled\tool\lsp_server.py"
    0	     8	 16588	       C:\WINDOWS\system32\conhost.exe 0x4
    0	    12	 35728	     "c:\Users\Tom Bertalan\.vscode\extensions\ms-python.vscode-python-envs-1.36.0-win32-x64\python-env-tools\bin\pet.exe" server
    0	     7	 30724	       C:\WINDOWS\system32\conhost.exe 0x4
    0	    73	 33612	file-watcher [2]
    0	    86	 34496	file-watcher [1]
    0	    98	 35620	shared-process

Remote: SSH: swiftie
CPU %	Mem MB	   PID	Process
    2	   217	1071813	remote-server
    0	    62	1071850	   ptyHost
    0	     0	1071983	     /bin/bash --init-file /home/tbertalan/.vscode-server/cli/servers/Stable-8a7abeba6e03ea3af87bfbce9a1b7e48fed567b8/server/out/vs/workbench/contrib/terminal/common/scripts/shellIntegration-bash.sh
    0	     0	1072407	     /bin/bash --init-file /home/tbertalan/.vscode-server/cli/servers/Stable-8a7abeba6e03ea3af87bfbce9a1b7e48fed567b8/server/out/vs/workbench/contrib/terminal/common/scripts/shellIntegration-bash.sh
    1	   744	1071856	   extension-host
    0	    62	1072004	     /home/tbertalan/.vscode-server/cli/servers/Stable-8a7abeba6e03ea3af87bfbce9a1b7e48fed567b8/server/node /home/tbertalan/.vscode-server/cli/servers/Stable-8a7abeba6e03ea3af87bfbce9a1b7e48fed567b8/server/extensions/markdown-language-features/dist/serverWorkerMain --node-ipc --clientProcessId=1071856
    0	   155	1072022	     /home/tbertalan/.vscode-server/cli/servers/Stable-8a7abeba6e03ea3af87bfbce9a1b7e48fed567b8/server/node /home/tbertalan/.vscode-server/cli/servers/Stable-8a7abeba6e03ea3af87bfbce9a1b7e48fed567b8/server/extensions/json-language-features/server/dist/node/jsonServerMain --node-ipc --clientProcessId=1071856
    0	     0	1072137	     /home/tbertalan/.vscode-server/extensions/ms-python.vscode-python-envs-1.36.0-linux-x64/python-env-tools/bin/pet server
    0	    31	1072423	     /home/tbertalan/lake/Projects/ModelCalibration-issue-118/.venv/bin/python /home/tbertalan/.vscode-server/extensions/ms-python.autopep8-2026.4.0/bundled/tool/lsp_server.py
    0	   527	1072802	     /home/tbertalan/.vscode-server/cli/servers/Stable-8a7abeba6e03ea3af87bfbce9a1b7e48fed567b8/server/node /home/tbertalan/.vscode-server/extensions/ms-python.vscode-pylance-2026.2.1/dist/server.bundle.js --cancellationReceive=file:bbb338299e1c2b081336cb52f5953bc490540afe89 --node-ipc --clientProcessId=1071856
    0	    62	1071873	   fileWatcher
    0	     0	1073210	   /bin/sh -c /usr/bin/ps -ax -o pid=,ppid=,pcpu=,pmem=,command=
    0	     0	1073211	     /usr/bin/ps -ax -o pid=,ppid=,pcpu=,pmem=,command=

Workspace Info

|  Window (tasks.json - Journal - Visual Studio Code)
|    Folder (Journal): 229 files
|      File types: json(134) md(84) png(4) pdf(3) jpg(1) py(1) gitignore(1)
|                  TAG(1)
|      Conf files: tasks.json(1)
|  Remote: SSH: swiftie|    Folder (ModelCalibration-issue-118): more than 20000 files|      File types: py(7391) pyi(5593) json(3018) pyc(1420) dat(1084) so(159)
|                  txt(132) typed(61) md(46) pxd(46) other(1050)
|      Conf files: dockerfile(1) devcontainer.json(1) github-actions(1)
|      Launch Configs: debugpy(3);

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Uh oh!

pytest runner overrides --dist=worksteal #26051

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Uh oh!

pytest runner overrides --dist=worksteal #26051

Description

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Steps to reproduce:

Diagnostic data

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