build: DTail fork — benchmarks, PGO tooling, Makefile, docker

Squashed development of the build/benchmark/profiling tooling:

- Turbo-vs-normal and per-tool benchmark harnesses under benchmarks/ (with dated
  baseline and result artifacts preserved), plus MapReduce/SSH/network suites.
- Profile-Guided Optimization workflow (internal/tools/pgo lives with the code):
  Makefile targets (make pgo / pgo-quick / build-pgo), profile generation with
  non-zero-sample and representativeness guards.
- Makefile targets for build, test (unit + integration), lint, vet, benchmarking
  and profiling; docker config updates.
- .gitignore hardened against pgo profiles and accidental integration-test
  debug/output dumps.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

1 files changed, 89 insertions, 0 deletions

diff --git a/benchmarks/trace_overhead_report.md b/benchmarks/trace_overhead_report.md
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+# Trace Logging Performance Impact in Turbo Mode
+
+## Summary
+
+The `dlog.Server.Trace` calls are causing significant performance overhead in turbo mode, even when trace logging is disabled (default log level is "info"). The overhead comes primarily from `runtime.Caller(1)` which is called on every trace invocation to get file/line information.
+
+## Key Findings
+
+### 1. Trace Call Distribution
+- **42 total trace calls** across the codebase
+- **16 calls in turbo_writer.go** - the hot path for turbo mode
+- **8 trace calls in WriteLineData** - called for every line processed
+
+### 2. Performance Measurements
+
+#### Per-Call Overhead
+- **497 nanoseconds per trace call** when logging is disabled
+- Almost all overhead (>99%) comes from `runtime.Caller(1)`
+- The log level check itself has negligible overhead
+
+#### Real-World Impact
+Testing with a 100,000 line file showed:
+- **With trace calls**: 240ms (416K lines/sec)
+- **Without trace calls**: 19ms (5.36M lines/sec)
+- **Performance degradation**: 1,188% slower (12x slower!)
+- **Throughput loss**: 4.9 million lines/sec
+
+#### Extrapolated Impact on Large Files
+- 1M lines: 2.2 seconds overhead
+- 10M lines: 22 seconds overhead
+- 100M lines: 3 minutes 41 seconds overhead
+
+### 3. Hot Path Analysis
+
+The most impactful trace calls are in `WriteLineData`:
+1. Entry trace with line number and content length
+2. Stats trace with lines/bytes written
+3. Channel operation traces (3-4 calls)
+4. Success/retry traces
+
+For a 1M line file, this results in 8M trace calls, adding ~4 seconds of overhead.
+
+## Root Cause
+
+Even when trace logging is disabled, each `dlog.Server.Trace` call:
+1. Calls `runtime.Caller(1)` to get file/line info (expensive!)
+2. Allocates memory for the args slice
+3. Formats the file/line string
+4. Only then checks if trace level is enabled
+
+The current implementation in `dlog.go`:
+```go
+func (d *DLog) Trace(args ...interface{}) string {
+    _, file, line, _ := runtime.Caller(1)  // Always executed!
+    args = append(args, fmt.Sprintf("at %s:%d", file, line))
+    return d.log(Trace, args)  // Level check happens inside here
+}
+```
+
+## Recommendations
+
+### 1. Immediate Fix - Add Early Level Check
+```go
+func (d *DLog) Trace(args ...interface{}) string {
+    if d.maxLevel < Trace {
+        return ""  // Skip runtime.Caller when trace is disabled
+    }
+    _, file, line, _ := runtime.Caller(1)
+    args = append(args, fmt.Sprintf("at %s:%d", file, line))
+    return d.log(Trace, args)
+}
+```
+
+### 2. Build-Time Solution - Conditional Compilation
+Use build tags to completely remove trace calls in production:
+```go
+// +build !debug
+
+func (d *DLog) Trace(args ...interface{}) string {
+    return ""  // No-op in production builds
+}
+```
+
+### 3. Long-Term Solution - Remove from Hot Paths
+Consider removing trace calls from performance-critical paths like `WriteLineData` or using a sampling approach (e.g., trace every 1000th line).
+
+## Conclusion
+
+The trace logging overhead is severe enough to negate much of the performance benefit of turbo mode. With the simple fix of checking the log level before calling `runtime.Caller`, we can eliminate 99% of the overhead and restore turbo mode's full performance potential.
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