Optionally track memory usage of each LruCache (#9881)

This will double count slightly in the presence of interned strings. It's off by default as it can consume a lot of resources.

2 files changed, 78 insertions, 1 deletions

diff --git a/synapse/util/caches/__init__.py b/synapse/util/caches/__init__.py

index 46af7fa473..ca36f07c20 100644
--- a/synapse/util/caches/__init__.py
+++ b/synapse/util/caches/__init__.py
@@ -24,6 +24,11 @@ from synapse.config.cache import add_resizable_cache
 
 logger = logging.getLogger(__name__)
 
+
+# Whether to track estimated memory usage of the LruCaches.
+TRACK_MEMORY_USAGE = False
+
+
 caches_by_name = {}  # type: Dict[str, Sized]
 collectors_by_name = {}  # type: Dict[str, CacheMetric]
 
@@ -32,6 +37,11 @@ cache_hits = Gauge("synapse_util_caches_cache:hits", "", ["name"])
 cache_evicted = Gauge("synapse_util_caches_cache:evicted_size", "", ["name"])
 cache_total = Gauge("synapse_util_caches_cache:total", "", ["name"])
 cache_max_size = Gauge("synapse_util_caches_cache_max_size", "", ["name"])
+cache_memory_usage = Gauge(
+    "synapse_util_caches_cache_size_bytes",
+    "Estimated memory usage of the caches",
+    ["name"],
+)
 
 response_cache_size = Gauge("synapse_util_caches_response_cache:size", "", ["name"])
 response_cache_hits = Gauge("synapse_util_caches_response_cache:hits", "", ["name"])
@@ -52,6 +62,7 @@ class CacheMetric:
     hits = attr.ib(default=0)
     misses = attr.ib(default=0)
     evicted_size = attr.ib(default=0)
+    memory_usage = attr.ib(default=None)
 
     def inc_hits(self):
         self.hits += 1
@@ -62,6 +73,19 @@ class CacheMetric:
     def inc_evictions(self, size=1):
         self.evicted_size += size
 
+    def inc_memory_usage(self, memory: int):
+        if self.memory_usage is None:
+            self.memory_usage = 0
+
+        self.memory_usage += memory
+
+    def dec_memory_usage(self, memory: int):
+        self.memory_usage -= memory
+
+    def clear_memory_usage(self):
+        if self.memory_usage is not None:
+            self.memory_usage = 0
+
     def describe(self):
         return []
 
@@ -81,6 +105,13 @@ class CacheMetric:
                 cache_total.labels(self._cache_name).set(self.hits + self.misses)
                 if getattr(self._cache, "max_size", None):
                     cache_max_size.labels(self._cache_name).set(self._cache.max_size)
+
+                if TRACK_MEMORY_USAGE:
+                    # self.memory_usage can be None if nothing has been inserted
+                    # into the cache yet.
+                    cache_memory_usage.labels(self._cache_name).set(
+                        self.memory_usage or 0
+                    )
             if self._collect_callback:
                 self._collect_callback()
         except Exception as e:
diff --git a/synapse/util/caches/lrucache.py b/synapse/util/caches/lrucache.py

index 10b0ec6b75..1be675e014 100644
--- a/synapse/util/caches/lrucache.py
+++ b/synapse/util/caches/lrucache.py
@@ -32,9 +32,36 @@ from typing import (
 from typing_extensions import Literal
 
 from synapse.config import cache as cache_config
+from synapse.util import caches
 from synapse.util.caches import CacheMetric, register_cache
 from synapse.util.caches.treecache import TreeCache
 
+try:
+    from pympler.asizeof import Asizer
+
+    def _get_size_of(val: Any, *, recurse=True) -> int:
+        """Get an estimate of the size in bytes of the object.
+
+        Args:
+            val: The object to size.
+            recurse: If true will include referenced values in the size,
+                otherwise only sizes the given object.
+        """
+        # Ignore singleton values when calculating memory usage.
+        if val in ((), None, ""):
+            return 0
+
+        sizer = Asizer()
+        sizer.exclude_refs((), None, "")
+        return sizer.asizeof(val, limit=100 if recurse else 0)
+
+
+except ImportError:
+
+    def _get_size_of(val: Any, *, recurse=True) -> int:
+        return 0
+
+
 # Function type: the type used for invalidation callbacks
 FT = TypeVar("FT", bound=Callable[..., Any])
 
@@ -56,7 +83,7 @@ def enumerate_leaves(node, depth):
 
 
 class _Node:
-    __slots__ = ["prev_node", "next_node", "key", "value", "callbacks"]
+    __slots__ = ["prev_node", "next_node", "key", "value", "callbacks", "memory"]
 
     def __init__(
         self,
@@ -84,6 +111,16 @@ class _Node:
 
         self.add_callbacks(callbacks)
 
+        self.memory = 0
+        if caches.TRACK_MEMORY_USAGE:
+            self.memory = (
+                _get_size_of(key)
+                + _get_size_of(value)
+                + _get_size_of(self.callbacks, recurse=False)
+                + _get_size_of(self, recurse=False)
+            )
+            self.memory += _get_size_of(self.memory, recurse=False)
+
     def add_callbacks(self, callbacks: Collection[Callable[[], None]]) -> None:
         """Add to stored list of callbacks, removing duplicates."""
 
@@ -233,6 +270,9 @@ class LruCache(Generic[KT, VT]):
             if size_callback:
                 cached_cache_len[0] += size_callback(node.value)
 
+            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
@@ -258,6 +298,9 @@ class LruCache(Generic[KT, VT]):
 
             node.run_and_clear_callbacks()
 
+            if caches.TRACK_MEMORY_USAGE and metrics:
+                metrics.dec_memory_usage(node.memory)
+
             return deleted_len
 
         @overload
@@ -373,6 +416,9 @@ class LruCache(Generic[KT, VT]):
             if size_callback:
                 cached_cache_len[0] = 0
 
+            if caches.TRACK_MEMORY_USAGE and metrics:
+                metrics.clear_memory_usage()
+
         @synchronized
         def cache_contains(key: KT) -> bool:
             return key in cache