Add support for evicting cache entries based on last access time. (#10205)

1 files changed, 202 insertions, 35 deletions

diff --git a/synapse/util/caches/lrucache.py b/synapse/util/caches/lrucache.py
index d89e9d9b1d..4b9d0433ff 100644
--- a/synapse/util/caches/lrucache.py
+++ b/synapse/util/caches/lrucache.py
@@ -12,9 +12,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import logging
 import threading
+import weakref
 from functools import wraps
 from typing import (
+    TYPE_CHECKING,
     Any,
     Callable,
     Collection,
@@ -31,10 +34,19 @@ from typing import (
 
 from typing_extensions import Literal
 
+from twisted.internet import reactor
+
 from synapse.config import cache as cache_config
-from synapse.util import caches
+from synapse.metrics.background_process_metrics import wrap_as_background_process
+from synapse.util import Clock, caches
 from synapse.util.caches import CacheMetric, register_cache
 from synapse.util.caches.treecache import TreeCache, iterate_tree_cache_entry
+from synapse.util.linked_list import ListNode
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
 
 try:
     from pympler.asizeof import Asizer
@@ -82,19 +94,126 @@ def enumerate_leaves(node, depth):
                 yield m
 
 
+P = TypeVar("P")
+
+
+class _TimedListNode(ListNode[P]):
+    """A `ListNode` that tracks last access time."""
+
+    __slots__ = ["last_access_ts_secs"]
+
+    def update_last_access(self, clock: Clock):
+        self.last_access_ts_secs = int(clock.time())
+
+
+# Whether to insert new cache entries to the global list. We only add to it if
+# time based eviction is enabled.
+USE_GLOBAL_LIST = False
+
+# A linked list of all cache entries, allowing efficient time based eviction.
+GLOBAL_ROOT = ListNode["_Node"].create_root_node()
+
+
+@wrap_as_background_process("LruCache._expire_old_entries")
+async def _expire_old_entries(clock: Clock, expiry_seconds: int):
+    """Walks the global cache list to find cache entries that haven't been
+    accessed in the given number of seconds.
+    """
+
+    now = int(clock.time())
+    node = GLOBAL_ROOT.prev_node
+    assert node is not None
+
+    i = 0
+
+    logger.debug("Searching for stale caches")
+
+    while node is not GLOBAL_ROOT:
+        # Only the root node isn't a `_TimedListNode`.
+        assert isinstance(node, _TimedListNode)
+
+        if node.last_access_ts_secs > now - expiry_seconds:
+            break
+
+        cache_entry = node.get_cache_entry()
+        next_node = node.prev_node
+
+        # The node should always have a reference to a cache entry and a valid
+        # `prev_node`, as we only drop them when we remove the node from the
+        # list.
+        assert next_node is not None
+        assert cache_entry is not None
+        cache_entry.drop_from_cache()
+
+        # If we do lots of work at once we yield to allow other stuff to happen.
+        if (i + 1) % 10000 == 0:
+            logger.debug("Waiting during drop")
+            await clock.sleep(0)
+            logger.debug("Waking during drop")
+
+        node = next_node
+
+        # If we've yielded then our current node may have been evicted, so we
+        # need to check that its still valid.
+        if node.prev_node is None:
+            break
+
+        i += 1
+
+    logger.info("Dropped %d items from caches", i)
+
+
+def setup_expire_lru_cache_entries(hs: "HomeServer"):
+    """Start a background job that expires all cache entries if they have not
+    been accessed for the given number of seconds.
+    """
+    if not hs.config.caches.expiry_time_msec:
+        return
+
+    logger.info(
+        "Expiring LRU caches after %d seconds", hs.config.caches.expiry_time_msec / 1000
+    )
+
+    global USE_GLOBAL_LIST
+    USE_GLOBAL_LIST = True
+
+    clock = hs.get_clock()
+    clock.looping_call(
+        _expire_old_entries, 30 * 1000, clock, hs.config.caches.expiry_time_msec / 1000
+    )
+
+
 class _Node:
-    __slots__ = ["prev_node", "next_node", "key", "value", "callbacks", "memory"]
+    __slots__ = [
+        "_list_node",
+        "_global_list_node",
+        "_cache",
+        "key",
+        "value",
+        "callbacks",
+        "memory",
+    ]
 
     def __init__(
         self,
-        prev_node,
-        next_node,
+        root: "ListNode[_Node]",
         key,
         value,
+        cache: "weakref.ReferenceType[LruCache]",
+        clock: Clock,
         callbacks: Collection[Callable[[], None]] = (),
     ):
-        self.prev_node = prev_node
-        self.next_node = next_node
+        self._list_node = ListNode.insert_after(self, root)
+        self._global_list_node = None
+        if USE_GLOBAL_LIST:
+            self._global_list_node = _TimedListNode.insert_after(self, GLOBAL_ROOT)
+            self._global_list_node.update_last_access(clock)
+
+        # We store a weak reference to the cache object so that this _Node can
+        # remove itself from the cache. If the cache is dropped we ensure we
+        # remove our entries in the lists.
+        self._cache = cache
+
         self.key = key
         self.value = value
 
@@ -116,11 +235,16 @@ class _Node:
             self.memory = (
                 _get_size_of(key)
                 + _get_size_of(value)
+                + _get_size_of(self._list_node, recurse=False)
                 + _get_size_of(self.callbacks, recurse=False)
                 + _get_size_of(self, recurse=False)
             )
             self.memory += _get_size_of(self.memory, recurse=False)
 
+            if self._global_list_node:
+                self.memory += _get_size_of(self._global_list_node, recurse=False)
+                self.memory += _get_size_of(self._global_list_node.last_access_ts_secs)
+
     def add_callbacks(self, callbacks: Collection[Callable[[], None]]) -> None:
         """Add to stored list of callbacks, removing duplicates."""
 
