dotnet-ci-benchmarking
SKILL.md
dotnet-ci-benchmarking
Continuous benchmarking guidance for detecting performance regressions in CI pipelines. Covers baseline file management with BenchmarkDotNet JSON exporters, GitHub Actions workflows for artifact-based baseline comparison, regression detection patterns with configurable thresholds, and alerting strategies for performance degradation.
Version assumptions: BenchmarkDotNet v0.14+ for JSON export, GitHub Actions runner environment. Examples use
actions/upload-artifact@v4 and actions/download-artifact@v4.
Scope
- Baseline file management with BenchmarkDotNet JSON exporters
- GitHub Actions workflows for artifact-based baseline comparison
- Regression detection with configurable thresholds
- Alerting strategies for performance degradation
Out of scope
- BenchmarkDotNet setup and benchmark class design -- see [skill:dotnet-benchmarkdotnet]
- Performance architecture patterns -- see [skill:dotnet-performance-patterns]
- Profiling tools (dotnet-counters, dotnet-trace, dotnet-dump) -- see [skill:dotnet-profiling]
- OpenTelemetry metrics and distributed tracing -- see [skill:dotnet-observability]
- Composable CI/CD workflow design -- see [skill:dotnet-gha-patterns]
Cross-references: [skill:dotnet-benchmarkdotnet] for benchmark class setup and JSON exporter configuration, [skill:dotnet-observability] for correlating benchmark regressions with runtime metrics changes, [skill:dotnet-gha-patterns] for composable workflow patterns (reusable workflows, composite actions, matrix builds).
Baseline File Management
BenchmarkDotNet JSON Export
BenchmarkDotNet's JSON exporter produces machine-readable results for automated comparison. Configure the exporter in benchmark classes:
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Exporters.Json;
[JsonExporterAttribute.Full]
[MemoryDiagnoser]
public class CriticalPathBenchmarks
{
[Benchmark(Baseline = true)]
public void ProcessOrder() { /* ... */ }
[Benchmark]
public void ProcessOrderOptimized() { /* ... */ }
}
```text
Or configure via custom config for all benchmark classes:
```csharp
using BenchmarkDotNet.Configs;
using BenchmarkDotNet.Exporters.Json;
using BenchmarkDotNet.Jobs;
using BenchmarkDotNet.Running;
var config = ManualConfig.Create(DefaultConfig.Instance)
.AddJob(Job.ShortRun) // fewer iterations for CI speed
.AddExporter(JsonExporter.Full)
.WithArtifactsPath("./benchmark-results");
BenchmarkSwitcher.FromAssembly(typeof(Program).Assembly).Run(args, config);
```json
### JSON Export Structure
The exported JSON file (`*-report-full.json`) contains structured benchmark results:
```json
{
"Title": "CriticalPathBenchmarks",
"Benchmarks": [
{
"FullName": "MyApp.Benchmarks.CriticalPathBenchmarks.ProcessOrder",
"Statistics": {
"Mean": 1234.5678,
"Median": 1230.1234,
"StandardDeviation": 15.234,
"StandardError": 4.812
},
"Memory": {
"BytesAllocatedPerOperation": 1024,
"Gen0Collections": 0.0012,
"Gen1Collections": 0,
"Gen2Collections": 0
}
}
]
}
```text
Key fields for regression comparison:
| Field | Purpose |
| ----------------------------------- | ----------------------------------------------- |
| `Statistics.Mean` | Average execution time (nanoseconds) |
| `Statistics.Median` | Middle execution time (more robust to outliers) |
| `Statistics.StandardDeviation` | Measurement variability |
| `Memory.BytesAllocatedPerOperation` | GC allocation per operation |
### Baseline Storage Strategies
| Strategy | Pros | Cons | Best For |
| -------------------------------- | -------------------------------------- | --------------------------------------------------------------- | --------------------------------------- |
| Git-committed baseline file | Versioned, auditable, no external deps | Repo size grows; must update deliberately | Small benchmark suites, stable hardware |
| GitHub Actions artifacts | No repo bloat; automatic retention | 90-day default retention; cross-workflow access requires tokens | Large benchmark suites, shared runners |
| External storage (S3/Azure Blob) | Unlimited history; cross-repo sharing | Extra infrastructure; credential management | Multi-repo benchmark comparison |
This skill focuses on the **GitHub Actions artifact** strategy as the default. For composable workflow patterns and
reusable actions, see [skill:dotnet-gha-patterns].
