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Add config flags to allow for cache auto-tuning (#12701)

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1
changelog.d/12701.feature Normal file
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@ -0,0 +1 @@
Add a config options to allow for auto-tuning of caches.

18
docs/sample_config.yaml
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@ -784,6 +784,24 @@ caches:
#
#cache_entry_ttl: 30m
# This flag enables cache autotuning, and is further specified by the sub-options `max_cache_memory_usage`,
# `target_cache_memory_usage`, `min_cache_ttl`. These flags work in conjunction with each other to maintain
# a balance between cache memory usage and cache entry availability. You must be using jemalloc to utilize
# this option, and all three of the options must be specified for this feature to work.
#cache_autotuning:
# This flag sets a ceiling on much memory the cache can use before caches begin to be continuously evicted.
# They will continue to be evicted until the memory usage drops below the `target_memory_usage`, set in
# the flag below, or until the `min_cache_ttl` is hit.
#max_cache_memory_usage: 1024M
# This flag sets a rough target for the desired memory usage of the caches.
#target_cache_memory_usage: 758M
# 'min_cache_ttl` sets a limit under which newer cache entries are not evicted and is only applied when
# caches are actively being evicted/`max_cache_memory_usage` has been exceeded. This is to protect hot caches
# from being emptied while Synapse is evicting due to memory.
#min_cache_ttl: 5m
# Controls how long the results of a /sync request are cached for after
# a successful response is returned. A higher duration can help clients with
# intermittent connections, at the cost of higher memory usage.

17
docs/usage/configuration/config_documentation.md
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@ -1119,7 +1119,17 @@ Caching can be configured through the following sub-options:
with intermittent connections, at the cost of higher memory usage.
By default, this is zero, which means that sync responses are not cached
at all.
* `cache_autotuning` and its sub-options `max_cache_memory_usage`, `target_cache_memory_usage`, and
`min_cache_ttl` work in conjunction with each other to maintain a balance between cache memory
usage and cache entry availability. You must be using [jemalloc](https://github.com/matrix-org/synapse#help-synapse-is-slow-and-eats-all-my-ramcpu)
to utilize this option, and all three of the options must be specified for this feature to work.
* `max_cache_memory_usage` sets a ceiling on how much memory the cache can use before caches begin to be continuously evicted.
They will continue to be evicted until the memory usage drops below the `target_memory_usage`, set in
the flag below, or until the `min_cache_ttl` is hit.
* `target_memory_usage` sets a rough target for the desired memory usage of the caches.
* `min_cache_ttl` sets a limit under which newer cache entries are not evicted and is only applied when
caches are actively being evicted/`max_cache_memory_usage` has been exceeded. This is to protect hot caches
from being emptied while Synapse is evicting due to memory.
Example configuration:
```yaml
@ -1127,8 +1137,11 @@ caches:
global_factor: 1.0
per_cache_factors:
get_users_who_share_room_with_user: 2.0
expire_caches: false
sync_response_cache_duration: 2m
cache_autotuning:
max_cache_memory_usage: 1024M
target_cache_memory_usage: 758M
min_cache_ttl: 5m
```
### Reloading cache factors

33
synapse/config/cache.py
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@ -176,6 +176,24 @@ class CacheConfig(Config):
#
#cache_entry_ttl: 30m
# This flag enables cache autotuning, and is further specified by the sub-options `max_cache_memory_usage`,
# `target_cache_memory_usage`, `min_cache_ttl`. These flags work in conjunction with each other to maintain
# a balance between cache memory usage and cache entry availability. You must be using jemalloc to utilize
# this option, and all three of the options must be specified for this feature to work.
#cache_autotuning:
# This flag sets a ceiling on much memory the cache can use before caches begin to be continuously evicted.
# They will continue to be evicted until the memory usage drops below the `target_memory_usage`, set in
# the flag below, or until the `min_cache_ttl` is hit.
#max_cache_memory_usage: 1024M
# This flag sets a rough target for the desired memory usage of the caches.
#target_cache_memory_usage: 758M
# 'min_cache_ttl` sets a limit under which newer cache entries are not evicted and is only applied when
# caches are actively being evicted/`max_cache_memory_usage` has been exceeded. This is to protect hot caches
# from being emptied while Synapse is evicting due to memory.
#min_cache_ttl: 5m
# Controls how long the results of a /sync request are cached for after
# a successful response is returned. A higher duration can help clients with
# intermittent connections, at the cost of higher memory usage.
@ -263,6 +281,21 @@ class CacheConfig(Config):
)
self.expiry_time_msec = self.parse_duration(expiry_time)
self.cache_autotuning = cache_config.get("cache_autotuning")
if self.cache_autotuning:
max_memory_usage = self.cache_autotuning.get("max_cache_memory_usage")
self.cache_autotuning["max_cache_memory_usage"] = self.parse_size(
max_memory_usage
)
target_mem_size = self.cache_autotuning.get("target_cache_memory_usage")
self.cache_autotuning["target_cache_memory_usage"] = self.parse_size(
target_mem_size
)
min_cache_ttl = self.cache_autotuning.get("min_cache_ttl")
self.cache_autotuning["min_cache_ttl"] = self.parse_duration(min_cache_ttl)
self.sync_response_cache_duration = self.parse_duration(
cache_config.get("sync_response_cache_duration", 0)
)

