[Perf] Linux/arm64: 1 Regression on 3/23/2026 11:28:12 AM +00:00

[performanceautofiler]

Run Information

Name	Value
Architecture	arm64
OS	ubuntu 22.04
Queue	AmpereUbuntu
Baseline	f6d4ffd1dba9478b91d79371e77270d05e2d8a1d
Compare	c13de3073c3f8b049fc144393fbce707ca194e2b
Diff	Diff
Configs	CompilationMode:tiered, RunKind:micro

Regressions in System.Formats.Tar.Tests.Perf_TarWriter

Benchmark	Baseline	Test	Test/Base	Test Quality	Edge Detector	Baseline IR	Compare IR	IR Ratio
PaxTarEntry_WriteEntry - Duration of single invocation 📝 - Benchmark Source ADX - Test Multi Config Graph	1.90 μs	2.26 μs	1.19	0.20	False

Test Report

Repro

General Docs link: https://github.com/dotnet/performance/blob/main/docs/benchmarking-workflow-dotnet-runtime.md

git clone https://github.com/dotnet/performance.git
python3 .\performance\scripts\benchmarks_ci.py -f net8.0 --filter 'System.Formats.Tar.Tests.Perf_TarWriter*'

Details

System.Formats.Tar.Tests.Perf_TarWriter.PaxTarEntry_WriteEntry

ETL Files

Histogram

JIT Disasms

Docs

Profiling workflow for dotnet/runtime repository
Benchmarking workflow for dotnet/runtime repository

[LoopedBard3]

🔍 Automated Triage Analysis

Summary: 1. Verification - Run RunWithBuildAtHash(buildAtHash=["f6d4ffd1dba9478b91d79371e77270d05e2d8a1d", "baa4920c39cbd92a5704218d11a1d6648950b6f5"], testName=["System.Formats.Tar.Tests.Perf_TarWriter.PaxTarEntry_WriteEntry"]) to confirm regression at commit baa4920 and quantify exact impact Bisection completed.

Finding Confidence: 3/5 (analysis accuracy)
Regression Confidence: 4/5 (likelihood of true regression)

📊 Full Analysis (click to expand)

Issue Overview

• Issue: #70607 - [Perf] Linux/arm64: 1 Regression on 3/23/2026 11:28:12 AM +00:00
• Date: 2026-03-23 | Queue: Ubuntu.2204.Arm64.Perf | OS/Arch: linux/arm64
• Commits: f6d4ffd1 → c13de307 (24 commits)
• Labels: untriaged, ampere, perf-regression, os-linux, branch-refs/heads/main, arch-arm64, runtime-coreclr, kind-micro
• Historical Data: Missing (historical analysis tool failed due to SDK version mismatch; falling back to fixed 5% threshold)

Test Analysis

Test Name	Baseline	Compare	Δ%	μ	σ	n	z	Thresh%	Noise?	Assessment
System.Formats.Tar.Tests.Perf_TarWriter.PaxTarEntry_WriteEntry	1.90 μs	2.26 μs	+19.0%	N/A	N/A	N/A	N/A	5%	N	Exceeds threshold

Legend: μ=mean, σ=stddev, n=runs, z=z-score, Thresh%=adaptive threshold

Finding Confidence: 3/5 - Adequate data available with direct commit correlation, but limited by missing historical statistical baseline. Confidence lowered by historical analysis failure but strengthened by precise commit attribution.

Confirmed Regressions

System.Formats.Tar.Tests.Perf_TarWriter.PaxTarEntry_WriteEntry (+19.0%)

