diff --git a/.github/workflows/code_changes.yaml b/.github/workflows/code_changes.yaml
index 6a0008f2ee..15d8b0a87b 100644
--- a/.github/workflows/code_changes.yaml
+++ b/.github/workflows/code_changes.yaml
@@ -22,7 +22,16 @@ jobs:
         - name: Check formatting
           run: uvx ruff format --check .
   Test:
-      runs-on: macos-latest
+      # Linux arm64 keeps the CPU architecture of the macOS runners this job
+      # used to run on. GitHub-hosted macOS runners are scarce, so PR runs
+      # queued for hours; the parameter tree and every test result are
+      # identical on both.
+      runs-on: ubuntu-24.04-arm
+      # The whole job takes about 18 minutes (YAML tests 2, pytest 15, the
+      # rest under 1). More than three times that means a run is hung or
+      # the suite needs attention, so fail it rather than wait for the
+      # 6-hour GitHub limit.
+      timeout-minutes: 60
       permissions:
         contents: "read"
         id-token: "write"
@@ -55,7 +64,9 @@ jobs:
               if [ -n "$HF_TOKEN_VALUE" ]; then
                 export HUGGING_FACE_TOKEN="$HF_TOKEN_VALUE"
               fi
-              make test
+              # Four pytest-xdist workers, one per core; together they peak at
+              # about 9 GB of the runner's 16 GB.
+              make test PYTEST_ARGS="-n 4 --dist worksteal"
             env:
               HF_TOKEN_VALUE: ${{ secrets.HUGGING_FACE_TOKEN }}
           - name: Upload coverage to Codecov
diff --git a/.github/workflows/pr_code_changes.yaml b/.github/workflows/pr_code_changes.yaml
index 9b7b313430..f039cfc1dd 100644
--- a/.github/workflows/pr_code_changes.yaml
+++ b/.github/workflows/pr_code_changes.yaml
@@ -14,6 +14,13 @@ on:
       - uv.lock
   workflow_dispatch:
 
+# A new push to a pull request supersedes the run on its previous head, so
+# cancel that run instead of letting it hold runners. Manual dispatches are
+# grouped by ref and never cancelled.
+concurrency:
+  group: pr-code-changes-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 jobs:
   Python-Compat:
     name: Install + smoke-import (py${{ matrix.python-version }})
@@ -43,7 +50,16 @@ jobs:
         - name: Check formatting
           run: uvx ruff format --check .
   Test:
-      runs-on: macos-latest
+      # Linux arm64 keeps the CPU architecture of the macOS runners this job
+      # used to run on. GitHub-hosted macOS runners are scarce, so PR runs
+      # queued for hours; the parameter tree and every test result are
+      # identical on both.
+      runs-on: ubuntu-24.04-arm
+      # The whole job takes about 18 minutes (YAML tests 2, pytest 15, the
+      # rest under 1). More than three times that means a run is hung or
+      # the suite needs attention, so fail it rather than wait for the
+      # 6-hour GitHub limit.
+      timeout-minutes: 60
       permissions:
         contents: "read"
         id-token: "write"
@@ -74,7 +90,9 @@ jobs:
               if [ -n "$HF_TOKEN_VALUE" ]; then
                 export HUGGING_FACE_TOKEN="$HF_TOKEN_VALUE"
               fi
-              make test
+              # Four pytest-xdist workers, one per core; together they peak at
+              # about 9 GB of the runner's 16 GB.
+              make test PYTEST_ARGS="-n 4 --dist worksteal"
             env:
               HF_TOKEN_VALUE: ${{ secrets.HUGGING_FACE_TOKEN }}
           - name: Upload coverage to Codecov
diff --git a/.github/workflows/pr_docs_changes.yaml b/.github/workflows/pr_docs_changes.yaml
index 5add446350..ce7fceb931 100644
--- a/.github/workflows/pr_docs_changes.yaml
+++ b/.github/workflows/pr_docs_changes.yaml
@@ -11,6 +11,13 @@ on:
       - .github/**
   workflow_dispatch:
 
+# A new push to a pull request supersedes the run on its previous head, so
+# cancel that run instead of letting it hold runners. Manual dispatches are
+# grouped by ref and never cancelled.
+concurrency:
+  group: pr-docs-changes-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: ${{ github.event_name == 'pull_request' }}
+
 jobs:
   Test:
       runs-on: ubuntu-latest
diff --git a/Makefile b/Makefile
index 764c102331..5723221068 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-.PHONY: docs
+.PHONY: docs test test-yaml test-python
 
 all: install
 	pip install build
@@ -14,9 +14,15 @@ format:
 	ruff format .
 	ruff check .
 
