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perf(metrics): record cache operations into per-thread cells (LAB-7117) - #661
Every backed cache operation records three Prometheus series and counts one operation for its collector. On a free-threaded build those writes serialised every thread on one function: record_cache_operation did an unlocked shared self._operation_count += 1 (which also loses updates with the GIL off), then either took the series' value locks (sync mode) or the pool lock and the queue mutex (batched mode).
Each thread now writes only state of its own. Scrapes, get_stats() and the mode switch still read exact totals at once, with no flush and no lag.
How
Per-thread cells. A threading.local holds each thread's _ThreadState: one cell (a list of floats) per series it records into, and one operation count per collector. record_cache_operation adds to them and takes no lock, queues nothing and writes no shared attribute. The first record of a label tuple on a thread binds its cells (_bind), which is the only locked step.
Values that sum the cells. The bound Prometheus children keep their names, labels and buckets. Their _value, _sum and _buckets become _ShardViews that return the live cells plus a retired total, read under one per-PID lock that no record takes. So registry collects, get_sample_value and direct ._value.get() reads all see every operation recorded before them. The child's own inc(), observe() and reset() still work.
Thread exit. Only the threading.local refers to a _ThreadState, so it is freed when its thread exits. __del__ then moves its counts into the retired totals and drops its cells in one locked step. Counts survive the exit, reads never go down, and cells stay bounded to the live threads.
Exact sums. Retired values are kept as exact partials (Shewchuk) and read with math.fsum. A plain float total would round differently once a cell joined it and could read one ulp lower than the previous scrape. rate() would treat that as a counter reset.
Operation count._operation_count is now a property that sums every thread's count for the collector, plus the counts of threads that exited. get_stats() and _maybe_switch_mode read the sum. The setter is kept, so code that assigns the count still works.
Dropped collectors. A collector's counts are keyed by a per-collector token. A weakref.finalize removes the token from every thread's counts and from the exited-thread totals when the collector is collected, so building and dropping collectors leaks nothing, as before.
Reentrancy. Python code runs under the new lock, so a garbage collection there can run a finalizer on the same thread, and that finalizer may record a cache operation, read a series or drop a collector. The lock is therefore an RLock, and every holder iterates snapshots, so a nested call can neither deadlock nor change a dict under an outer loop.
Fork. The at-fork hooks hold the new lock across os.fork, and the child gets a fresh one, as _metrics_cache_lock does. A forked child starts from the counts it inherited. A child forked from C gets a fresh lock by PID, and _bind runs _reset_metric_locks_once before it touches prometheus_client locks.
Multiprocess mode. With PROMETHEUS_MULTIPROC_DIR set, a scrape reads each process's mmap files, not in-process values. In that mode cache operations record into the series directly, as before.
Batched mode. Cache operations never queue now, so the batched cache-operation path (_record_cache_operation_async and its _flush_batch branch) is deleted. Batching still covers record_counter and record_histogram, and the mode switch is unchanged.
Behaviour change: a cache operation with a malformed duration (one that cannot be compared with 0) now raises TypeError to the caller in batched mode too, as it always did in sync mode. cachekit's own call sites always pass floats.
Measurements
Free-threaded scaling (CPython 3.15.1t, GIL off, l1hit cell of tests/performance/ft_scaling_bench.py). 22 interleaved ABBA blocks compared a venv holding this branch with one holding main. A second set of 22 blocks, interleaved with the first, compared main against a second main venv (A/A). The 95% CIs resample whole blocks.
threads
this branch vs main, calls/s
95% CI
A/A (95% CI)
1
+17.1%
+12.2% to +23.1%
+0.3% (−3.0% to +3.7%)
4
+34.2%
+30.3% to +38.7%
+0.0% (−2.2% to +2.3%)
16
+52.3%
+43.9% to +61.0%
+4.2% (−1.7% to +10.3%)
Each figure is the mean over blocks of the block's ratio. At 4 threads the A/A difference is under 0.1 points, against 34.2.
