diff --git a/synapse/metrics/_gc.py b/synapse/metrics/_gc.py
new file mode 100644
index 0000000000..2bc909efa0
--- /dev/null
+++ b/synapse/metrics/_gc.py
@@ -0,0 +1,203 @@
+# Copyright 2015-2022 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.
+
+
+import gc
+import logging
+import platform
+import time
+from typing import Iterable
+
+from prometheus_client.core import (
+ REGISTRY,
+ CounterMetricFamily,
+ Gauge,
+ GaugeMetricFamily,
+ Histogram,
+ Metric,
+)
+
+from twisted.internet import task
+
+"""Prometheus metrics for garbage collection"""
+
+
+logger = logging.getLogger(__name__)
+
+# The minimum time in seconds between GCs for each generation, regardless of the current GC
+# thresholds and counts.
+MIN_TIME_BETWEEN_GCS = (1.0, 10.0, 30.0)
+
+running_on_pypy = platform.python_implementation() == "PyPy"
+
+#
+# Python GC metrics
+#
+
+gc_unreachable = Gauge("python_gc_unreachable_total", "Unreachable GC objects", ["gen"])
+gc_time = Histogram(
+ "python_gc_time",
+ "Time taken to GC (sec)",
+ ["gen"],
+ buckets=[
+ 0.0025,
+ 0.005,
+ 0.01,
+ 0.025,
+ 0.05,
+ 0.10,
+ 0.25,
+ 0.50,
+ 1.00,
+ 2.50,
+ 5.00,
+ 7.50,
+ 15.00,
+ 30.00,
+ 45.00,
+ 60.00,
+ ],
+)
+
+
+class GCCounts:
+ def collect(self) -> Iterable[Metric]:
+ cm = GaugeMetricFamily("python_gc_counts", "GC object counts", labels=["gen"])
+ for n, m in enumerate(gc.get_count()):
+ cm.add_metric([str(n)], m)
+
+ yield cm
+
+
+def install_gc_manager() -> None:
+ """Disable automatic GC, and replace it with a task that runs every 100ms
+
+ This means that (a) we can limit how often GC runs; (b) we can get some metrics
+ about GC activity.
+
+ It does nothing on PyPy.
+ """
+
+ if running_on_pypy:
+ return
+
+ REGISTRY.register(GCCounts())
+
+ gc.disable()
+
+ # The time (in seconds since the epoch) of the last time we did a GC for each generation.
+ _last_gc = [0.0, 0.0, 0.0]
+
+ def _maybe_gc() -> None:
+ # Check if we need to do a manual GC (since its been disabled), and do
+ # one if necessary. Note we go in reverse order as e.g. a gen 1 GC may
+ # promote an object into gen 2, and we don't want to handle the same
+ # object multiple times.
+ threshold = gc.get_threshold()
+ counts = gc.get_count()
+ end = time.time()
+ for i in (2, 1, 0):
+ # We check if we need to do one based on a straightforward
+ # comparison between the threshold and count. We also do an extra
+ # check to make sure that we don't a GC too often.
+ if threshold[i] < counts[i] and MIN_TIME_BETWEEN_GCS[i] < end - _last_gc[i]:
+ if i == 0:
+ logger.debug("Collecting gc %d", i)
+ else:
+ logger.info("Collecting gc %d", i)
+
+ start = time.time()
+ unreachable = gc.collect(i)
+ end = time.time()
+
+ _last_gc[i] = end
+
+ gc_time.labels(i).observe(end - start)
+ gc_unreachable.labels(i).set(unreachable)
+
+ gc_task = task.LoopingCall(_maybe_gc)
+ gc_task.start(0.1)
+
+
+#
+# PyPy GC / memory metrics
+#
+
+
+class PyPyGCStats:
+ def collect(self) -> Iterable[Metric]:
+
+ # @stats is a pretty-printer object with __str__() returning a nice table,
+ # plus some fields that contain data from that table.
+ # unfortunately, fields are pretty-printed themselves (i. e. '4.5MB').
+ stats = gc.get_stats(memory_pressure=False) # type: ignore
+ # @s contains same fields as @stats, but as actual integers.
+ s = stats._s # type: ignore
+
+ # also note that field naming is completely braindead
+ # and only vaguely correlates with the pretty-printed table.
+ # >>>> gc.get_stats(False)
+ # Total memory consumed:
+ # GC used: 8.7MB (peak: 39.0MB) # s.total_gc_memory, s.peak_memory
+ # in arenas: 3.0MB # s.total_arena_memory
+ # rawmalloced: 1.7MB # s.total_rawmalloced_memory
+ # nursery: 4.0MB # s.nursery_size
+ # raw assembler used: 31.0kB # s.jit_backend_used
+ # -----------------------------
+ # Total: 8.8MB # stats.memory_used_sum
+ #
+ # Total memory allocated:
+ # GC allocated: 38.7MB (peak: 41.1MB) # s.total_allocated_memory, s.peak_allocated_memory
+ # in arenas: 30.9MB # s.peak_arena_memory
+ # rawmalloced: 4.1MB # s.peak_rawmalloced_memory
+ # nursery: 4.0MB # s.nursery_size
+ # raw assembler allocated: 1.0MB # s.jit_backend_allocated
+ # -----------------------------
+ # Total: 39.7MB # stats.memory_allocated_sum
+ #
+ # Total time spent in GC: 0.073 # s.total_gc_time
+
+ pypy_gc_time = CounterMetricFamily(
+ "pypy_gc_time_seconds_total",
+ "Total time spent in PyPy GC",
+ labels=[],
+ )
+ pypy_gc_time.add_metric([], s.total_gc_time / 1000)
+ yield pypy_gc_time
+
+ pypy_mem = GaugeMetricFamily(
+ "pypy_memory_bytes",
+ "Memory tracked by PyPy allocator",
+ labels=["state", "class", "kind"],
+ )
+ # memory used by JIT assembler
+ pypy_mem.add_metric(["used", "", "jit"], s.jit_backend_used)
+ pypy_mem.add_metric(["allocated", "", "jit"], s.jit_backend_allocated)
+ # memory used by GCed objects
+ pypy_mem.add_metric(["used", "", "arenas"], s.total_arena_memory)
+ pypy_mem.add_metric(["allocated", "", "arenas"], s.peak_arena_memory)
+ pypy_mem.add_metric(["used", "", "rawmalloced"], s.total_rawmalloced_memory)
+ pypy_mem.add_metric(["allocated", "", "rawmalloced"], s.peak_rawmalloced_memory)
+ pypy_mem.add_metric(["used", "", "nursery"], s.nursery_size)
+ pypy_mem.add_metric(["allocated", "", "nursery"], s.nursery_size)
+ # totals
+ pypy_mem.add_metric(["used", "totals", "gc"], s.total_gc_memory)
+ pypy_mem.add_metric(["allocated", "totals", "gc"], s.total_allocated_memory)
+ pypy_mem.add_metric(["used", "totals", "gc_peak"], s.peak_memory)
+ pypy_mem.add_metric(["allocated", "totals", "gc_peak"], s.peak_allocated_memory)
+ yield pypy_mem
+
+
+if running_on_pypy:
+ REGISTRY.register(PyPyGCStats())
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