bench(pk-default): add diagnostic config variants and correct the P3 analysis

Single-knob isolation (SHARPCOREDB_PK_DEFAULT_VARIANT=async|bufferedio|novalidate|
noadaptive|hsinsert|plain|tuned) shows the earlier FullSync-dominance claim was
wrong: WalDurabilityMode is only honored by GroupCommitWAL (off by default) and
the async variant measures the same as default. NoEncryptMode moves default
UPDATE ~1.3x (plain ~145K vs tuned ~109K median) and machine drift spans
275-315K on SQLite itself across the same window.

docs/benchmarks/default-config-pk.md corrected: no blind 'FullSync commit-flush
optimization'; the next step is a same-window interleaved A/B harness mode so
default-vs-tuned deltas are attributable before any code change.

Author: MPCoreDeveloper

docs/CHANGELOG.md

@@ -9,6 +9,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Hardening
 
+- **Default-config benchmark follow-up: FullSync hypothesis falsified (P3b)** - single-knob
+  isolation via `SHARPCOREDB_PK_DEFAULT_VARIANT` (`async`, `bufferedio`, `novalidate`,
+  `noadaptive`, `hsinsert`, `plain`, `tuned`) disproved the earlier “FullSync dominates the
+  default gap” claim: the durability mode is only honored by GroupCommitWAL (off by default) and
+  the `async` variant measured the same. `NoEncryptMode` moves default UPDATE ~1.3x (144K vs
+  109K), machine drift spans ~275-315K on SQLite itself. `docs/benchmarks/default-config-pk.md`
+  is corrected accordingly; the next step is a same-window interleaved A/B mode before any
+  further optimization is implemented.
+
 - **Default-config benchmark published (P3)** - `docs/benchmarks/default-config-pk.md` records a
   median-of-3 fair-PK run where the SharpCoreDB arm uses a PURE default `DatabaseConfig`
   (NoEncryptMode=false, no harness flags). Honest result: the default path engages the Columnar

docs/benchmarks/default-config-pk.md

@@ -1,7 +1,7 @@
 # Fair-PK benchmark with DEFAULT DatabaseConfig (P3 hardening)
 
 Run: `dotnet run --project tests/benchmarks/SharpCoreDB.Benchmarks.Comparative -- -c Release -- --pk-default`
-Date: 2026-09-04 · Machine: local dev box · median of 3 runs per phase.
+Date: 2026-09-04 · Machine: local dev box · median of 3 runs per phase unless noted.
 
 The SharpCoreDB arm uses a **pure default** `DatabaseConfig` (only the engine type is pinned to
 `AppendOnly` — `NoEncryptMode` stays at its default `false`, no page-cache/query-cache/durability
@@ -17,23 +17,35 @@ contiguous UPDATE/DELETE paths do engage (verified separately by `DefaultEngineS
 | SQLite | 188,247 | 107,122 | 291,414 | 389,120 |
 | gap vs SQLite | 1.7x | 1.6x | **3.5x** | **4.2x** |
 
-For comparison, the same schema with the tuned benchmark config (NoEncryptMode + Async WAL + larger
-batches) measures UPDATE ~245K ops/s and DELETE ~172K ops/s (gaps ~1.2x / ~2.1x).
+For comparison, the tuned benchmark config measured earlier in the day (UPDATE ~245K ops/s and
+DELETE ~172K ops/s, gaps ~1.2x / ~2.1x).
+
+## Knob isolation (diagnostic single-knob variants, single-shot and median)
+
+The harness supports `SHARPCOREDB_PK_DEFAULT_VARIANT=<name>` for the default arm:
+`async`, `bufferedio`, `novalidate`, `noadaptive`, `hsinsert`, `plain` (= full tuned knob set with
+`NoEncryptMode=true`), `tuned` (= full tuned knob set, `NoEncryptMode=false`).
+
+Observed (2026-09-04 afternoon):
+
+| variant | UPDATE ops/s | notes |
+|---|---:|---|
+| pure default | ~84K (median) | reference |
+| `async` (WalDurabilityMode) | ~74K (single) | **disproves the FullSync hypothesis** — the mode is only honored by GroupCommitWAL (default off) |
+| `bufferedio` / `novalidate` | ~74-76K (single) | within noise |
+| `tuned` (no NoEncryptMode) | ~109K (median) | knob set alone is not the gap |
+| `plain` (= tuned + NoEncryptMode=true) | ~145K (median) | `NoEncryptMode` moves UPDATE ~1.3x; still below the morning's ~212-245K |
+
+Machine drift is significant: SQLite's own UPDATE varied 275K-315K across these runs. Single-knob
+deltas below ~1.3x are not reliably attributable outside a same-window interleaved A/B.
 
 ## Honest conclusion
 
-The out-of-the-box default is **correct and engages the fast paths, but is not yet at the tuned
-throughput**: UPDATE/DELETE land ~3.5-4.2x behind SQLite (vs ~1.2-2.1x tuned). The dominant
-difference is the **default `WalDurabilityMode.FullSync`** (per-commit flush) versus the benchmark
-arm's `Async`; the per-batch commit cost explains most of the gap. Fast-path counters prove the
-contiguous code is running — the throughput is limited by durability flushing, not by resolution.
-
-## Recommendation / follow-up
-
-1. Do **not** silently weaken the durability default (`FullSync` is the safe choice for
-   production). Instead, optimize the **FullSync commit flush** path (fewer/single flush per
-   commit, group-commit of the commit markers + overwrites that already batch per page) so a
-   synchronous commit costs a few ms instead of tens of ms.
-2. Re-run this `--pk-default` harness after that change and require the default-config UPDATE/DELETE
-   gap to move from ~3.5-4.2x toward ~2x before the release-cut.
-3. Keep this file updated with the latest median-of-3 numbers.
+1. The earlier claim that the default `WalDurabilityMode.FullSync` dominates the gap is **wrong**
+   (disproven by the `async` variant). Do **not** implement a “FullSync commit-flush optimization”
+   based on it.
+2. The default path is correct and engages the fast paths; part of the remaining gap correlates
+   with `NoEncryptMode` (record/at-rest toggles and file-format decisions), part is machine drift.
+3. Next step: add a **same-window interleaved A/B** mode to this harness (arms round-robin within
+   one process) so default-vs-tuned deltas are attributable, then re-open the optimization only on
+   a measured knob.

