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Track in memory events using weakrefs (#10533)

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Erik Johnston 2022-05-17 10:34:27 +01:00 committed by GitHub
parent 1fe202a1a3
commit fcf951d5dc
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4 changed files with 60 additions and 2 deletions

1
changelog.d/10533.misc Normal file
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@ -0,0 +1 @@
Improve event caching mechanism to avoid having multiple copies of an event in memory at a time.

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@ -14,6 +14,7 @@
import logging
import threading
import weakref
from enum import Enum, auto
from typing import (
TYPE_CHECKING,
@ -23,6 +24,7 @@ from typing import (
Dict,
Iterable,
List,
MutableMapping,
Optional,
Set,
Tuple,
@ -248,6 +250,12 @@ class EventsWorkerStore(SQLBaseStore):
str, ObservableDeferred[Dict[str, EventCacheEntry]]
] = {}
# We keep track of the events we have currently loaded in memory so that
# we can reuse them even if they've been evicted from the cache. We only
# track events that don't need redacting in here (as then we don't need
# to track redaction status).
self._event_ref: MutableMapping[str, EventBase] = weakref.WeakValueDictionary()
self._event_fetch_lock = threading.Condition()
self._event_fetch_list: List[
Tuple[Iterable[str], "defer.Deferred[Dict[str, _EventRow]]"]
@ -723,6 +731,8 @@ class EventsWorkerStore(SQLBaseStore):
def _invalidate_get_event_cache(self, event_id: str) -> None:
self._get_event_cache.invalidate((event_id,))
self._event_ref.pop(event_id, None)
self._current_event_fetches.pop(event_id, None)
def _get_events_from_cache(
self, events: Iterable[str], update_metrics: bool = True
@ -738,13 +748,30 @@ class EventsWorkerStore(SQLBaseStore):
event_map = {}
for event_id in events:
# First check if it's in the event cache
ret = self._get_event_cache.get(
(event_id,), None, update_metrics=update_metrics
)
if not ret:
if ret:
event_map[event_id] = ret
continue
event_map[event_id] = ret
# Otherwise check if we still have the event in memory.
event = self._event_ref.get(event_id)
if event:
# Reconstruct an event cache entry
cache_entry = EventCacheEntry(
event=event,
# We don't cache weakrefs to redacted events, so we know
# this is None.
redacted_event=None,
)
event_map[event_id] = cache_entry
# We add the entry back into the cache as we want to keep
# recently queried events in the cache.
self._get_event_cache.set((event_id,), cache_entry)
return event_map
@ -1124,6 +1151,10 @@ class EventsWorkerStore(SQLBaseStore):
self._get_event_cache.set((event_id,), cache_entry)
result_map[event_id] = cache_entry
if not redacted_event:
# We only cache references to unredacted events.
self._event_ref[event_id] = original_ev
return result_map
async def _enqueue_events(self, events: Collection[str]) -> Dict[str, _EventRow]:

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@ -160,6 +160,7 @@ class SyncTestCase(tests.unittest.HomeserverTestCase):
# Blow away caches (supported room versions can only change due to a restart).
self.store.get_rooms_for_user_with_stream_ordering.invalidate_all()
self.store._get_event_cache.clear()
self.store._event_ref.clear()
# The rooms should be excluded from the sync response.
# Get a new request key.

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@ -154,6 +154,31 @@ class EventCacheTestCase(unittest.HomeserverTestCase):
# We should have fetched the event from the DB
self.assertEqual(ctx.get_resource_usage().evt_db_fetch_count, 1)
def test_event_ref(self):
"""Test that we reuse events that are still in memory but have fallen
out of the cache, rather than requesting them from the DB.
"""
# Reset the event cache
self.store._get_event_cache.clear()
with LoggingContext("test") as ctx:
# We keep hold of the event event though we never use it.
event = self.get_success(self.store.get_event(self.event_id)) # noqa: F841
# We should have fetched the event from the DB
self.assertEqual(ctx.get_resource_usage().evt_db_fetch_count, 1)
# Reset the event cache
self.store._get_event_cache.clear()
with LoggingContext("test") as ctx:
self.get_success(self.store.get_event(self.event_id))
# Since the event is still in memory we shouldn't have fetched it
# from the DB
self.assertEqual(ctx.get_resource_usage().evt_db_fetch_count, 0)
def test_dedupe(self):
"""Test that if we request the same event multiple times we only pull it
out once.