perf: improve DependencyCache lru_cache hit rate, avoid iterating thr…

…ough lots of levels (python-poetry#7950)

src/poetry/mixology/version_solver.py

@@ -5,6 +5,8 @@
 import time
 
 from typing import TYPE_CHECKING
+from typing import Optional
+from typing import Tuple
 
 from poetry.core.packages.dependency import Dependency
 
@@ -29,6 +31,11 @@
 _conflict = object()
 
 
+DependencyCacheKey = Tuple[
+    str, Optional[str], Optional[str], Optional[str], Optional[str]
+]
+
+
 class DependencyCache:
     """
     A cache of the valid dependencies.
@@ -40,36 +47,37 @@ class DependencyCache:
 
     def __init__(self, provider: Provider) -> None:
         self._provider = provider
-        self._cache: dict[
-            int,
-            dict[
-                tuple[str, str | None, str | None, str | None, str | None],
-                list[DependencyPackage],
-            ],
-        ] = collections.defaultdict(dict)
 
-        self.search_for = functools.lru_cache(maxsize=128)(self._search_for)
+        # self._cache maps a package name to a stack of cached package lists,
+        # ordered by the decision level which added them to the cache. This is
+        # done so that when backtracking we can maintain cache entries from
+        # previous decision levels, while clearing cache entries from only the
+        # rolled back levels.
+        #
+        # In order to maintain the integrity of the cache, `clear_level()`
+        # needs to be called in descending order as decision levels are
+        # backtracked so that the correct items can be popped from the stack.
+        self._cache: dict[DependencyCacheKey, list[list[DependencyPackage]]] = (
+            collections.defaultdict(list)
+        )
+        self._cached_dependencies_by_level: dict[int, list[DependencyCacheKey]] = (
+            collections.defaultdict(list)
+        )
+
+        self._search_for_cached = functools.lru_cache(maxsize=128)(self._search_for)
 
     def _search_for(
-        self, dependency: Dependency, level: int
+        self,
+        dependency: Dependency,
+        key: DependencyCacheKey,
     ) -> list[DependencyPackage]:
-        key = (
-            dependency.complete_name,
-            dependency.source_type,
-            dependency.source_url,
-            dependency.source_reference,
-            dependency.source_subdirectory,
-        )
-
-        for check_level in range(level, -1, -1):
-            packages = self._cache[check_level].get(key)
-            if packages is not None:
-                packages = [
-                    p
-                    for p in packages
-                    if dependency.constraint.allows(p.package.version)
-                ]
-                break
+        cache_entries = self._cache[key]
+        if cache_entries:
+            packages = [
+                p
+                for p in cache_entries[-1]
+                if dependency.constraint.allows(p.package.version)
+            ]
         else:
             packages = None
 
@@ -83,12 +91,33 @@ def _search_for(
         if not packages:
             packages = self._provider.search_for(dependency)
 
-        self._cache[level][key] = packages
+        return packages
+
+    def search_for(
+        self,
+        dependency: Dependency,
+        decision_level: int,
+    ) -> list[DependencyPackage]:
+        key = (
+            dependency.complete_name,
+            dependency.source_type,
+            dependency.source_url,
+            dependency.source_reference,
+            dependency.source_subdirectory,
+        )
+
+        packages = self._search_for_cached(dependency, key)
+        if not self._cache[key] or self._cache[key][-1] is not packages:
+            self._cache[key].append(packages)
+            self._cached_dependencies_by_level[decision_level].append(key)
+
         return packages
 
     def clear_level(self, level: int) -> None:
-        self.search_for.cache_clear()
-        self._cache.pop(level, None)
+        if level in self._cached_dependencies_by_level:
+            self._search_for_cached.cache_clear()
+            for key in self._cached_dependencies_by_level.pop(level):
+                self._cache[key].pop()
 
 
 class VersionSolver:
@@ -327,9 +356,10 @@ def _resolve_conflict(self, incompatibility: Incompatibility) -> Incompatibility
                 for level in range(
                     self._solution.decision_level, previous_satisfier_level, -1
                 ):
-                    self._contradicted_incompatibilities.difference_update(
-                        self._contradicted_incompatibilities_by_level.pop(level, set()),
-                    )
+                    if level in self._contradicted_incompatibilities_by_level:
+                        self._contradicted_incompatibilities.difference_update(
+                            self._contradicted_incompatibilities_by_level.pop(level),
+                        )
                     self._dependency_cache.clear_level(level)
 
                 self._solution.backtrack(previous_satisfier_level)

tests/mixology/version_solver/test_dependency_cache.py

@@ -30,20 +30,20 @@ def test_solver_dependency_cache_respects_source_type(
     add_to_repo(repo, "demo", "1.0.0")
 
     cache = DependencyCache(provider)
-    cache.search_for.cache_clear()
+    cache._search_for_cached.cache_clear()
 
     # ensure cache was never hit for both calls
     cache.search_for(dependency_pypi, 0)
     cache.search_for(dependency_git, 0)
-    assert not cache.search_for.cache_info().hits
+    assert not cache._search_for_cached.cache_info().hits
 
     # increase test coverage by searching for copies
     # (when searching for the exact same object, __eq__ is never called)
     packages_pypi = cache.search_for(deepcopy(dependency_pypi), 0)
     packages_git = cache.search_for(deepcopy(dependency_git), 0)
 