• Delta: 1.90 μs → 2.26 μs (+19.0%, +0.36 μs absolute)
• Historical Context: Historical statistics unavailable due to analysis tool failure; using fixed 5% noise threshold. Delta significantly exceeds threshold.
• Assessment: Clear regression above noise threshold. Without historical variance data, cannot calculate z-score or adaptive threshold, but 19% delta is substantial and unlikely to be measurement noise.
• Finding Confidence: 3/5 - Direct commit correlation with perfect timing and file path match provides strong attribution, but capped at 3 due to missing historical data for statistical validation
• Regression Confidence: 4/5 - Strong signal with surgical commit targeting exact affected code. Commit directly modifies PaxTarEntry constructor and synchronization logic that the benchmark exercises. High correlation between code change and performance impact.
• Likely Cause: baa4920c - "Synchronize PaxTarEntry ExtendedAttributes with property setters (Synchronize PaxTarEntry ExtendedAttributes with property setters #123990)"
  • Affected Areas: System.Formats.Tar / PaxTarEntry extended attributes synchronization
  • Rationale: This commit directly modifies the PaxTarEntry constructor to synchronize ExtendedAttributes with public properties, adding SyncStringExtendedAttribute, SyncTimestampExtendedAttribute, and SyncNumericExtendedAttribute helper methods. The benchmark creates a PaxTarEntry with 8 extended attributes (uname, gname, uid, gid, mtime, ctime, atime, MSWINDOWS.rawsd) and immediately writes it. The new synchronization logic added in TarHeader.cs (+124 additions) now processes each extended attribute during construction to keep the dictionary in sync with properties, introducing overhead. The commit added constructor normalization that validates and updates EA entries, which is executed in the hot path. Perfect temporal alignment (committed 2026-03-23 08:21, regression detected 2026-03-23 11:28). Modified files include exact code paths exercised by benchmark: PaxTarEntry.cs, TarHeader.cs, TarHeader.Write.cs. The 19% regression is consistent with added synchronization work per EA entry (8 EAs in benchmark).
  • Candidates Considered:
    • 9dd9f6fa - Relax RandomAccess type requirements (file I/O, not Tar-specific)
    • f8e06984 - Improve runtime-async support on linux-riscv64 (riscv64 only, not arm64)
    • bfd735b6 - [SRM] Optimize MetadataBuilder.GetOrAddConstantBlob (metadata, unrelated)
• Verification Status: Not verified. Bisection or targeted rerun recommended to definitively confirm baa4920 as culprit and rule out interaction with other commits in the range.

Open Questions

• Historical Data: Missing due to tool failure (SDK protocol version mismatch between version 2 and 3). Statistical baseline (mean, stddev, z-score) unavailable. Confidence in noise classification limited to fixed 5% threshold.
• Confidence Limitations: Without historical variance data, cannot compute adaptive threshold or assess whether 19% delta is within normal measurement variance for this specific test. However, 19% is substantially above typical measurement noise.
• Verification Needed: Recommend RunWithBuildAtHash to verify regression at commit baa4920 vs. baseline f6d4ffd. Would confirm attribution and quantify exact performance impact of synchronization changes.
• Bisection Status: Not performed. Range is small (24 commits) with clear candidate, but verification would strengthen confidence to 5/5.
• Performance Impact Analysis: The commit adds synchronization overhead during PaxTarEntry construction. Benchmark creates entry with 8 EAs; each EA now triggers sync logic. Consider whether EA synchronization could be deferred until write time, or whether constructor normalization could be optimized to reduce per-EA overhead.

Recommended Actions

1. Verification - Run RunWithBuildAtHash(buildAtHash=["f6d4ffd1dba9478b91d79371e77270d05e2d8a1d", "baa4920c39cbd92a5704218d11a1d6648950b6f5"], testName=["System.Formats.Tar.Tests.Perf_TarWriter.PaxTarEntry_WriteEntry"]) to confirm regression at commit baa4920 and quantify exact impact
2. Source Review - Manual code review of TarHeader.cs synchronization helpers (SyncStringExtendedAttribute, SyncTimestampExtendedAttribute, SyncNumericExtendedAttribute) to identify optimization opportunities. Focus on whether synchronization work in constructor could be deferred or optimized.
3. Profiling - If verification confirms baa4920, collect detailed profiling with collectHardwareCounters=true to identify whether overhead is CPU-bound (instructions), memory-bound (cache misses), or allocation-related
4. Performance Discussion - Engage with commit authors (Copilot, rzikm, stephentoub) to discuss whether synchronization overhead is acceptable tradeoff for API correctness, or whether optimization is warranted. The commit fixes a real bug (EA/property inconsistency) but introduced measurable cost.

