Perf: off-thread alert-check DuckDB queries (Lite UI freeze under load)

[erikdarlingdata]

Problem

Active UI profiling under a HammerDB TPC-C load against SQL2025 surfaced intermittent ~1s UI freezes in Lite (an external dispatcher-latency probe caught blocks of 842–1174ms while the app was otherwise at p99 < 4ms).

Root cause

Found with dotnet-trace --profile dotnet-sampled-thread-time (wall-clock thread-time sampling — catches blocked/native frames a CPU profiler misses). The alert-check paths run synchronous DuckDB queries directly on the WPF dispatcher:

• MainWindow.CheckPerformanceAlerts → GetLatestPoisonWaitAvgs, GetRecentBlockedProcessReports, GetRecentDeadlocks, GetLongRunningQueries, GetLatestTempDbSpace, GetAnomalousJobs
• ServerTab.RefreshAlertCountsAsync → GetAlertCounts

DuckDB.NET is synchronous, so await _dataService.X() completes on the calling thread. Under load a single DuckDB connection open is ~766ms. These fire on the 60s overview auto-refresh and on every Blocking-tab refresh, so they freeze whatever tab the user is on (the timing coincidence sent me chasing the Queries and Blocking tabs first — see below). The earlier off-thread sweep (#1110–1120) missed the alert/overview paths.

Fix

Wrap the 9 calls in Task.Run(() => _dataService.X(...)) — the same off-thread pattern used across the other refresh paths. 9 insertions / 9 deletions, 2 files.

How it was ruled in

Method-level Stopwatch instrumentation first disproved the obvious suspects — chart build + .Refresh() = 0–3ms (even 1,362 pts × 6 series), GC = 0 collections, grid bind < 30ms, DataGrid columns fixed-width. Only thread-time tracing revealed the synchronous DuckDB on the UI thread, which is invisible to method timers because await sync_call() doesn't show as a stall in the awaiting method.

Proof (Lite vs SQL2025 under HammerDB TPC-C, 75s spanning the overview auto-refresh)

metric	before	after
dispatcher latency MAX	1174 ms	9.2 ms
p99	2.0 ms	1.5 ms
stalls > 250 ms	2	0
stalls > 1 s	1	0

1,584 samples, nothing over 9.2 ms.

Validation

• Lite builds clean (net10, 0 errors).
• Lite.Tests: 434/434 pass.

🤖 Generated with Claude Code