114
synapse/metrics/jemalloc.py
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@ -18,6 +18,7 @@ import os
import re
from typing import Iterable, Optional, overload
import attr
from prometheus_client import REGISTRY, Metric
from typing_extensions import Literal
@ -27,52 +28,24 @@ from synapse.metrics._types import Collector
logger = logging.getLogger(__name__)
def _setup_jemalloc_stats() -> None:
"""Checks to see if jemalloc is loaded, and hooks up a collector to record
statistics exposed by jemalloc.
"""
# Try to find the loaded jemalloc shared library, if any. We need to
# introspect into what is loaded, rather than loading whatever is on the
# path, as if we load a *different* jemalloc version things will seg fault.
# We look in `/proc/self/maps`, which only exists on linux.
if not os.path.exists("/proc/self/maps"):
logger.debug("Not looking for jemalloc as no /proc/self/maps exist")
return
# We're looking for a path at the end of the line that includes
# "libjemalloc".
regex = re.compile(r"/\S+/libjemalloc.*$")
jemalloc_path = None
with open("/proc/self/maps") as f:
for line in f:
match = regex.search(line.strip())
if match:
jemalloc_path = match.group()
if not jemalloc_path:
# No loaded jemalloc was found.
logger.debug("jemalloc not found")
return
logger.debug("Found jemalloc at %s", jemalloc_path)
jemalloc = ctypes.CDLL(jemalloc_path)
@attr.s(slots=True, frozen=True, auto_attribs=True)
class JemallocStats:
jemalloc: ctypes.CDLL
@overload
def _mallctl(
name: str, read: Literal[True] = True, write: Optional[int] = None
self, name: str, read: Literal[True] = True, write: Optional[int] = None
) -> int:
...
@overload
def _mallctl(name: str, read: Literal[False], write: Optional[int] = None) -> None:
def _mallctl(
self, name: str, read: Literal[False], write: Optional[int] = None
) -> None:
...
def _mallctl(
name: str, read: bool = True, write: Optional[int] = None
self, name: str, read: bool = True, write: Optional[int] = None
) -> Optional[int]:
"""Wrapper around `mallctl` for reading and writing integers to
jemalloc.
@ -120,7 +93,7 @@ def _setup_jemalloc_stats() -> None:
# Where oldp/oldlenp is a buffer where the old value will be written to
# (if not null), and newp/newlen is the buffer with the new value to set
# (if not null). Note that they're all references *except* newlen.
result = jemalloc.mallctl(
result = self.jemalloc.mallctl(
name.encode("ascii"),
input_var_ref,
input_len_ref,
@ -136,21 +109,80 @@ def _setup_jemalloc_stats() -> None:
return input_var.value
def _jemalloc_refresh_stats() -> None:
def refresh_stats(self) -> None:
"""Request that jemalloc updates its internal statistics. This needs to
be called before querying for stats, otherwise it will return stale
values.
"""
try:
_mallctl("epoch", read=False, write=1)
self._mallctl("epoch", read=False, write=1)
except Exception as e:
logger.warning("Failed to reload jemalloc stats: %s", e)
def get_stat(self, name: str) -> int:
"""Request the stat of the given name at the time of the last
`refresh_stats` call. This may throw if we fail to read
the stat.
"""
return self._mallctl(f"stats.{name}")
_JEMALLOC_STATS: Optional[JemallocStats] = None
def get_jemalloc_stats() -> Optional[JemallocStats]:
"""Returns an interface to jemalloc, if it is being used.
Note that this will always return None until `setup_jemalloc_stats` has been
called.
"""
return _JEMALLOC_STATS
def _setup_jemalloc_stats() -> None:
"""Checks to see if jemalloc is loaded, and hooks up a collector to record
statistics exposed by jemalloc.
"""
global _JEMALLOC_STATS
# Try to find the loaded jemalloc shared library, if any. We need to
# introspect into what is loaded, rather than loading whatever is on the
# path, as if we load a *different* jemalloc version things will seg fault.
# We look in `/proc/self/maps`, which only exists on linux.
if not os.path.exists("/proc/self/maps"):
logger.debug("Not looking for jemalloc as no /proc/self/maps exist")
return
# We're looking for a path at the end of the line that includes
# "libjemalloc".
regex = re.compile(r"/\S+/libjemalloc.*$")
jemalloc_path = None
with open("/proc/self/maps") as f:
for line in f:
match = regex.search(line.strip())
if match:
jemalloc_path = match.group()
if not jemalloc_path:
# No loaded jemalloc was found.
logger.debug("jemalloc not found")
return
logger.debug("Found jemalloc at %s", jemalloc_path)
jemalloc_dll = ctypes.CDLL(jemalloc_path)
stats = JemallocStats(jemalloc_dll)
_JEMALLOC_STATS = stats
class JemallocCollector(Collector):
"""Metrics for internal jemalloc stats."""
def collect(self) -> Iterable[Metric]:
_jemalloc_refresh_stats()
stats.refresh_stats()
g = GaugeMetricFamily(
"jemalloc_stats_app_memory_bytes",
@ -184,7 +216,7 @@ def _setup_jemalloc_stats() -> None:
"metadata",
):
try:
value = _mallctl(f"stats.{t}")
value = stats.get_stat(t)
except Exception as e:
# There was an error fetching the value, skip.
logger.warning("Failed to read jemalloc stats.%s: %s", t, e)