Instruction counts (tests/performance/ir_budget.py --base against main, CPython 3.14.3): l2_hit −7.9%, l2_hit_async_metrics −3.1%, miss −4.1%, secure_l1_hit −9.6%. l1_hit and minimal_l1_hit record no metric and stay within 0.1%. Editing ir_workload.py moves every measured heap layout, so all budgets are re-recorded on 3.12.12 and 3.14.3 (--update --allow-increase). Apart from the metric-recording paths, every path moves by 0.4% or less.
Tests
New tests in tests/unit/test_async_metrics_per_thread.py:
16 threads × 250 records: a scrape taken while every thread is still alive returns exactly 4,000 for the counter and both histograms' counts, with exact sums, in sync and batched mode. get_stats()["total_operations"] matches.
Bucket placement matches a reference prometheus_client.Histogram fed the same values, including bounds, values just past a bound, values past the last bound, and inf.
get_stats() keeps its keys and sums every thread. The mode switch goes batched on the summed rate when no single thread's rate would trigger it.
After warm-up, a record calls no lock helper, no metric registration, no queue put and no pool, and leaves the collector's attributes unchanged.
16 threads record while new ones start and exit, and two readers scrape and call get_stats() throughout. No read ever goes down.
An exited thread's counts survive and its cells go. 10,000 thread lifetimes leave one live cell per series.
Retiring cells never changes a total, over randomised float sums.
500 collectors built, used on two threads and dropped leave no token behind, while the series keep every operation.
A thread that holds the lock, as a finalizer run by a collection inside a scrape would, can still bind a new label tuple, drop a collector, call get_stats() and read a series.
inc() and reset() on the child still reach the series. Multiprocess mode records directly.
A forked child starts from its parent's counts, including those of a parent thread that is still live, and its records stay in the child. A fork waits for a thread that is mid-read under the lock, and the child does not hang.
Each test was checked against a mutant: a float retired total, no cell retirement on exit, an operation count that sums only the calling thread, a retirement that does not deregister, no rebind on a detached series, a plain Lock, and no collector finalizer. Every mutant is killed. The snapshot copies have no test of their own, because a finalizer would have to fire mid-iteration.
Existing tests that pinned the removed batched cache-operation path or the replaced value locks now pin the new contract: test_async_metrics_mode_switch.py, test_single_metrics_record.py, test_async_metrics_histogram_buckets.py and test_ir_budget.py. The fork lock tests hold the new series lock in the child of a fork made from C. A Python fork cannot inherit that lock held, because its at-fork hook takes it.
Results:
CPython 3.14.3: pytest tests --ignore=tests/fuzzing -m "not slow" gives 6,747 passed. The only failures are Redis-backed tests that need redis-server, which this environment lacks.
3.14.7t and 3.15.1t with the GIL off: the metrics tests pass. The per-thread, mode-switch and telemetry tests passed 20 runs in a row on 3.14t.
ruff check, ruff format --check and basedpyright report nothing new.
Docs
docs/free-threading.md: a concurrency-audit row for the per-thread cells. The mode-switch row notes that cache operations never queue.
docs/features/prometheus-metrics.md: how recording works, why a scrape is exact, and what multiprocess mode does.
docs/api-reference.md: the AsyncMetricsCollector description, which claimed queue-based collection with "zero impact on critical path latency".
docs/performance.md: the batched-mode note, re-recorded budgets, and the batched-vs-sync instruction comparison, which no longer holds.
Reviewing files that changed from the base of the PR and between 4d7514c and d0eb05f.
Files selected for processing: 3
docs/free-threading.md
src/cachekit/reliability/async_metrics.py
tests/unit/test_async_metrics_per_thread.py
Summary by CodeRabbit
Performance
Cache-operation metrics are now recorded independently per thread in Prometheus in-process mode, avoiding inter-thread lock waits during recording. Scrapes include operations recorded before the scrape.
Cache-operation metrics no longer use the background batching queue. Generic counters and histograms continue to support synchronous or batched recording.
Documentation
Updated metrics and performance guidance to reflect recording behaviour, scrape totals and revised benchmark results.
Walkthrough
Cache-operation metrics now use per-thread cells in in-process Prometheus mode and aggregate at scrape time. Generic counters and histograms retain synchronous or batched recording. The changes also update fork handling, tests, performance budgets and documentation.