-test:
+test: test-yaml test-python
+
+test-yaml:
 	policyengine-core test policyengine_uk/tests/policy -c policyengine_uk
-	pytest policyengine_uk/tests/ --cov=policyengine_uk --cov-report=xml --maxfail=0 -v
+
+# CI passes PYTEST_ARGS="-n 4 --dist worksteal" (pytest-xdist) to spread the
+# tests over the runner's four cores. Without it this runs serially, as before.
+test-python:
+	pytest policyengine_uk/tests/ --cov=policyengine_uk --cov-report=xml --maxfail=0 -v $(PYTEST_ARGS)
 
 test-all:
 	policyengine-core test policyengine_uk/tests/policy -c policyengine_uk
diff --git a/changelog.d/ci-linux-parallel-tests.changed.md b/changelog.d/ci-linux-parallel-tests.changed.md
new file mode 100644
index 0000000000..e6c1f87c91
--- /dev/null
+++ b/changelog.d/ci-linux-parallel-tests.changed.md
@@ -0,0 +1 @@
+Run the CI Test job on Linux arm64 with four pytest-xdist workers (pytest-xdist is now a dev dependency), cancel superseded pull request runs, and time the job out after 60 minutes.
diff --git a/policyengine_uk/tests/conftest.py b/policyengine_uk/tests/conftest.py
index 56ee3b207b..deee0cc756 100644
--- a/policyengine_uk/tests/conftest.py
+++ b/policyengine_uk/tests/conftest.py
@@ -1,6 +1,10 @@
+import gc
 import os
+import re
+import sys
 
 import pytest
+from policyengine_core.taxbenefitsystems import TaxBenefitSystem
 
 DEFAULT_TEST_DATASET_URL = (
     "hf://policyengine/policyengine-uk-data-private/enhanced_frs_2023_24.h5@1.40.3"
@@ -26,3 +30,48 @@ def pytest_collection_modifyitems(config, items):
     for item in items:
         if "microsimulation" in item.keywords:
             item.add_marker(skip_microsimulation)
+
+
+# policyengine-core loads every variable file of each new tax-benefit system as
+# a module named "<id(system)>_<path hash>_<file name>" and registers it in
+# sys.modules, where it stays after the system is garbage collected. Every
+# Simulation builds its own system, so each one leaves about 1,000 modules
+# (roughly 10 MB) behind, and the property tests build hundreds: one test
+# process grew to 15.7 GB on a 16 GB CI runner. After each test, drop the
+# modules of systems that no longer exist; live systems keep theirs. Remove
+# this once policyengine-core stops leaking them
+# (PolicyEngine/policyengine-core#520).
+_VARIABLE_MODULE_NAME = re.compile(r"(\d+)_-?\d+_")
+
+
+def release_dead_variable_modules(max_owners: int = 8) -> int:
+    """Remove dead tax-benefit systems' variable modules from sys.modules.
+
+    Does nothing while at most ``max_owners`` systems own loaded variable
+    modules, since finding the live systems scans every tracked object.
+    Returns the number of modules removed.
+    """
+    owners = {}
+    for name in list(sys.modules):
+        match = _VARIABLE_MODULE_NAME.match(name)
+        if match:
+            owners.setdefault(match.group(1), []).append(name)
+    if len(owners) <= max_owners:
+        return 0
+    gc.collect()
+    live = {
+        str(id(obj)) for obj in gc.get_objects() if isinstance(obj, TaxBenefitSystem)
+    }
+    released = 0
+    for owner, names in owners.items():
+        if owner not in live:
+            for name in names:
+                sys.modules.pop(name, None)
+            released += len(names)
+    return released
+
+
+@pytest.fixture(autouse=True)
+def _release_variable_modules_of_dead_systems():
+    yield
+    release_dead_variable_modules()
diff --git a/policyengine_uk/tests/test_latest_data_smoke.py b/policyengine_uk/tests/test_latest_data_smoke.py
index e80e96ec2e..aa9639cab9 100644
--- a/policyengine_uk/tests/test_latest_data_smoke.py
+++ b/policyengine_uk/tests/test_latest_data_smoke.py
@@ -16,8 +16,6 @@
 
 from __future__ import annotations
 
-import os
-
 import numpy as np
 import pytest
 
@@ -35,10 +33,13 @@ def sim() -> Microsimulation:
     """Simulation built against the unpinned latest dataset.
 
