Simplify GC prometheus metrics (#11723)

Rather than hooking into the reactor loop, just add a timed task that runs every 100 ms to do the garbage collection.

Part 1 of a quest to simplify the reactor monkey-patching.

4 files changed, 209 insertions, 160 deletions

diff --git a/changelog.d/11723.misc b/changelog.d/11723.misc
new file mode 100644
index 0000000000..f99e02070a
--- /dev/null
+++ b/changelog.d/11723.misc
@@ -0,0 +1 @@
+Simplify calculation of prometheus metrics for garbage collection.
diff --git a/synapse/app/_base.py b/synapse/app/_base.py
index 5fc59c1be1..579adbbca0 100644
--- a/synapse/app/_base.py
+++ b/synapse/app/_base.py
@@ -60,7 +60,7 @@ from synapse.events.spamcheck import load_legacy_spam_checkers
 from synapse.events.third_party_rules import load_legacy_third_party_event_rules
 from synapse.handlers.auth import load_legacy_password_auth_providers
 from synapse.logging.context import PreserveLoggingContext
-from synapse.metrics import register_threadpool
+from synapse.metrics import install_gc_manager, register_threadpool
 from synapse.metrics.background_process_metrics import wrap_as_background_process
 from synapse.metrics.jemalloc import setup_jemalloc_stats
 from synapse.types import ISynapseReactor
@@ -159,6 +159,7 @@ def start_reactor(
         change_resource_limit(soft_file_limit)
         if gc_thresholds:
             gc.set_threshold(*gc_thresholds)
+        install_gc_manager()
         run_command()
 
     # make sure that we run the reactor with the sentinel log context,
diff --git a/synapse/metrics/__init__.py b/synapse/metrics/__init__.py
index 269c2b989e..ba7ca0f2d4 100644
--- a/synapse/metrics/__init__.py
+++ b/synapse/metrics/__init__.py
@@ -13,7 +13,6 @@
 # limitations under the License.
 
 import functools
-import gc
 import itertools
 import logging
 import os
@@ -41,7 +40,6 @@ import attr
 from prometheus_client import CollectorRegistry, Counter, Gauge, Histogram, Metric
 from prometheus_client.core import (
     REGISTRY,
-    CounterMetricFamily,
     GaugeHistogramMetricFamily,
     GaugeMetricFamily,
 )
@@ -56,13 +54,13 @@ from synapse.metrics._exposition import (
     generate_latest,
     start_http_server,
 )
+from synapse.metrics._gc import MIN_TIME_BETWEEN_GCS, install_gc_manager
 from synapse.util.versionstring import get_version_string
 
 logger = logging.getLogger(__name__)
 
 METRICS_PREFIX = "/_synapse/metrics"
 
-running_on_pypy = platform.python_implementation() == "PyPy"
 all_gauges: "Dict[str, Union[LaterGauge, InFlightGauge]]" = {}
 
 HAVE_PROC_SELF_STAT = os.path.exists("/proc/self/stat")
@@ -369,121 +367,6 @@ class CPUMetrics:
 
 REGISTRY.register(CPUMetrics())
 
-#
-# 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
-
-
-if not running_on_pypy:
-    REGISTRY.register(GCCounts())
-
-
-#
-# 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())
-
 
 #
 # Twisted reactor metrics
@@ -612,14 +495,6 @@ class ReactorLastSeenMetric:
 
 REGISTRY.register(ReactorLastSeenMetric())
 
-# 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)
-
-# 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]
-
-
 F = TypeVar("F", bound=Callable[..., Any])
 
 
@@ -658,34 +533,6 @@ def runUntilCurrentTimer(reactor: ReactorBase, func: F) -> F:
         global last_ticked
         last_ticked = end
 
-        if running_on_pypy:
-            return ret
-
-        # 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()
-        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)
-
         return ret
 
     return cast(F, f)
@@ -701,11 +548,6 @@ try:
     # runUntilCurrent is called when we have pending calls. It is called once
     # per iteratation after fd polling.
     reactor.runUntilCurrent = runUntilCurrentTimer(reactor, reactor.runUntilCurrent)  # type: ignore
-
-    # We manually run the GC each reactor tick so that we can get some metrics
-    # about time spent doing GC,
-    if not running_on_pypy:
-        gc.disable()
 except AttributeError:
     pass
 
@@ -717,4 +559,6 @@ __all__ = [
     "LaterGauge",
     "InFlightGauge",
     "GaugeBucketCollector",
+    "MIN_TIME_BETWEEN_GCS",
+    "install_gc_manager",
 ]
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())