---
## GitHub Actions Benchmark Workflow
### Basic Benchmark Workflow
```yaml
name: Benchmarks
on:
pull_request:
paths:
- 'src/**'
- 'benchmarks/**'
workflow_dispatch:
permissions:
contents: read
actions: read # required for artifact download
jobs:
benchmark:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v4
- name: Setup .NET
uses: actions/setup-dotnet@v4
with:
dotnet-version: '8.0.x'
- name: Run benchmarks
run: dotnet run -c Release --project benchmarks/MyBenchmarks.csproj -- --exporters json
- name: Upload benchmark results
uses: actions/upload-artifact@v4
with:
name: benchmark-results-${{ github.sha }}
path: benchmarks/BenchmarkDotNet.Artifacts/results/
retention-days: 90
```text
### Baseline Comparison Workflow
This workflow downloads the baseline from a previous run and compares against current results:
```yaml
name: Benchmark Regression Check
on:
pull_request:
paths:
- 'src/**'
- 'benchmarks/**'
permissions:
contents: read
actions: read
env:
BENCHMARK_PROJECT: benchmarks/MyBenchmarks.csproj
RESULTS_DIR: benchmarks/BenchmarkDotNet.Artifacts/results
jobs:
benchmark:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v4
- name: Setup .NET
uses: actions/setup-dotnet@v4
with:
dotnet-version: '8.0.x'
- name: Download baseline results
uses: actions/download-artifact@v4
with:
name: benchmark-baseline
path: ./baseline-results
continue-on-error: true
id: download-baseline
- name: Run benchmarks
run: dotnet run -c Release --project ${{ env.BENCHMARK_PROJECT }} -- --exporters json
- name: Compare with baseline
if: steps.download-baseline.outcome == 'success'
shell: bash
run: |
set -euo pipefail
python3 scripts/compare-benchmarks.py \
--baseline ./baseline-results \
--current "${{ env.RESULTS_DIR }}" \
--threshold 10 \
--output benchmark-comparison.md
- name: Upload current results as new baseline
if: github.ref == 'refs/heads/main'
uses: actions/upload-artifact@v4
with:
name: benchmark-baseline
path: ${{ env.RESULTS_DIR }}/
retention-days: 90
overwrite: true
- name: Upload comparison report
if: steps.download-baseline.outcome == 'success'
uses: actions/upload-artifact@v4
with:
name: benchmark-comparison-${{ github.sha }}
path: benchmark-comparison.md
retention-days: 30
```markdown
**Key design decisions:**
- `continue-on-error: true` on baseline download handles first-run (no baseline exists yet)
- Baseline is only updated from `main` branch merges to prevent PR branches from polluting the baseline
- `overwrite: true` replaces the previous baseline artifact
For converting these inline workflows into reusable `workflow_call` patterns, see [skill:dotnet-gha-patterns].
---
## Regression Detection Patterns
### Threshold-Based Comparison
Compare current benchmark results against baseline using percentage thresholds. A regression is flagged when the current
mean exceeds the baseline mean by more than the configured threshold:
```python
#!/usr/bin/env python3
"""compare-benchmarks.py -- Detect benchmark regressions from BenchmarkDotNet JSON exports."""
import json
import sys
from pathlib import Path
def load_benchmarks(results_dir: str) -> dict:
"""Load benchmark results from BenchmarkDotNet JSON export files."""
benchmarks = {}
for json_file in Path(results_dir).glob("*-report-full.json"):
with open(json_file) as f:
data = json.load(f)
for bm in data.get("Benchmarks", []):
name = bm["FullName"]
benchmarks[name] = {
"mean": bm["Statistics"]["Mean"],
"median": bm["Statistics"]["Median"],
"stddev": bm["Statistics"]["StandardDeviation"],
"allocated": bm.get("Memory", {}).get("BytesAllocatedPerOperation", 0),
}
return benchmarks
def compare(baseline_dir: str, current_dir: str, threshold_pct: float) -> list:
"""Compare current results against baseline. Returns list of regressions."""