tests/benchmarks/SharpCoreDB.Benchmarks.Comparative/Program.cs

@@ -874,9 +874,80 @@ static BenchmarkResult RunSharpCoreDBPk(
             var sp = services.BuildServiceProvider();
 
             var factory = sp.GetRequiredService<DatabaseFactory>();
-            var config = useDefaultConfig
-                ? new DatabaseConfig { StorageEngineType = engineType }
-                : BuildConfig(engineType, fixedWidth);
+            DatabaseConfig config;
+            if (useDefaultConfig)
+            {
+                var variant = Environment.GetEnvironmentVariable("SHARPCOREDB_PK_DEFAULT_VARIANT")?.ToLowerInvariant();
+                config = variant switch
+                {
+                    "async" => new DatabaseConfig { StorageEngineType = engineType, WalDurabilityMode = SharpCoreDB.Services.DurabilityMode.Async },
+                    "bufferedio" => new DatabaseConfig { StorageEngineType = engineType, UseBufferedIO = true },
+                    "novalidate" => new DatabaseConfig
+                    {
+                        StorageEngineType = engineType,
+                        SqlValidationMode = SharpCoreDB.Services.SqlQueryValidator.ValidationMode.Disabled,
+                        StrictParameterValidation = false,
+                    },
+                    "noadaptive" => new DatabaseConfig { StorageEngineType = engineType, EnableAdaptiveWalBatching = false },
+                    "hsinsert" => new DatabaseConfig { StorageEngineType = engineType, HighSpeedInsertMode = true },
+                    "plain" => new DatabaseConfig
+                    {
+                        StorageEngineType = engineType,
+                        NoEncryptMode = true,
+                        AutoFixedWidthRecords = false,
+                        FixedWidthRecordLayout = true,
+                        UseGroupCommitWal = false,
+                        EnableAdaptiveWalBatching = false,
+                        HighSpeedInsertMode = true,
+                        GroupCommitSize = 1000,
+                        WalDurabilityMode = SharpCoreDB.Services.DurabilityMode.Async,
+                        EnablePageCache = true,
+                        PageCacheCapacity = 10_000,
+                        UseMemoryMapping = true,
+                        UseBufferedIO = true,
+                        EnableHashIndexes = true,
+                        EnableQueryCache = true,
+                        QueryCacheSize = 4096,
+                        EnableCompiledPlanCache = true,
+                        EnableBTreeSelection = true,
+                        EnableSimdAndProjectionPushdown = true,
+                        WalBufferSize = 8 * 1024 * 1024,
+                        CollectGCAfterBatches = false,
+                        SqlValidationMode = SharpCoreDB.Services.SqlQueryValidator.ValidationMode.Disabled,
+                        StrictParameterValidation = false,
+                    },
+                    "tuned" => new DatabaseConfig
+                    {
+                        StorageEngineType = engineType,
+                        AutoFixedWidthRecords = false,
+                        FixedWidthRecordLayout = true,
+                        UseGroupCommitWal = false,
+                        EnableAdaptiveWalBatching = false,
+                        HighSpeedInsertMode = true,
+                        GroupCommitSize = 1000,
+                        WalDurabilityMode = SharpCoreDB.Services.DurabilityMode.Async,
+                        EnablePageCache = true,
+                        PageCacheCapacity = 10_000,
+                        UseMemoryMapping = true,
+                        UseBufferedIO = true,
+                        EnableHashIndexes = true,
+                        EnableQueryCache = true,
+                        QueryCacheSize = 4096,
+                        EnableCompiledPlanCache = true,
+                        EnableBTreeSelection = true,
+                        EnableSimdAndProjectionPushdown = true,
+                        WalBufferSize = 8 * 1024 * 1024,
+                        CollectGCAfterBatches = false,
+                        SqlValidationMode = SharpCoreDB.Services.SqlQueryValidator.ValidationMode.Disabled,
+                        StrictParameterValidation = false,
+                    },
+                    _ => new DatabaseConfig { StorageEngineType = engineType },
+                };
+            }
+            else
+            {
+                config = BuildConfig(engineType, fixedWidth);
+            }
 
             using var db = (SharpCoreDB.Database)factory.Create(
                 dbPath: dbPath,
@@ -979,6 +1050,12 @@ data TEXT
     /// </summary>
     private static BenchmarkResult RunPkMedian(Func<BenchmarkResult> arm, int reps = 3)
     {
+        // Allow quick single-shot profiling via SHARPCOREDB_BENCH_REPS (diagnostic only).
+        if (int.TryParse(Environment.GetEnvironmentVariable("SHARPCOREDB_BENCH_REPS"), out int envReps) && envReps > 0)
+        {
+            reps = envReps;
+        }
+
         var runs = new List<BenchmarkResult>(reps);
         for (int r = 0; r < reps; r++)
         {