     Overrides any pinned-version dataset set in conftest.py so the test
-    exercises whatever is on HuggingFace ``main`` right now.
+    exercises whatever is on HuggingFace ``main`` right now. The override
+    lasts only while the simulation is built, so tests that run later in the
+    same process still get the pinned dataset whatever order they run in.
     """
-    os.environ["POLICYENGINE_UK_DEFAULT_DATASET"] = LATEST_DATASET_URL
-    return Microsimulation()
+    with pytest.MonkeyPatch.context() as monkeypatch:
+        monkeypatch.setenv("POLICYENGINE_UK_DEFAULT_DATASET", LATEST_DATASET_URL)
+        return Microsimulation()
 
 
 def _weighted(sim: Microsimulation, variable: str, period: int = YEAR) -> float:
diff --git a/policyengine_uk/tests/test_variable_module_release.py b/policyengine_uk/tests/test_variable_module_release.py
new file mode 100644
index 0000000000..61d2047ccd
--- /dev/null
+++ b/policyengine_uk/tests/test_variable_module_release.py
@@ -0,0 +1,62 @@
+"""The conftest releases the variable modules of dead tax-benefit systems.
+
+policyengine-core registers each system's variable modules in sys.modules
+under the system's id and never removes them. The conftest's
+``release_dead_variable_modules`` drops those of systems that no longer exist,
+which keeps a long test process's memory flat. If core stops leaking the
+modules (PolicyEngine/policyengine-core#520), the precondition below fails
+and the workaround can go.
+"""
+
+import gc
+import sys
+from pathlib import Path
+
+from policyengine_uk import Simulation
+
+SITUATION = {
+    "people": {"adult": {"age": {2026: 40}, "employment_income": {2026: 30_000}}},
+    "benunits": {"benunit": {"members": ["adult"]}},
+    "households": {"household": {"members": ["adult"]}},
+}
+
+
+def _conftest(request):
+    path = Path(__file__).with_name("conftest.py")
+    for plugin in request.config.pluginmanager.get_plugins():
+        if getattr(plugin, "__file__", None) and Path(plugin.__file__) == path:
+            return plugin
+    raise AssertionError(f"{path} is not loaded as a plugin")
+
+
+def _modules_of(owner: str) -> list:
+    return [name for name in sys.modules if name.startswith(owner + "_")]
+
+
+def test_dead_systems_modules_are_released_and_live_ones_kept(request):
+    release_dead_variable_modules = _conftest(request).release_dead_variable_modules
+    kept = Simulation(situation=SITUATION)
+    kept_owner = str(id(kept.tax_benefit_system))
+    dropped = Simulation(situation=SITUATION)
+    dropped_owner = str(id(dropped.tax_benefit_system))
+    expected = dropped.calculate("income_tax", 2026)
+
+    assert _modules_of(dropped_owner), (
+        "policyengine-core no longer registers variable modules under the "
+        "system's id; the conftest workaround may no longer be needed"
+    )
+    del dropped
+    gc.collect()
+
+    released = release_dead_variable_modules(max_owners=0)
+
+    assert released >= 1
+    assert not _modules_of(dropped_owner)
+    assert _modules_of(kept_owner)
+    assert kept.calculate("income_tax", 2026) == expected
+    assert Simulation(situation=SITUATION).calculate("income_tax", 2026) == expected
+
+
+def test_release_waits_until_enough_systems_own_modules(request):
+    release_dead_variable_modules = _conftest(request).release_dead_variable_modules
+    assert release_dead_variable_modules(max_owners=10**9) == 0
diff --git a/pyproject.toml b/pyproject.toml
index 6453b944b6..e670f561cf 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -61,6 +61,7 @@ dev = [
     "coverage",
     "hypothesis",
     "pytest-cov",
+    "pytest-xdist",
     "ruff>=0.9.0",
     "towncrier>=24.8.0",
     "furo<2023",
diff --git a/uv.lock b/uv.lock
index 47b04e0136..3200859772 100644
--- a/uv.lock
+++ b/uv.lock
@@ -69,7 +69,7 @@ resolution-markers = [
     "python_full_version >= '3.14'",
 ]
 dependencies = [
-    { name = "cffi", marker = "python_full_version >= '3.14'" },
+    { name = "cffi" },
 ]
 sdist = { url = "https://files.pythonhosted.org/packages/b9/e9/184b8ccce6683b0aa2fbb7ba5683ea4b9c5763f1356347f1312c32e3c66e/argon2-cffi-bindings-21.2.0.tar.gz", hash = "sha256:bb89ceffa6c791807d1305ceb77dbfacc5aa499891d2c55661c6459651fc39e3", size = 1779911, upload-time = "2021-12-01T08:52:55.68Z" }
 wheels = [
@@ -94,7 +94,7 @@ resolution-markers = [
     "python_full_version < '3.12'",
 ]
 dependencies = [
-    { name = "cffi", marker = "python_full_version < '3.14'" },
+    { name = "cffi" },
 ]
 sdist = { url = "https://files.pythonhosted.org/packages/5c/2d/db8af0df73c1cf454f71b2bbe5e356b8c1f8041c979f505b3d3186e520a9/argon2_cffi_bindings-25.1.0.tar.gz", hash = "sha256:b957f3e6ea4d55d820e40ff76f450952807013d361a65d7f28acc0acbf29229d", size = 1783441, upload-time = "2025-07-30T10:02:05.147Z" }
 wheels = [
@@ -483,6 +483,15 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/05/d1/8952806fbf9583004ab479d8f58a9496c3d35f6b6009ddd458bdd9978eaf/dpath-2.2.0-py3-none-any.whl", hash = "sha256:b330a375ded0a0d2ed404440f6c6a715deae5313af40bbb01c8a41d891900576", size = 17618, upload-time = "2024-06-12T22:08:01.881Z" },
 ]
 
