Make inflight background metrics more efficient. (#7597)

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>
This commit is contained in:
Erik Johnston 2020-05-29 13:25:32 +01:00 committed by GitHub
parent 47db2c3673
commit f5353eff21
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2 changed files with 71 additions and 34 deletions

1
changelog.d/7597.bugfix Normal file
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@ -0,0 +1 @@
Fix metrics failing when there is a large number of active background processes.

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@ -17,16 +17,18 @@ import logging
import threading
from asyncio import iscoroutine
from functools import wraps
from typing import Dict, Set
from typing import TYPE_CHECKING, Dict, Optional, Set
import six
from prometheus_client.core import REGISTRY, Counter, GaugeMetricFamily
from prometheus_client.core import REGISTRY, Counter, Gauge
from twisted.internet import defer
from synapse.logging.context import LoggingContext, PreserveLoggingContext
if TYPE_CHECKING:
import resource
logger = logging.getLogger(__name__)
@ -36,6 +38,12 @@ _background_process_start_count = Counter(
["name"],
)
_background_process_in_flight_count = Gauge(
"synapse_background_process_in_flight_count",
"Number of background processes in flight",
labelnames=["name"],
)
# we set registry=None in all of these to stop them getting registered with
# the default registry. Instead we collect them all via the CustomCollector,
# which ensures that we can update them before they are collected.
@ -83,13 +91,17 @@ _background_process_db_sched_duration = Counter(
# it's much simpler to do so than to try to combine them.)
_background_process_counts = {} # type: Dict[str, int]
# map from description to the currently running background processes.
# Set of all running background processes that became active active since the
# last time metrics were scraped (i.e. background processes that performed some
# work since the last scrape.)
#
# it's kept as a dict of sets rather than a big set so that we can keep track
# of process descriptions that no longer have any active processes.
_background_processes = {} # type: Dict[str, Set[_BackgroundProcess]]
# We do it like this to handle the case where we have a large number of
# background processes stacking up behind a lock or linearizer, where we then
# only need to iterate over and update metrics for the process that have
# actually been active and can ignore the idle ones.
_background_processes_active_since_last_scrape = set() # type: Set[_BackgroundProcess]
# A lock that covers the above dicts
# A lock that covers the above set and dict
_bg_metrics_lock = threading.Lock()
@ -101,26 +113,17 @@ class _Collector(object):
"""
def collect(self):
background_process_in_flight_count = GaugeMetricFamily(
"synapse_background_process_in_flight_count",
"Number of background processes in flight",
labels=["name"],
)
global _background_processes_active_since_last_scrape
# We copy the dict so that it doesn't change from underneath us.
# We also copy the process lists as that can also change
# We swap out the _background_processes set with an empty one so that
# we can safely iterate over the set without holding the lock.
with _bg_metrics_lock:
_background_processes_copy = {
k: list(v) for k, v in six.iteritems(_background_processes)
}
_background_processes_copy = _background_processes_active_since_last_scrape
_background_processes_active_since_last_scrape = set()
for desc, processes in six.iteritems(_background_processes_copy):
background_process_in_flight_count.add_metric((desc,), len(processes))
for process in processes:
for process in _background_processes_copy:
process.update_metrics()
yield background_process_in_flight_count
# now we need to run collect() over each of the static Counters, and
# yield each metric they return.
for m in (
@ -191,13 +194,10 @@ def run_as_background_process(desc, func, *args, **kwargs):
_background_process_counts[desc] = count + 1
_background_process_start_count.labels(desc).inc()
_background_process_in_flight_count.labels(desc).inc()
with LoggingContext(desc) as context:
with BackgroundProcessLoggingContext(desc) as context:
context.request = "%s-%i" % (desc, count)
proc = _BackgroundProcess(desc, context)
with _bg_metrics_lock:
_background_processes.setdefault(desc, set()).add(proc)
try:
result = func(*args, **kwargs)
@ -214,10 +214,7 @@ def run_as_background_process(desc, func, *args, **kwargs):
except Exception:
logger.exception("Background process '%s' threw an exception", desc)
finally:
proc.update_metrics()
with _bg_metrics_lock:
_background_processes[desc].remove(proc)
_background_process_in_flight_count.labels(desc).dec()
with PreserveLoggingContext():
return run()
@ -238,3 +235,42 @@ def wrap_as_background_process(desc):
return wrap_as_background_process_inner_2
return wrap_as_background_process_inner
class BackgroundProcessLoggingContext(LoggingContext):
"""A logging context that tracks in flight metrics for background
processes.
"""
__slots__ = ["_proc"]
def __init__(self, name: str):
super().__init__(name)
self._proc = _BackgroundProcess(name, self)
def start(self, rusage: "Optional[resource._RUsage]"):
"""Log context has started running (again).
"""
super().start(rusage)
# We've become active again so we make sure we're in the list of active
# procs. (Note that "start" here means we've become active, as opposed
# to starting for the first time.)
with _bg_metrics_lock:
_background_processes_active_since_last_scrape.add(self._proc)
def __exit__(self, type, value, traceback) -> None:
"""Log context has finished.
"""
super().__exit__(type, value, traceback)
# The background process has finished. We explictly remove and manually
# update the metrics here so that if nothing is scraping metrics the set
# doesn't infinitely grow.
with _bg_metrics_lock:
_background_processes_active_since_last_scrape.discard(self._proc)
self._proc.update_metrics()