baseline = load_benchmarks(baseline_dir)
current = load_benchmarks(current_dir)
regressions = []
for name, curr in current.items():
if name not in baseline:
continue # new benchmark, no comparison possible
base = baseline[name]
if base["mean"] == 0:
continue # avoid division by zero
time_change_pct = ((curr["mean"] - base["mean"]) / base["mean"]) * 100
alloc_change = curr["allocated"] - base["allocated"]
if time_change_pct > threshold_pct:
regressions.append({
"name": name,
"baseline_mean": base["mean"],
"current_mean": curr["mean"],
"change_pct": time_change_pct,
"alloc_change": alloc_change,
})
return regressions
if __name__ == "__main__":
import argparse
parser = argparse.ArgumentParser(description="Compare BenchmarkDotNet results")
parser.add_argument("--baseline", required=True, help="Path to baseline results directory")
parser.add_argument("--current", required=True, help="Path to current results directory")
parser.add_argument("--threshold", type=float, default=10.0,
help="Regression threshold percentage (default: 10)")
parser.add_argument("--output", default="comparison.md", help="Output markdown file")
args = parser.parse_args()
regressions = compare(args.baseline, args.current, args.threshold)
with open(args.output, "w") as f:
if regressions:
f.write("## Benchmark Regressions Detected\n\n")
f.write("| Benchmark | Baseline (ns) | Current (ns) | Change | Alloc Delta |\n")
f.write("|-----------|--------------|-------------|--------|-------------|\n")
for r in regressions:
f.write(f"| `{r['name']}` | {r['baseline_mean']:.2f} | "
f"{r['current_mean']:.2f} | +{r['change_pct']:.1f}% | "
f"{r['alloc_change']:+d} B |\n")
f.write(f"\nThreshold: {args.threshold}%\n")
else:
f.write("## Benchmark Results\n\nNo regressions detected ")
f.write(f"(threshold: {args.threshold}%).\n")
if regressions:
print(f"REGRESSION: {len(regressions)} benchmark(s) exceeded "
f"{args.threshold}% threshold", file=sys.stderr)
sys.exit(1)
```text
### Choosing Thresholds
| Environment | Suggested Threshold | Rationale |
| ----------------------------- | ------------------- | ------------------------------------------------------------ |
| Dedicated benchmark hardware | 5% | Low noise floor; small regressions are signal |
| GitHub Actions shared runners | 10-15% | Shared runners introduce 5-10% variance from noisy neighbors |
| Self-hosted runners | 5-10% | More stable than shared, but still monitor variance |
**Calibrate thresholds empirically:** Run the same benchmark suite 5-10 times on your CI environment without code
changes. The maximum observed variance sets your noise floor. Set the threshold above this noise floor (typically 2x the
observed variance).
### Allocation Regression Detection
Memory allocation regressions are more reliable signals than timing regressions because allocations are deterministic
(not affected by noisy neighbors):
```python
# Add to the compare function:
if alloc_change > 0:
regressions.append({
"name": name,
"type": "allocation",
"baseline_alloc": base["allocated"],
"current_alloc": curr["allocated"],
"alloc_change": alloc_change,
})
```text
Use allocation changes as a **hard gate** (zero tolerance for new allocations in zero-alloc paths) and timing changes as
a **soft gate** (warning with threshold).
---
## Alerting Strategies
### PR Comment with Regression Summary
Post benchmark comparison results as a PR comment for reviewer visibility:
```yaml
- name: Comment PR with results
if: steps.download-baseline.outcome == 'success' && github.event_name == 'pull_request'
uses: actions/github-script@v7
with:
script: |
const fs = require('fs');
const body = fs.readFileSync('benchmark-comparison.md', 'utf8');
await github.rest.issues.createComment({
owner: context.repo.owner,
repo: context.repo.repo,
issue_number: context.issue.number,
body: body
});
```text
### Fail the Build on Regression
Exit with non-zero status from the comparison script to fail the GitHub Actions job. This prevents merging PRs that
introduce performance regressions:
```yaml
- name: Check for regressions
if: steps.download-baseline.outcome == 'success'
shell: bash
run: |
set -euo pipefail
python3 scripts/compare-benchmarks.py \
--baseline ./baseline-results \
--current "${{ env.RESULTS_DIR }}" \
--threshold 10
# Script exits non-zero if regressions found -- fails the job
```text
For required status checks and branch protection integration with benchmark gates, see [skill:dotnet-gha-patterns].