+[[package]]
+name = "execnet"
+version = "2.1.2"
+source = { registry = "https://pypi.org/simple" }
+sdist = { url = "https://files.pythonhosted.org/packages/bf/89/780e11f9588d9e7128a3f87788354c7946a9cbb1401ad38a48c4db9a4f07/execnet-2.1.2.tar.gz", hash = "sha256:63d83bfdd9a23e35b9c6a3261412324f964c2ec8dcd8d3c6916ee9373e0befcd", size = 166622, upload-time = "2025-11-12T09:56:37.75Z" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/ab/84/02fc1827e8cdded4aa65baef11296a9bbe595c474f0d6d758af082d849fd/execnet-2.1.2-py3-none-any.whl", hash = "sha256:67fba928dd5a544b783f6056f449e5e3931a5c378b128bc18501f7ea79e296ec", size = 40708, upload-time = "2025-11-12T09:56:36.333Z" },
+]
+
 [[package]]
 name = "executing"
 version = "2.2.0"
@@ -1471,7 +1480,7 @@ wheels = [
 
 [[package]]
 name = "policyengine-uk"
-version = "2.102.3"
+version = "2.107.0"
 source = { editable = "." }
 dependencies = [
     { name = "microdf-python" },
@@ -1487,6 +1496,7 @@ dev = [
     { name = "hypothesis" },
     { name = "jupyter-book" },
     { name = "pytest-cov" },
+    { name = "pytest-xdist" },
     { name = "rich" },
     { name = "ruff" },
     { name = "setuptools" },
@@ -1507,6 +1517,7 @@ requires-dist = [
     { name = "policyengine-core", specifier = ">=3.32.9" },
     { name = "pydantic", specifier = ">=2.11.7" },
     { name = "pytest-cov", marker = "extra == 'dev'" },
+    { name = "pytest-xdist", marker = "extra == 'dev'" },
     { name = "rich", marker = "extra == 'dev'" },
     { name = "ruff", marker = "extra == 'dev'", specifier = ">=0.9.0" },
     { name = "setuptools", marker = "extra == 'dev'" },
@@ -1710,6 +1721,19 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/bc/16/4ea354101abb1287856baa4af2732be351c7bee728065aed451b678153fd/pytest_cov-6.2.1-py3-none-any.whl", hash = "sha256:f5bc4c23f42f1cdd23c70b1dab1bbaef4fc505ba950d53e0081d0730dd7e86d5", size = 24644, upload-time = "2025-06-12T10:47:45.932Z" },
 ]
 
+[[package]]
+name = "pytest-xdist"
+version = "3.8.0"
+source = { registry = "https://pypi.org/simple" }
+dependencies = [
+    { name = "execnet" },
+    { name = "pytest" },
+]
+sdist = { url = "https://files.pythonhosted.org/packages/78/b4/439b179d1ff526791eb921115fca8e44e596a13efeda518b9d845a619450/pytest_xdist-3.8.0.tar.gz", hash = "sha256:7e578125ec9bc6050861aa93f2d59f1d8d085595d6551c2c90b6f4fad8d3a9f1", size = 88069, upload-time = "2025-07-01T13:30:59.346Z" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/ca/31/d4e37e9e550c2b92a9cbc2e4d0b7420a27224968580b5a447f420847c975/pytest_xdist-3.8.0-py3-none-any.whl", hash = "sha256:202ca578cfeb7370784a8c33d6d05bc6e13b4f25b5053c30a152269fd10f0b88", size = 46396, upload-time = "2025-07-01T13:30:56.632Z" },
+]
+
 [[package]]
 name = "python-dateutil"
 version = "2.9.0.post0"