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

This commit is contained in:
Erik Johnston 2021-07-05 16:32:12 +01:00 committed by GitHub
parent 10671da05b
commit 7a5873277e
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9 changed files with 483 additions and 93 deletions
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1
changelog.d/10205.feature Normal file
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@ -0,0 +1 @@
Add support for evicting cache entries based on last access time.

6
docs/sample_config.yaml
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@ -703,6 +703,12 @@ caches:
per_cache_factors:
#get_users_who_share_room_with_user: 2.0
# Controls how long an entry can be in a cache without having been
# accessed before being evicted. Defaults to None, which means
# entries are never evicted based on time.
#
#expiry_time: 30m
## Database ##

1
mypy.ini
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@ -75,6 +75,7 @@ files =
synapse/util/daemonize.py,
synapse/util/hash.py,
synapse/util/iterutils.py,
synapse/util/linked_list.py,
synapse/util/metrics.py,
synapse/util/macaroons.py,
synapse/util/module_loader.py,

11
synapse/app/_base.py
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@ -21,7 +21,7 @@ import socket
import sys
import traceback
import warnings
from typing import Awaitable, Callable, Iterable
from typing import TYPE_CHECKING, Awaitable, Callable, Iterable
from cryptography.utils import CryptographyDeprecationWarning
from typing_extensions import NoReturn
@ -41,10 +41,14 @@ from synapse.events.spamcheck import load_legacy_spam_checkers
from synapse.logging.context import PreserveLoggingContext
from synapse.metrics.background_process_metrics import wrap_as_background_process
from synapse.metrics.jemalloc import setup_jemalloc_stats
from synapse.util.caches.lrucache import setup_expire_lru_cache_entries
from synapse.util.daemonize import daemonize_process
from synapse.util.rlimit import change_resource_limit
from synapse.util.versionstring import get_version_string
if TYPE_CHECKING:
from synapse.server import HomeServer
logger = logging.getLogger(__name__)
# list of tuples of function, args list, kwargs dict
@ -312,7 +316,7 @@ def refresh_certificate(hs):
logger.info("Context factories updated.")
async def start(hs: "synapse.server.HomeServer"):
async def start(hs: "HomeServer"):
"""
Start a Synapse server or worker.
@ -365,6 +369,9 @@ async def start(hs: "synapse.server.HomeServer"):
load_legacy_spam_checkers(hs)
# If we've configured an expiry time for caches, start the background job now.
setup_expire_lru_cache_entries(hs)
# It is now safe to start your Synapse.
hs.start_listening()
hs.get_datastore().db_pool.start_profiling()

2
synapse/config/_base.pyi
View File

@ -5,6 +5,7 @@ from synapse.config import (
api,
appservice,
auth,
cache,
captcha,
cas,
consent,
@ -88,6 +89,7 @@ class RootConfig:
tracer: tracer.TracerConfig
redis: redis.RedisConfig
modules: modules.ModulesConfig
caches: cache.CacheConfig
federation: federation.FederationConfig
config_classes: List = ...

12
synapse/config/cache.py
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@ -145,6 +145,12 @@ class CacheConfig(Config):
#
per_cache_factors:
#get_users_who_share_room_with_user: 2.0
# Controls how long an entry can be in a cache without having been
# accessed before being evicted. Defaults to None, which means
# entries are never evicted based on time.
#
#expiry_time: 30m
"""
def read_config(self, config, **kwargs):
@ -200,6 +206,12 @@ class CacheConfig(Config):
e.message # noqa: B306, DependencyException.message is a property
)
expiry_time = cache_config.get("expiry_time")
if expiry_time:
self.expiry_time_msec = self.parse_duration(expiry_time)
else:
self.expiry_time_msec = None
# Resize all caches (if necessary) with the new factors we've loaded
self.resize_all_caches()

235
synapse/util/caches/lrucache.py
View File

@ -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 move_node_to_front(node: _Node):
node.move_to_front(real_clock, list_root)
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 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()