Changes
Cache-operation metrics
Layer / File(s)
Summary
Per-thread cells and Prometheus aggregation src/cachekit/reliability/async_metrics.py, tests/unit/test_async_metrics_per_thread.py, tests/unit/test_hot_path_telemetry.py, docs/features/prometheus-metrics.md
Cache-operation metrics use per-thread cells in supported in-process mode. Shard-backed views aggregate live cells and retired totals. Tests cover recording, aggregation, cell retirement and Prometheus value types.
Collector recording and batch processing src/cachekit/reliability/async_metrics.py, tests/unit/test_async_metrics_histogram_buckets.py, tests/unit/test_async_metrics_mode_switch.py, tests/unit/test_single_metrics_record.py, docs/api-reference.md
Cache operations no longer use the generic-metric queue. Generic counters and histograms retain synchronous or batched recording. The collector aggregates operation counts across threads.
Multiprocess and fork handling src/cachekit/reliability/async_metrics.py, tests/unit/test_async_metrics_mode_switch.py, tests/unit/test_async_metrics_per_thread.py, docs/free-threading.md
Fork hooks coordinate metric-cache and shard locks. Tests cover inherited counts and recording in forked processes.
Performance budgets and workload checks docs/performance.md, tests/performance/*, tests/unit/test_ir_budget.py
The instruction budgets and performance comparison are revised. The batched workload checks that cache operations do not queue records.
sequenceDiagram
participant RecordingThread
participant AsyncMetricsCollector
participant _Shards
participant PrometheusScrape
RecordingThread->>AsyncMetricsCollector: record_cache_operation(...)
AsyncMetricsCollector->>_Shards: update the thread cell
PrometheusScrape->>_Shards: read live and retired totals
_Shards-->>PrometheusScrape: return summed metric values
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Suggested reviewers: kodus-27b
Merge Risk: Low · previous review 4d751
The change moves cache-operation metrics to per-thread cells, and the core design looks sound. An infinite duration recorded on a short-lived thread would make the duration sum read NaN permanently. One budget unit test also fails when PROMETHEUS_MULTIPROC_DIR is set. Both are small, localized fixes worth making before or soon after merge.
Pre-merge checks: 4 · 1
Failed checks: 1 warning
Check name
Status
Explanation
Resolution
Docstring Coverage
Docstring coverage is 63.27% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documenta…
Write docstrings for the functions missing them to satisfy the coverage threshold.
Passed checks: 4 passed
Check name
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Explanation
Title check
The title clearly and concisely identifies the main performance change: recording cache operations into per-thread cells.
Description check
The description is detailed and relevant. It explains the motivation, implementation, behaviour change, performance results, tests, documentation updates, and compatibility impact. It does not reprodu…
Linked Issues check
Check skipped because no linked issues were found for this pull request.
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Check skipped because no linked issues were found for this pull request.
Full details: Description check
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The description is detailed and relevant. It explains the motivation, implementation, behaviour change, performance results, tests, documentation updates, and compatibility impact. It does not reproduce every template heading or checklist item, but the required review information is substantially present.
Full details: Docstring Coverage
Explanation
Docstring coverage is 63.27% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 49 functions across 8 files. (5 skipped: 5 unsupported.)
Resolution
Write docstrings for the functions missing them to satisfy the coverage threshold.
Finishing Touches: 💡 1
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Kody Code Review — 1 suggested fix.
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A code review identified the following issues in this pull request.
Each section describes what was found and includes a reference implementation where available.
Files involved:
- src/cachekit/reliability/async_metrics.py:497
---
### [1/1] src/cachekit/reliability/async_metrics.py:497
Issue identified during code review:
Unbounded memory growth in record_cache_operation and _ThreadState.__del__: each collector's token is added to every recording thread's `ops` dict and copied into the module-level `_retired_ops` when that thread exits, and nothing removes these entries after the collector is garbage-collected. Before this change, the count was a field on the collector and was freed with it. FeatureOrchestrator builds a new AsyncMetricsCollector for every decorated function (orchestrator.py:88), so an app that decorates closures or per-request wrappers keeps a token object and a dict entry for every collector it ever created, in every long-lived thread's `ops` and in `_retired_ops`. Fix: register `weakref.finalize(self, _forget_token, self._token)` in `__init__` to pop the token from `_retired_ops` and from each `_thread_ops` dict under `_shard_lock()`, or keep `_retired_ops` on the collector instance instead.
