perf(plugins): stop rewriting a plugin's metrics file on every call - #480
perf(plugins): stop rewriting a plugin's metrics file on every call#480ChuckBuilds wants to merge 2 commits into
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Plugin metrics were persisted to the cache inside monitor_call, so every call by every plugin rewrote a small JSON file. Measured on a running rig: one plugin's plugin_metrics file changed nine times a minute, with fourteen such files active. Each is around 350 bytes, which on ext4 costs a 4KB block plus a journal entry, so the cost is dominated by the write itself rather than the payload. Cache writes accounted for essentially all of that device's 2.4 MB/min of SD traffic, on a card that wears out and has already failed twice on the other rig. Metrics cannot be de-duplicated the way health state can, because call_count changes on every call and the timings usually do too. So they are rate-limited instead: at most one write per plugin per 30 seconds. The in-memory copy stays authoritative and exact -- a plugin's call_count is still precise the instant after it runs. Only the cross-process snapshot the web UI reads is delayed, and telemetry up to half a minute old is still a fair description of a long-running plugin. reset_metrics clears the throttle timestamp, so a reset is not left showing a deleted key for the rest of the interval. Extrapolating the sampled rate, this takes metric writes from roughly 126 a minute to 28. Health persistence, the other half of the churn, is handled separately in #475. Verified by reverting the throttle: the churn test then reports 50 writes for 50 calls. 88 tests pass across resource monitor, plugin system and web API. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01STMbQE4YctTacQXfbYqKuW
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📝 WalkthroughWalkthroughMetric persistence now occurs at most once every 30 seconds per plugin. Metric resets clear the throttle state. New tests cover interval checks, skipped writes, and immediate post-reset persistence. ChangesMetrics persistence
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🔵 Low · up to The change reduces metrics-file writes while keeping in-memory metrics exact, but clock adjustments could make snapshots persist too early or too late, and a failed cache write could leave cross-process telemetry stale until the interval expires. The PR is mergeable with explicit owner follow-up on these bounded persistence behaviors. Sequence Diagram(s)sequenceDiagram
participant Plugin
participant ResourceMonitor
participant MetricsCache
Plugin->>ResourceMonitor: Execute monitored call
ResourceMonitor->>ResourceMonitor: Check the 30-second interval
ResourceMonitor->>MetricsCache: Persist complete metric snapshot
ResourceMonitor->>ResourceMonitor: Clear timestamp during reset_metrics
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Up to standards ✅🟢 Issues
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| Metric | Results |
|---|---|
| Complexity | 0 |
| Duplication | 0 |
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Inline comments:
In `@src/plugin_system/resource_monitor.py`:
- Around line 380-382: Update the persistence timing logic in the resource
monitor to use time.monotonic() for both _metrics_persisted_at values and the
interval comparison, while preserving the force override and
_METRICS_PERSIST_INTERVAL threshold behavior.
- Around line 384-386: Move the assignment to _metrics_persisted_at[plugin_id]
in the metrics persistence method to after cache_manager.set() completes
successfully, while keeping the existing cache key and payload flow unchanged.
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src/plugin_system/resource_monitor.pytest/test_resource_monitor.py
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Two review findings on the throttle, both right. The interval compared wall-clock timestamps. These devices have no RTC, so the clock jumps by however far off boot-time was the moment NTP first syncs -- a forward jump would allow an early write, a backward one would stall the snapshot well past the interval. time.monotonic() is not subject to either. The timestamp was also recorded before cache_manager.set(). A set() that raised would buy the next interval's silence without leaving a snapshot behind, which is the one case where skipping the write is least affordable. Recorded after the write lands instead, so a failure is retried on the next call. Verified by restoring the original ordering: the new test then reports one write where two are expected. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01STMbQE4YctTacQXfbYqKuW
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Both findings were right; fixed in the latest commit. Monotonic clock — more relevant here than it looks. These are Raspberry Pis with no RTC, so the clock does not drift gradually, it jumps by however wrong boot-time was the moment NTP first syncs. That is a routine event on every boot, not an edge case, and it lands squarely on a 30-second interval comparison. Switched to Timestamp before the write — also right, and the failure mode is the worst-timed one: a Added |
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Superseded by #486, which combines the three SD-write/log-volume PRs. Every change from this PR is verified present on that branch; the branch here is untouched if you want to compare. |
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Where the SD writes were going
devpi writes 2.4 MB/min to the SD card (~3.4 GB/day). Attributed it this iteration:
and then to the files:
That count is of files changed, not writes. Watching a single file directly:
So ~9 writes/min per plugin, 14 plugins, two files each — roughly 250 small writes a minute. Each is ~350 bytes but costs a 4KB block plus an ext4 journal entry, which is why the block-layer figure is ~5x the application figure. Cache persistence accounts for essentially all of the device's write volume.
This matters beyond wear: ledpi has now destroyed two SD cards.
The fix
Metrics can't be de-duplicated the way health state can —
call_countchanges on every call and the timings usually do too. So they're rate-limited instead: at most one write per plugin per 30 seconds.The in-memory copy stays authoritative and exact; a plugin's
call_countis precise the instant after it runs. Only the cross-process snapshot the web UI reads is delayed, and telemetry up to half a minute old is still a fair description of a long-running plugin.reset_metricsclears the throttle timestamp so a reset isn't left showing a deleted key for the rest of the interval.Extrapolating the sampled rate: ~126 metric writes/min → 28.
Scope
This is the metrics half. Health persistence — the other 14 files — is fixed separately in #475, which is still open. Together they should take the bulk of that 2.4 MB/min out.
Verification
test_repeated_calls_persist_once_per_interval— 50 calls, 1 write. Reverting the throttle makes it report 50.test_in_memory_metrics_stay_exact_while_writes_are_skipped— guards the thing the throttle could plausibly break.test_the_interval_elapsing_allows_the_next_writeandtest_reset_lets_the_next_call_persist_immediately.Summary by CodeRabbit