150
synapse/util/linked_list.py Normal file
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@ -0,0 +1,150 @@
# Copyright 2021 The Matrix.org Foundation C.I.C.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
# http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
"""A circular doubly linked list implementation.
"""
import threading
from typing import Generic, Optional, Type, TypeVar
P = TypeVar("P")
LN = TypeVar("LN", bound="ListNode")
class ListNode(Generic[P]):
"""A node in a circular doubly linked list, with an (optional) reference to
a cache entry.
The reference should only be `None` for the root node or if the node has
been removed from the list.
"""
# A lock to protect mutating the list prev/next pointers.
_LOCK = threading.Lock()
# We don't use attrs here as in py3.6 you can't have `attr.s(slots=True)`
# and inherit from `Generic` for some reason
__slots__ = [
"cache_entry",
"prev_node",
"next_node",
]
def __init__(self, cache_entry: Optional[P] = None) -> None:
self.cache_entry = cache_entry
self.prev_node: Optional[ListNode[P]] = None
self.next_node: Optional[ListNode[P]] = None
@classmethod
def create_root_node(cls: Type["ListNode[P]"]) -> "ListNode[P]":
"""Create a new linked list by creating a "root" node, which is a node
that has prev_node/next_node pointing to itself and no associated cache
entry.
"""
root = cls()
root.prev_node = root
root.next_node = root
return root
@classmethod
def insert_after(
cls: Type[LN],
cache_entry: P,
node: "ListNode[P]",
) -> LN:
"""Create a new list node that is placed after the given node.
Args:
cache_entry: The associated cache entry.
node: The existing node in the list to insert the new entry after.
"""
new_node = cls(cache_entry)
with cls._LOCK:
new_node._refs_insert_after(node)
return new_node
def remove_from_list(self):
"""Remove this node from the list."""
with self._LOCK:
self._refs_remove_node_from_list()
# We drop the reference to the cache entry to break the reference cycle
# between the list node and cache entry, allowing the two to be dropped
# immediately rather than at the next GC.
self.cache_entry = None
def move_after(self, node: "ListNode"):
"""Move this node from its current location in the list to after the
given node.
"""
with self._LOCK:
# We assert that both this node and the target node is still "alive".
assert self.prev_node
assert self.next_node
assert node.prev_node
assert node.next_node
assert self is not node
# Remove self from the list
self._refs_remove_node_from_list()
# Insert self back into the list, after target node
self._refs_insert_after(node)
def _refs_remove_node_from_list(self):
"""Internal method to *just* remove the node from the list, without
e.g. clearing out the cache entry.
"""
if self.prev_node is None or self.next_node is None:
# We've already been removed from the list.
return
prev_node = self.prev_node
next_node = self.next_node
prev_node.next_node = next_node
next_node.prev_node = prev_node
# We set these to None so that we don't get circular references,
# allowing us to be dropped without having to go via the GC.
self.prev_node = None
self.next_node = None
def _refs_insert_after(self, node: "ListNode"):
"""Internal method to insert the node after the given node."""
# This method should only be called when we're not already in the list.
assert self.prev_node is None
assert self.next_node is None
# We expect the given node to be in the list and thus have valid
# prev/next refs.
assert node.next_node
assert node.prev_node
prev_node = node
next_node = node.next_node
self.prev_node = prev_node
self.next_node = next_node
prev_node.next_node = self
next_node.prev_node = self
def get_cache_entry(self) -> Optional[P]:
"""Get the cache entry, returns None if this is the root node (i.e.
cache_entry is None) or if the entry has been dropped.
"""
return self.cache_entry

46
tests/util/test_lrucache.py
View File

@ -15,7 +15,7 @@
from unittest.mock import Mock
from synapse.util.caches.lrucache import LruCache
from synapse.util.caches.lrucache import LruCache, setup_expire_lru_cache_entries
from synapse.util.caches.treecache import TreeCache
from tests import unittest
@ -260,3 +260,47 @@ class LruCacheSizedTestCase(unittest.HomeserverTestCase):
self.assertEquals(cache["key3"], [3])
self.assertEquals(cache["key4"], [4])
self.assertEquals(cache["key5"], [5, 6])
class TimeEvictionTestCase(unittest.HomeserverTestCase):
"""Test that time based eviction works correctly."""
def default_config(self):
config = super().default_config()
config.setdefault("caches", {})["expiry_time"] = "30m"
return config
def test_evict(self):
setup_expire_lru_cache_entries(self.hs)
cache = LruCache(5, clock=self.hs.get_clock())
# Check that we evict entries we haven't accessed for 30 minutes.
cache["key1"] = 1
cache["key2"] = 2
self.reactor.advance(20 * 60)
self.assertEqual(cache.get("key1"), 1)
self.reactor.advance(20 * 60)
# We have only touched `key1` in the last 30m, so we expect that to
# still be in the cache while `key2` should have been evicted.
self.assertEqual(cache.get("key1"), 1)
self.assertEqual(cache.get("key2"), None)
# Check that re-adding an expired key works correctly.
cache["key2"] = 3
self.assertEqual(cache.get("key2"), 3)
self.reactor.advance(20 * 60)
self.assertEqual(cache.get("key2"), 3)
self.reactor.advance(20 * 60)
self.assertEqual(cache.get("key1"), None)
self.assertEqual(cache.get("key2"), 3)