-    assert cache.search_for.cache_info().hits == 2
-    assert cache.search_for.cache_info().currsize == 2
+    assert cache._search_for_cached.cache_info().hits == 2
+    assert cache._search_for_cached.cache_info().currsize == 2
 
     assert len(packages_pypi) == len(packages_git) == 1
     assert packages_pypi != packages_git
@@ -65,38 +65,59 @@ def test_solver_dependency_cache_pulls_from_prior_level_cache(
     root: ProjectPackage, provider: Provider, repo: Repository
 ) -> None:
     dependency_pypi = Factory.create_dependency("demo", ">=0.1.0")
+    dependency_pypi_constrained = Factory.create_dependency("demo", ">=0.1.0,<2.0.0")
     root.add_dependency(dependency_pypi)
+    root.add_dependency(dependency_pypi_constrained)
     add_to_repo(repo, "demo", "1.0.0")
 
     wrapped_provider = mock.Mock(wraps=provider)
     cache = DependencyCache(wrapped_provider)
-    cache.search_for.cache_clear()
+    cache._search_for_cached.cache_clear()
 
-    # On first call, provider.search_for() should be called and the level-0
-    # cache populated.
+    # On first call, provider.search_for() should be called and the cache
+    # populated.
     cache.search_for(dependency_pypi, 0)
     assert len(wrapped_provider.search_for.mock_calls) == 1
-    assert ("demo", None, None, None, None) in cache._cache[0]
-    assert cache.search_for.cache_info().hits == 0
-    assert cache.search_for.cache_info().misses == 1
-
-    # On second call at level 1, provider.search_for() should not be called
-    # again and the level-1 cache should be populated from the level-0 cache.
+    assert ("demo", None, None, None, None) in cache._cache
+    assert ("demo", None, None, None, None) in cache._cached_dependencies_by_level[0]
+    assert cache._search_for_cached.cache_info().hits == 0
+    assert cache._search_for_cached.cache_info().misses == 1
+
+    # On second call at level 1, neither provider.search_for() nor
+    # cache._search_for_cached() should have been called again, and the cache
+    # should remain the same.
     cache.search_for(dependency_pypi, 1)
     assert len(wrapped_provider.search_for.mock_calls) == 1
-    assert ("demo", None, None, None, None) in cache._cache[1]
-    assert cache._cache[0] == cache._cache[1]
-    assert cache.search_for.cache_info().hits == 0
-    assert cache.search_for.cache_info().misses == 2
-
-    # Clearing the level 1 cache should invalidate the lru_cache on
-    # cache.search_for and wipe out the level 1 cache while preserving the
+    assert ("demo", None, None, None, None) in cache._cache
+    assert ("demo", None, None, None, None) in cache._cached_dependencies_by_level[0]
+    assert set(cache._cached_dependencies_by_level.keys()) == {0}
+    assert cache._search_for_cached.cache_info().hits == 1
+    assert cache._search_for_cached.cache_info().misses == 1
+
+    # On third call at level 2 with an updated constraint for the `demo`
+    # package should not call provider.search_for(), but should call
+    # cache._search_for_cached() and update the cache.
+    cache.search_for(dependency_pypi_constrained, 2)
+    assert len(wrapped_provider.search_for.mock_calls) == 1
+    assert ("demo", None, None, None, None) in cache._cache
+    assert ("demo", None, None, None, None) in cache._cached_dependencies_by_level[0]
+    assert ("demo", None, None, None, None) in cache._cached_dependencies_by_level[2]
+    assert set(cache._cached_dependencies_by_level.keys()) == {0, 2}
+    assert cache._search_for_cached.cache_info().hits == 1
+    assert cache._search_for_cached.cache_info().misses == 2
+
+    # Clearing the level 2 and level 1 caches should invalidate the lru_cache
+    # on cache.search_for and wipe out the level 2 cache while preserving the
     # level 0 cache.
+    cache.clear_level(2)
     cache.clear_level(1)
-    assert set(cache._cache.keys()) == {0}
-    assert ("demo", None, None, None, None) in cache._cache[0]
-    assert cache.search_for.cache_info().hits == 0
-    assert cache.search_for.cache_info().misses == 0
+    cache.search_for(dependency_pypi, 0)
+    assert len(wrapped_provider.search_for.mock_calls) == 1
+    assert ("demo", None, None, None, None) in cache._cache
+    assert ("demo", None, None, None, None) in cache._cached_dependencies_by_level[0]
+    assert set(cache._cached_dependencies_by_level.keys()) == {0}
+    assert cache._search_for_cached.cache_info().hits == 0
+    assert cache._search_for_cached.cache_info().misses == 1
 
 
 def test_solver_dependency_cache_respects_subdirectories(
@@ -123,20 +144,20 @@ def test_solver_dependency_cache_respects_subdirectories(
     root.add_dependency(dependency_one_copy)
 
     cache = DependencyCache(provider)
-    cache.search_for.cache_clear()
+    cache._search_for_cached.cache_clear()
 
     # ensure cache was never hit for both calls
     cache.search_for(dependency_one, 0)
     cache.search_for(dependency_one_copy, 0)
-    assert not cache.search_for.cache_info().hits
+    assert not cache._search_for_cached.cache_info().hits
 
     # increase test coverage by searching for copies
     # (when searching for the exact same object, __eq__ is never called)
     packages_one = cache.search_for(deepcopy(dependency_one), 0)
     packages_one_copy = cache.search_for(deepcopy(dependency_one_copy), 0)
 
-    assert cache.search_for.cache_info().hits == 2
-    assert cache.search_for.cache_info().currsize == 2
+    assert cache._search_for_cached.cache_info().hits == 2
+    assert cache._search_for_cached.cache_info().currsize == 2
 
     assert len(packages_one) == len(packages_one_copy) == 1