Reference implementation (from code review):
// src/cachekit/reliability/async_metrics.py:497
self._token = object()
self._ops_offset = 0
weakref.finalize(self, _forget_token, self._token)
def _forget_token(token):
with _shard_lock():
_retired_ops.pop(token, None)
for ops in _thread_ops.values():
ops.pop(token, None)
---
Review each issue in context, use the reference implementations as guidance, and apply fixes that are consistent with the surrounding codebase.
Head d0eb05f38c0b9900862b2c09e6df4abb13f3372a measured against base 99047e11c6ee4d706003bf5fc09e3ca192ce62c3 on the same runner, on cpython-3.12-x86_64 (3.12.3). Workflow run
Verdict
Path
Head Ir/op
Base Ir/op
Delta
WARN
serializer_arrow
2,219,823
2,212,045
+0.35%
WARN
serializer_default
64,492
64,235
+0.40%
LOWER
l2_hit
195,473
215,934
-9.48%
LOWER
l2_hit_async_metrics
195,532
204,043
-4.17%
LOWER
miss
240,326
252,891
-4.97%
LOWER
secure_l1_hit
156,066
176,936
-11.80%
ok
byte_storage_store_40b
2,756
2,755
+0.04%
ok
byte_storage_store_64k
724,751
724,753
-0.00%
ok
file_set
86,895
86,904
-0.01%
ok
l1_hit
13,130
13,128
+0.02%
ok
minimal_l1_hit
11,986
11,985
+0.01%
ok
object_cache_put_full
198,462
198,212
+0.13%
ok
serializer_auto
72,527
72,429
+0.14%
ok
serializer_default_records
1,482,186
1,482,662
-0.03%
ok
serializer_encrypted
109,193
109,122
+0.07%
ok
serializer_orjson
24,878
24,885
-0.03%
A path fails at +1% or more against the base (+2% for serializer_orjson). This check is not required: a deliberate cost increase is accepted at merge. Method: docs/performance.md, Instruction budgets.
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Actionable comments posted: 2
🪄 Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @src/cachekit/reliability/async_metrics.py:
- Around line 372-382: Update _add_exact to use the exact partials loop only
when x and all existing partials are finite; otherwise collapse them with
math.fsum, mapping its ValueError for opposing infinities to math.nan. Add a
regression test that records math.inf on a short-lived thread and verifies the
_sum sample remains inf after the thread exits.
Review comments at @tests/unit/test_ir_budget.py:
- Line 444: Update the test containing the async_metrics._PER_THREAD assertion
to skip before building its workload when multiprocess mode is enabled,
following the existing per-thread metrics tests’ skip pattern; retain the
assertion for single-process mode.
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Stale: d0eb05f fixes the finding this review raised (_add_exact now keeps an infinite sum infinite, with a regression test). The other finding (tests/unit/test_ir_budget.py) was replied to in its thread. Every CodeRabbit thread is resolved, and CodeRabbit's review of d0eb05f raised nothing new. Dismissal is not approval; a human reviews and merges.