### Trend Tracking
For long-term trend analysis beyond single-PR comparison, upload results to a persistent store and track metrics over
time:
| Approach | Tool | Complexity |
| ---------------------------------- | --------------------------------------------- | ------------------------------------- |
| GitHub Actions artifacts | Built-in, 90-day retention | Low -- artifact download/upload only |
| GitHub Pages with benchmark-action | `benchmark-action/github-action-benchmark@v1` | Medium -- auto-generates trend charts |
| External time-series DB | InfluxDB, Prometheus + Grafana | High -- full observability stack |
The simplest approach for most projects is the artifact-based baseline comparison shown in this skill. Graduate to trend
tracking when you need historical regression analysis across many releases.
---
## CI-Specific BenchmarkDotNet Configuration
### ShortRun for CI Speed
Full benchmark runs take 10-30+ minutes. Use `Job.ShortRun` in CI to reduce iteration counts while retaining regression
detection capability:
```csharp
using BenchmarkDotNet.Configs;
using BenchmarkDotNet.Jobs;
public class CiConfig : ManualConfig
{
public CiConfig()
{
AddJob(Job.ShortRun
.WithWarmupCount(3)
.WithIterationCount(5)
.WithInvocationCount(1));
AddExporter(BenchmarkDotNet.Exporters.Json.JsonExporter.Full);
}
}
```json
Apply conditionally based on environment:
```csharp
var config = Environment.GetEnvironmentVariable("CI") is not null
? new CiConfig()
: DefaultConfig.Instance;
BenchmarkRunner.Run<CriticalPathBenchmarks>(config);
```text
### Filtering Benchmarks for CI
Run only critical-path benchmarks in CI to reduce pipeline duration:
```bash
# Run only benchmarks in the "Critical" category
dotnet run -c Release --project benchmarks/MyBenchmarks.csproj -- \
--filter *Critical* --exporters json
```bash
```csharp
[BenchmarkCategory("Critical")]
[MemoryDiagnoser]
[JsonExporterAttribute.Full]
public class CriticalPathBenchmarks
{
[Benchmark]
public void ProcessOrder() { /* ... */ }
}
[BenchmarkCategory("Extended")]
[MemoryDiagnoser]
public class ExtendedBenchmarks
{
[Benchmark]
public void RareCodePath() { /* ... */ }
}
```text
Run `Critical` benchmarks on every PR; run `Extended` benchmarks on a nightly schedule.
### Nightly Benchmark Schedule
```yaml
name: Nightly Benchmarks (Full Suite)
on:
schedule:
- cron: '0 3 * * *' # 3 AM UTC daily
workflow_dispatch:
jobs:
benchmark-full:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v4
- name: Setup .NET
uses: actions/setup-dotnet@v4
with:
dotnet-version: '8.0.x'
- name: Run full benchmark suite
run: dotnet run -c Release --project benchmarks/MyBenchmarks.csproj -- --exporters json
# No --filter: runs all benchmarks including Extended category
- name: Upload full results
uses: actions/upload-artifact@v4
with:
name: benchmark-full-${{ github.run_number }}
path: benchmarks/BenchmarkDotNet.Artifacts/results/
retention-days: 90
```text
For scheduled workflow patterns and matrix builds across TFMs, see [skill:dotnet-gha-patterns].
---
## Agent Gotchas
1. **Use `Job.ShortRun` in CI, not `Job.Default`** -- default benchmark jobs run many iterations for statistical
precision, taking 10-30+ minutes per benchmark class. CI pipelines need faster feedback with `ShortRun` (3 warmup, 5
iteration).
2. **Set threshold above measured noise floor** -- shared CI runners introduce 5-10% timing variance from noisy
neighbors. A 5% threshold on shared runners produces false positives. Calibrate by running the same code multiple
times and measuring variance.
3. **Use allocation changes as hard gates** -- allocation counts are deterministic and unaffected by runner noise. A
zero-to-nonzero allocation change is always a real regression, unlike timing variations.
4. **Only update baselines from main branch** -- if PR branches can update the baseline, a regression in one PR becomes
the new baseline, masking it from subsequent comparisons.
5. **Always set `set -euo pipefail` in bash steps** -- without `pipefail`, a regression detection script that exits
non-zero in a pipeline (e.g., `script | tee`) does not fail the GitHub Actions step.
6. **Handle missing baselines gracefully** -- the first CI run has no baseline to compare against. Use
`continue-on-error: true` on the baseline download step and skip comparison when no baseline exists.
7. **Export JSON, not just Markdown** -- Markdown reports are human-readable but not machine-parseable for automated
regression detection. Always include `[JsonExporterAttribute.Full]` or `JsonExporter.Full` in the config.
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