Avoid leaking a MeterListener per Cache in DEBUG builds#19995
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In DEBUG builds, every Cache instance created a CacheMetrics.CacheMetricsListener, which starts a System.Diagnostics.Metrics.MeterListener registered in the process-global metrics registry. These were never disposed, so they accumulated for the lifetime of the process. Because every cache hit/miss/add publishes a measurement to all registered listeners, the per-operation cost grew linearly with the number of leaked listeners, so workloads that create many caches (for example repeated ParseAndCheckProject / per-file checks) slowed down steadily. Track the per-cache totals used by DebugDisplay directly, incrementing a small Stats object alongside the existing global Meter counters, instead of via a per-cache MeterListener. No listener is created, so nothing leaks, and DebugDisplay still works. The now-unused CacheMetrics.Hit/Miss/Add/Update/Eviction/EvictionFail helpers are replaced by a single recordMetric helper.
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- Remove the CacheMetrics.CacheMetricsListener type. Its only per-cache use was the #if DEBUG debugListener each Cache created and never disposed, which was the leak this PR set out to fix. (majocha) - Drop the per-instance cacheId tag (and nextCacheId). Measurements now carry only the cache name, shrinking the payload published to any connected exporter and removing the per-instance filtering that was cacheId's only purpose. (majocha) - DebugDisplay and the cache tests read the existing name-aggregated stats via CacheMetrics.getTotalsByName / getRatioByName, populated by the single process-wide ListenToAll listener. No per-cache listener is created and no per-operation cost is added in any configuration, so there is no DEBUG-only overhead left to gate behind a separate directive. (T-Gro) - Overload-cache tests enable ListenToAll and snapshot totals before/after to stay scoped to their own compilation; FSharpChecker .CreateOverloadCacheMetricsListener is removed.
CacheMetricsListener was a public type, so dropping it changes the recorded public surface. Remove its 10 entries from FSharp.Compiler.Service.SurfaceArea.netstandard20.bsl; the SurfaceArea test now passes. Also note the single-listener assumption the cache metric tests rely on.
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Summary
In
DEBUGbuilds,Cache(src/Compiler/Utilities/Caches.fs) created aCacheMetrics.CacheMetricsListenerper instance. Each listener starts aSystem.Diagnostics.Metrics.MeterListenerthat registers in the process-global metrics registry and was never disposed, so the listeners accumulated for the lifetime of the process.Because every cache hit/miss/add publishes a measurement to all registered listeners, the cost of each cache operation grows linearly with the number of leaked listeners. Workloads that create many caches over time (repeated
ParseAndCheckProject, per-file checks, long IDE sessions, FCS test runs) therefore slow down steadily.How it shows up
I found this while measuring repeated project checks against a Debug FCS. Driving
ParseAndCheckProjecton the same project across 10 edits (only one file changed each time, so the incremental check should be roughly constant):ComputeProjectExtras)A heap dump after those 10 edits showed the accumulation directly:
System.Diagnostics.Metrics.MeterListenerCacheMetrics.CacheMetricsListenerSystem.Diagnostics.DiagNode<...Instrument>+ 98,028DiagNode<...ListenerSubscription>(the global metrics subscription lists)The obvious suspects were ruled out first: cache size factor (1 vs 100 both grow), the project snapshot, a fresh
FSharpCheckerper edit (still grows, so it is process-global), and GC (a forced full collection each check, still grows). The dump named the accumulator.Fix
The per-cache
CacheMetricsListenerexisted only to backDebugDisplay(). Instead of standing up aMeterListenerper cache, the cache now tracks those totals directly in a smallStatsobject, incremented alongside the existing globalMetercounters via a singlerecordMetrichelper. NoMeterListeneris created, so nothing leaks and there is no per-operation "publish to all listeners" cost.DebugDisplay()keeps working with no configuration, and Release builds are unchanged. The now-unusedCacheMetrics.Hit/Miss/Add/Update/Eviction/EvictionFailhelpers are removed in favour ofrecordMetric.With the listener removed, the same loop is flat:
Notes
#if DEBUG, so Release builds were already unaffected. The impact is on Debug FCS: the compiler-dev inner loop, profiling accuracy (a Debug profile is dominated by the metrics publish-to-all-listeners cost), and Debug FCS test runs.mainand is independent of any feature work. The per-cachedebugListenerwas introduced in Print cache stats with --times #18930.