79
synapse/util/caches/lrucache.py
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@ -13,6 +13,7 @@
# limitations under the License.
import logging
import math
import threading
import weakref
from enum import Enum
@ -40,6 +41,7 @@ from twisted.internet.interfaces import IReactorTime
from synapse.config import cache as cache_config
from synapse.metrics.background_process_metrics import wrap_as_background_process
from synapse.metrics.jemalloc import get_jemalloc_stats
from synapse.util import Clock, caches
from synapse.util.caches import CacheMetric, EvictionReason, register_cache
from synapse.util.caches.treecache import TreeCache, iterate_tree_cache_entry
@ -106,10 +108,16 @@ 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) -> None:
async def _expire_old_entries(
clock: Clock, expiry_seconds: int, autotune_config: Optional[dict]
) -> None:
"""Walks the global cache list to find cache entries that haven't been
accessed in the given number of seconds.
accessed in the given number of seconds, or if a given memory threshold has been breached.
"""
if autotune_config:
max_cache_memory_usage = autotune_config["max_cache_memory_usage"]
target_cache_memory_usage = autotune_config["target_cache_memory_usage"]
min_cache_ttl = autotune_config["min_cache_ttl"] / 1000
now = int(clock.time())
node = GLOBAL_ROOT.prev_node
@ -119,11 +127,36 @@ async def _expire_old_entries(clock: Clock, expiry_seconds: int) -> None:
logger.debug("Searching for stale caches")
evicting_due_to_memory = False
# determine if we're evicting due to memory
jemalloc_interface = get_jemalloc_stats()
if jemalloc_interface and autotune_config:
try:
jemalloc_interface.refresh_stats()
mem_usage = jemalloc_interface.get_stat("allocated")
if mem_usage > max_cache_memory_usage:
logger.info("Begin memory-based cache eviction.")
evicting_due_to_memory = True
except Exception:
logger.warning(
"Unable to read allocated memory, skipping memory-based cache eviction."
)
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:
# if node has not aged past expiry_seconds and we are not evicting due to memory usage, there's
# nothing to do here
if (
node.last_access_ts_secs > now - expiry_seconds
and not evicting_due_to_memory
):
break
# if entry is newer than min_cache_entry_ttl then do not evict and don't evict anything newer
if evicting_due_to_memory and now - node.last_access_ts_secs < min_cache_ttl:
break
cache_entry = node.get_cache_entry()
@ -136,10 +169,29 @@ async def _expire_old_entries(clock: Clock, expiry_seconds: int) -> None:
assert cache_entry is not None
cache_entry.drop_from_cache()
# Check mem allocation periodically if we are evicting a bunch of caches
if jemalloc_interface and evicting_due_to_memory and (i + 1) % 100 == 0:
try:
jemalloc_interface.refresh_stats()
mem_usage = jemalloc_interface.get_stat("allocated")
if mem_usage < target_cache_memory_usage:
evicting_due_to_memory = False
logger.info("Stop memory-based cache eviction.")
except Exception:
logger.warning(
"Unable to read allocated memory, this may affect memory-based cache eviction."
)
# If we've failed to read the current memory usage then we
# should stop trying to evict based on memory usage
evicting_due_to_memory = False
# 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)
if node.last_access_ts_secs > now - expiry_seconds:
await clock.sleep(0.5)
else:
await clock.sleep(0)
logger.debug("Waking during drop")
node = next_node
@ -156,21 +208,28 @@ async def _expire_old_entries(clock: Clock, expiry_seconds: int) -> None:
def setup_expire_lru_cache_entries(hs: "HomeServer") -> None:
"""Start a background job that expires all cache entries if they have not
been accessed for the given number of seconds.
been accessed for the given number of seconds, or if a given memory usage threshold has been
breached.
"""
if not hs.config.caches.expiry_time_msec:
if not hs.config.caches.expiry_time_msec and not hs.config.caches.cache_autotuning:
return
logger.info(
"Expiring LRU caches after %d seconds", hs.config.caches.expiry_time_msec / 1000
)
if hs.config.caches.expiry_time_msec:
expiry_time = hs.config.caches.expiry_time_msec / 1000
logger.info("Expiring LRU caches after %d seconds", expiry_time)
else:
expiry_time = math.inf
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
_expire_old_entries,
30 * 1000,
clock,
expiry_time,
hs.config.caches.cache_autotuning,
)