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Summary
Every backed cache operation records three Prometheus series and counts one operation for its collector. On a free-threaded build those writes serialised every thread on one function:
record_cache_operationdid an unlocked sharedself._operation_count += 1(which also loses updates with the GIL off), then either took the series' value locks (sync mode) or the pool lock and the queue mutex (batched mode).Each thread now writes only state of its own. Scrapes,
get_stats()and the mode switch still read exact totals at once, with no flush and no lag.How
threading.localholds each thread's_ThreadState: one cell (a list of floats) per series it records into, and one operation count per collector.record_cache_operationadds to them and takes no lock, queues nothing and writes no shared attribute. The first record of a label tuple on a thread binds its cells (_bind), which is the only locked step._value,_sumand_bucketsbecome_ShardViews that return the live cells plus a retired total, read under one per-PID lock that no record takes. So registry collects,get_sample_valueand direct._value.get()reads all see every operation recorded before them. The child's owninc(),observe()andreset()still work.threading.localrefers to a_ThreadState, so it is freed when its thread exits.__del__then moves its counts into the retired totals and drops its cells in one locked step. Counts survive the exit, reads never go down, and cells stay bounded to the live threads.math.fsum. A plain float total would round differently once a cell joined it and could read one ulp lower than the previous scrape.rate()would treat that as a counter reset._operation_countis now a property that sums every thread's count for the collector, plus the counts of threads that exited.get_stats()and_maybe_switch_moderead the sum. The setter is kept, so code that assigns the count still works.weakref.finalizeremoves the token from every thread's counts and from the exited-thread totals when the collector is collected, so building and dropping collectors leaks nothing, as before.RLock, and every holder iterates snapshots, so a nested call can neither deadlock nor change a dict under an outer loop.os.fork, and the child gets a fresh one, as_metrics_cache_lockdoes. A forked child starts from the counts it inherited. A child forked from C gets a fresh lock by PID, and_bindruns_reset_metric_locks_oncebefore it touches prometheus_client locks.PROMETHEUS_MULTIPROC_DIRset, a scrape reads each process's mmap files, not in-process values. In that mode cache operations record into the series directly, as before._record_cache_operation_asyncand its_flush_batchbranch) is deleted. Batching still coversrecord_counterandrecord_histogram, and the mode switch is unchanged.Behaviour change: a cache operation with a malformed duration (one that cannot be compared with
0) now raisesTypeErrorto the caller in batched mode too, as it always did in sync mode. cachekit's own call sites always pass floats.Measurements
Free-threaded scaling (CPython 3.15.1t, GIL off,
l1hitcell oftests/performance/ft_scaling_bench.py). 22 interleaved ABBA blocks compared a venv holding this branch with one holdingmain. A second set of 22 blocks, interleaved with the first, comparedmainagainst a secondmainvenv (A/A). The 95% CIs resample whole blocks.main, calls/sEach figure is the mean over blocks of the block's ratio. At 4 threads the A/A difference is under 0.1 points, against 34.2.
Instruction counts (
tests/performance/ir_budget.py --baseagainstmain, CPython 3.14.3):l2_hit−7.9%,l2_hit_async_metrics−3.1%,miss−4.1%,secure_l1_hit−9.6%.l1_hitandminimal_l1_hitrecord no metric and stay within 0.1%. Editingir_workload.pymoves every measured heap layout, so all budgets are re-recorded on 3.12.12 and 3.14.3 (--update --allow-increase). Apart from the metric-recording paths, every path moves by 0.4% or less.Tests
New tests in
tests/unit/test_async_metrics_per_thread.py:get_stats()["total_operations"]matches.prometheus_client.Histogramfed the same values, including bounds, values just past a bound, values past the last bound, andinf.get_stats()keeps its keys and sums every thread. The mode switch goes batched on the summed rate when no single thread's rate would trigger it.get_stats()throughout. No read ever goes down.get_stats()and read a series.inc()andreset()on the child still reach the series. Multiprocess mode records directly.Each test was checked against a mutant: a float retired total, no cell retirement on exit, an operation count that sums only the calling thread, a retirement that does not deregister, no rebind on a detached series, a plain
Lock, and no collector finalizer. Every mutant is killed. The snapshot copies have no test of their own, because a finalizer would have to fire mid-iteration.Existing tests that pinned the removed batched cache-operation path or the replaced value locks now pin the new contract:
test_async_metrics_mode_switch.py,test_single_metrics_record.py,test_async_metrics_histogram_buckets.pyandtest_ir_budget.py. The fork lock tests hold the new series lock in the child of a fork made from C. A Python fork cannot inherit that lock held, because its at-fork hook takes it.Results:
pytest tests --ignore=tests/fuzzing -m "not slow"gives 6,747 passed. The only failures are Redis-backed tests that needredis-server, which this environment lacks.ruff check,ruff format --checkand basedpyright report nothing new.Docs
docs/free-threading.md: a concurrency-audit row for the per-thread cells. The mode-switch row notes that cache operations never queue.docs/features/prometheus-metrics.md: how recording works, why a scrape is exact, and what multiprocess mode does.docs/api-reference.md: theAsyncMetricsCollectordescription, which claimed queue-based collection with "zero impact on critical path latency".docs/performance.md: the batched-mode note, re-recorded budgets, and the batched-vs-sync instruction comparison, which no longer holds.