@@ -147,6 +271,32 @@ class _Node:
 
         self.callbacks = None
 
+    def drop_from_cache(self) -> None:
+        """Drop this node from the cache.
+
+        Ensures that the entry gets removed from the cache and that we get
+        removed from all lists.
+        """
+        cache = self._cache()
+        if not cache or not cache.pop(self.key, None):
+            # `cache.pop` should call `drop_from_lists()`, unless this Node had
+            # already been removed from the cache.
+            self.drop_from_lists()
+
+    def drop_from_lists(self) -> None:
+        """Remove this node from the cache lists."""
+        self._list_node.remove_from_list()
+
+        if self._global_list_node:
+            self._global_list_node.remove_from_list()
+
+    def move_to_front(self, clock: Clock, cache_list_root: ListNode) -> None:
+        """Moves this node to the front of all the lists its in."""
+        self._list_node.move_after(cache_list_root)
+        if self._global_list_node:
+            self._global_list_node.move_after(GLOBAL_ROOT)
+            self._global_list_node.update_last_access(clock)
+
 
 class LruCache(Generic[KT, VT]):
     """
@@ -163,6 +313,7 @@ class LruCache(Generic[KT, VT]):
         size_callback: Optional[Callable] = None,
         metrics_collection_callback: Optional[Callable[[], None]] = None,
         apply_cache_factor_from_config: bool = True,
+        clock: Optional[Clock] = None,
     ):
         """
         Args:
@@ -188,6 +339,13 @@ class LruCache(Generic[KT, VT]):
             apply_cache_factor_from_config (bool): If true, `max_size` will be
                 multiplied by a cache factor derived from the homeserver config
         """
+        # Default `clock` to something sensible. Note that we rename it to
+        # `real_clock` so that mypy doesn't think its still `Optional`.
+        if clock is None:
+            real_clock = Clock(reactor)
+        else:
+            real_clock = clock
+
         cache = cache_type()
         self.cache = cache  # Used for introspection.
         self.apply_cache_factor_from_config = apply_cache_factor_from_config
@@ -219,17 +377,31 @@ class LruCache(Generic[KT, VT]):
         # this is exposed for access from outside this class
         self.metrics = metrics
 
-        list_root = _Node(None, None, None, None)
-        list_root.next_node = list_root
-        list_root.prev_node = list_root
+        # We create a single weakref to self here so that we don't need to keep
+        # creating more each time we create a `_Node`.
+        weak_ref_to_self = weakref.ref(self)
+
+        list_root = ListNode[_Node].create_root_node()
 
         lock = threading.Lock()
 
         def evict():
             while cache_len() > self.max_size:
+                # Get the last node in the list (i.e. the oldest node).
                 todelete = list_root.prev_node
-                evicted_len = delete_node(todelete)
-                cache.pop(todelete.key, None)
+
+                # The list root should always have a valid `prev_node` if the
+                # cache is not empty.
+                assert todelete is not None
+
+                # The node should always have a reference to a cache entry, as
+                # we only drop the cache entry when we remove the node from the
+                # list.
+                node = todelete.get_cache_entry()
+                assert node is not None
+
+                evicted_len = delete_node(node)
+                cache.pop(node.key, None)
                 if metrics:
                     metrics.inc_evictions(evicted_len)
 
@@ -255,11 +427,7 @@ class LruCache(Generic[KT, VT]):
         self.len = synchronized(cache_len)
 
         def add_node(key, value, callbacks: Collection[Callable[[], None]] = ()):
-            prev_node = list_root
-            next_node = prev_node.next_node
-            node = _Node(prev_node, next_node, key, value, callbacks)
-            prev_node.next_node = node
-            next_node.prev_node = node
+            node = _Node(list_root, key, value, weak_ref_to_self, real_clock, callbacks)
             cache[key] = node
 
             if size_callback:
@@ -268,23 +436,11 @@ class LruCache(Generic[KT, VT]):
             if caches.TRACK_MEMORY_USAGE and metrics:
                 metrics.inc_memory_usage(node.memory)
 
-        def move_node_to_front(node):
-            prev_node = node.prev_node
-            next_node = node.next_node
-            prev_node.next_node = next_node
-            next_node.prev_node = prev_node
-            prev_node = list_root
-            next_node = prev_node.next_node
-            node.prev_node = prev_node
-            node.next_node = next_node
-            prev_node.next_node = node
-            next_node.prev_node = node
-
-        def delete_node(node):
-            prev_node = node.prev_node
-            next_node = node.next_node
-            prev_node.next_node = next_node
-            next_node.prev_node = prev_node
+        def move_node_to_front(node: _Node):
+            node.move_to_front(real_clock, list_root)
+
+        def delete_node(node: _Node) -> int:
+            node.drop_from_lists()
 
             deleted_len = 1
             if size_callback:
@@ -411,10 +567,13 @@ class LruCache(Generic[KT, VT]):
 
         @synchronized
         def cache_clear() -> None:
-            list_root.next_node = list_root
-            list_root.prev_node = list_root
             for node in cache.values():
                 node.run_and_clear_callbacks()
+                node.drop_from_lists()
+
+            assert list_root.next_node == list_root
+            assert list_root.prev_node == list_root
+
             cache.clear()
             if size_callback:
                 cached_cache_len[0] = 0
@@ -484,3 +643,11 @@ class LruCache(Generic[KT, VT]):
                 self._on_resize()
             return True
         return False
+
+    def __del__(self) -> None:
+        # We're about to be deleted, so we make sure to clear up all the nodes
+        # and run callbacks, etc.
+        #
+        # This happens e.g. in the sync code where we have an expiring cache of
+        # lru caches.
+        self.clear()