58
tests/util/test_lrucache.py
View File

@ -14,8 +14,9 @@
from typing import List
from unittest.mock import Mock
from unittest.mock import Mock, patch
from synapse.metrics.jemalloc import JemallocStats
from synapse.util.caches.lrucache import LruCache, setup_expire_lru_cache_entries
from synapse.util.caches.treecache import TreeCache
@ -316,3 +317,58 @@ class TimeEvictionTestCase(unittest.HomeserverTestCase):
self.assertEqual(cache.get("key1"), None)
self.assertEqual(cache.get("key2"), 3)
class MemoryEvictionTestCase(unittest.HomeserverTestCase):
@override_config(
{
"caches": {
"cache_autotuning": {
"max_cache_memory_usage": "700M",
"target_cache_memory_usage": "500M",
"min_cache_ttl": "5m",
}
}
}
)
@patch("synapse.util.caches.lrucache.get_jemalloc_stats")
def test_evict_memory(self, jemalloc_interface) -> None:
mock_jemalloc_class = Mock(spec=JemallocStats)
jemalloc_interface.return_value = mock_jemalloc_class
# set the return value of get_stat() to be greater than max_cache_memory_usage
mock_jemalloc_class.get_stat.return_value = 924288000
setup_expire_lru_cache_entries(self.hs)
cache = LruCache(4, clock=self.hs.get_clock())
cache["key1"] = 1
cache["key2"] = 2
# advance the reactor less than the min_cache_ttl
self.reactor.advance(60 * 2)
# our items should still be in the cache
self.assertEqual(cache.get("key1"), 1)
self.assertEqual(cache.get("key2"), 2)
# advance the reactor past the min_cache_ttl
self.reactor.advance(60 * 6)
# the items should be cleared from cache
self.assertEqual(cache.get("key1"), None)
self.assertEqual(cache.get("key2"), None)
# add more stuff to caches
cache["key1"] = 1
cache["key2"] = 2
# set the return value of get_stat() to be lower than target_cache_memory_usage
mock_jemalloc_class.get_stat.return_value = 10000
# advance the reactor past the min_cache_ttl
self.reactor.advance(60 * 6)
# the items should still be in the cache
self.assertEqual(cache.get("key1"), 1)
self.assertEqual(cache.get("key2"), 2)