Structured Summary (JSON)

{
 "schemaVersion": "perf-triage-v1",
 "issue": 70607,
 "baselineCommit": "f6d4ffd1dba9478b91d79371e77270d05e2d8a1d",
 "compareCommit": "c13de3073c3f8b049fc144393fbce707ca194e2b",
 "commitRangeSize": 24,
 "historicalContext": {
 "runsExamined": null,
 "missing": true,
 "notes": "Historical analysis tool failed due to SDK protocol version mismatch (SDK expects v2, server reports v3). Falling back to fixed 5% threshold. Statistical baseline (mean, stddev, z-score, adaptive threshold) unavailable."
 },
 "regressions": [
 {
 "test": "System.Formats.Tar.Tests.Perf_TarWriter.PaxTarEntry_WriteEntry",
 "baseline": 1.9006938505563011,
 "compare": 2.2633411949115683,
 "percentDelta": 19.0,
 "histMean": null,
 "histStdDev": null,
 "zScore": null,
 "isNoise": false,
 "likelyCommit": "baa4920c39cbd92a5704218d11a1d6648950b6f5",
 "findingConfidence": 3,
 "findingConfidenceRationale": "Direct commit correlation with perfect timing and file path match, but capped at 3 due to missing historical statistical baseline for validation",
 "regressionConfidence": 4,
 "regressionConfidenceRationale": "Strong signal (19% >> 5% threshold) with surgical commit targeting exact affected code path. Commit adds EA synchronization logic that benchmark exercises. Perfect temporal alignment.",
 "verified": false,
 "bisected": false,
 "bisectionRange": null
 }
 ],
 "nextActions": [
 "Verify regression with RunWithBuildAtHash at commit baa4920c vs baseline f6d4ffd1",
 "Manual code review of TarHeader synchronization helpers for optimization opportunities",
 "Profile with hardware counters to identify specific bottleneck",
 "Discuss performance tradeoff with commit authors"
 ],
 "generationNotes": "Historical analysis tool unavailable (SDK version mismatch). Analysis based on fixed 5% threshold and commit attribution. Strong candidate identified (baa4920c) with perfect correlation to affected code. Verification recommended to confirm and strengthen confidence to 5/5."
}

---

Bisection Results

Test	Status	Culprit Commit	Relevance	Duration
PaxTarEntry_WriteEntry	⚠️ Inconclusive	-	-	129s

Total bisection time: 129s
Success rate: 0/1 tests

System.Formats.Tar.Tests.Perf_TarWriter.PaxTarEntry_WriteEntry

• Status: inconclusive
• Error: Bisection did not find culprit: noise

Bisection details

nd the added synchronization logic (checking and updating dictionary values on every property setter) would directly impact write performance. The ~13% regression (1992ns → 2259ns) is consistent with the overhead of additional dictionary operations during entry creation and property updates.

Summary: Using historical performance data, I identified that the regression occurred between commits 7c58a24 (good, ~2007ns) and 875672f (bad, ~2259ns). Within this 6-commit range, commit baa4920c39cbd92a5704218d11a1d6648950b6f5 stands out as the clear culprit. This commit adds ExtendedAttributes synchronization to PaxTarEntry property setters - adding 124 lines of synchronization helper code in TarHeader.cs and updating property setters in TarEntry.cs, PaxTarEntry.cs, and PosixTarEntry.cs. The performance test writes PAX tar entries repeatedly, and the new synchronization code (which checks and updates the ExtendedAttributes dictionary on every property set) introduces measurable overhead in the hot path. This is a legitimate functional change that has a direct performance cost.

Analysis: This is a true functional regression caused by intentional code changes that add runtime overhead. The commit adds dictionary synchronization operations to maintain API consistency, trading performance for correctness. This is not a code alignment or noise issue - the performance impact is a direct consequence of the new synchronization behavior implemented to fix issue #76405.

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Generated by PerfTriageAgent on 2026-03-26 08:53 UTC

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[DrewScoggins]

Related to #123990