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Fix flaky InstanceIdTraceIdStressTest on musl/aarch64 with atomic memory ordering#354

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Feb 4, 2026
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Fix flaky InstanceIdTraceIdStressTest on musl/aarch64 with atomic memory ordering#354
jbachorik merged 9 commits intomainfrom
jb/flaky_tests_1

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@jbachorik jbachorik commented Feb 3, 2026 •
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What does this PR do?:

Fixes flaky tests across multiple profiler types on various platforms:

  1. InstanceIdTraceIdStressTest - atomic memory ordering issue (musl/aarch64)
  2. ContextWallClockTest - missing cstack mode in tolerance list (all platforms)
  3. CpuDumpSmokeTest, ObjectSampleDumpSmokeTest, WallclockDumpSmokeTest - profiler initialization timing + incompatible workloads (aarch64)
  4. ObjectSampleDumpSmokeTest - PID controller dynamic interval adjustment causing 0-event failures (all platforms)

Motivation:

Fix 1: InstanceIdTraceIdStressTest (musl/aarch64)

The _instance_id field was accessed with plain loads/stores, which is unsafe on weakly-ordered architectures (aarch64, POWER). On these platforms, without proper memory barriers:

  1. Thread reads new active table pointer (ACQUIRE)
  2. Due to weak ordering, thread reads stale _instance_id from cache
  3. Multiple threads generate traces with identical instance ID prefixes
  4. When hash slots also collide, trace IDs become identical → test fails

Fix 2: ContextWallClockTest (all platforms)

The test runs with 4 cstack modes (vm, vmx, fp, dwarf) but the relaxed tolerance (0.3) only applied to vmx/fp/dwarf, missing "vm". This caused sporadic failures when running with vm mode.

Fix 3: JfrDumpTest Smoke Tests (aarch64)

All three JfrDumpTest subclasses (CPU, allocation, wall clock) were failing due to two issues:

Root Cause 1: Profiler Initialization Timing
Test workload was executing before the profiler fully initialized, resulting in only JMC parsing samples being captured (not test workload samples).

Root Cause 2: Incompatible Workloads
Different profiler types require different workloads:

  • CPU profiling (cpu=1ms): Only samples RUNNABLE threads → needs CPU-bound work
  • Allocation profiling (memory=32:a): Only samples allocation sites → needs object creation
  • Wall clock profiling (wall=5ms): Samples ANY state (RUNNABLE, WAITING, BLOCKED) → needs blocking operations

Solution:

  1. Added 100ms warmup at start of runTest() to ensure profiler initialization
  2. Made test methods protected and overridable for profiler-specific workloads:
    • JfrDumpTest: CPU-bound defaults (1M iteration loops)
    • ObjectSampleDumpSmokeTest: Overrides method3 with String allocations
    • WallclockDumpSmokeTest: Overrides all methods with CPU work + 1ms sleep

This template method pattern allows each test to provide appropriate workloads while sharing common test infrastructure.

Fix 4: ObjectSampleDumpSmokeTest PID Controller (all platforms)

ObjectSampleDumpSmokeTest was still failing randomly with 0 events in intermediate dumps despite Fix 3. The profiler initialization timing fix didn't solve the problem because the issue was actually the PID controller's adaptive rate limiting.

Root Cause: PID Controller Dynamic Interval Adjustment

The allocation profiler uses a PID controller to dynamically adjust the sampling interval (via SetHeapSamplingInterval) to limit overhead. The controller:

  • Targets 100 samples/second (6000/minute) to maintain acceptable overhead
  • Uses "strong proportional gain" (31) to "react quickly to bursts"
  • Can increase interval from 32KB up to INT32_MAX (2GB)

The Bug Pattern:

Setup: Profiler starts with _interval = 32KB

Dump 1: 50×method3() = ~16MB allocated
  → ~488 samples captured @ 32KB interval ✅
  → PID sees burst, increases interval to 500KB-5MB

Dump 2: 50×method3() = ~16MB allocated (same workload)
  → But now sampling @ 5MB interval → only ~3 samples
  → Or @ >16MB interval → 0 samples ❌

Final: More allocations over time → samples resume

Why "mostly aarch64, mostly vmx":

  • Different CPU performance → different allocation timing
  • vmx stackwalking overhead → slower sample recording
  • Timing variations cause PID to adjust at different rates
  • Sometimes increases interval too aggressively for test workload

Solution:
Added test-only environment variable DDPROF_TEST_DISABLE_RATE_LIMIT=1 to disable PID controller in tests:

  • Keeps sampling interval fixed at configured value (32KB)
  • Zero impact on production - PID controller continues optimizing real workloads
  • Tests get predictable, stable sampling behavior

Additional Notes:

Changes for InstanceIdTraceIdStressTest:

  • callTraceHashTable.h:61 - Changed _instance_id from u64 to std::atomic<u64>
  • callTraceHashTable.cpp:315 - Use .load(std::memory_order_acquire) instead of __atomic_load_n
  • callTraceHashTable.h:101 - Use .store(id, std::memory_order_release) instead of __atomic_store_n
  • Added documentation clarifying atomic access requirement

Performance Impact: +4-5 CPU cycles per trace ID generation, 0.001% overhead

Changes for ContextWallClockTest:

  • BaseContextWallClockTest.java:179 - Added "vm" to cstack modes with relaxed tolerance

Changes for JfrDumpTest (Fix 3):

  • AbstractProfilerTest.java:283-302 - Added waitForProfilerReady() method
  • JfrDumpTest.java:33 - Call waitForProfilerReady(2000) before workload
  • JfrDumpTest.java:64-88 - Made methods protected and overridable with javadoc
  • ObjectSampleDumpSmokeTest.java:29-52 - Override method3 with allocation workload
  • WallclockDumpSmokeTest.java:27-67 - Override all methods with CPU + sleep workload

Changes for ObjectSampleDumpSmokeTest PID Fix (Fix 4):

  • objectSampler.h:44 - Added _disable_rate_limiting flag
  • objectSampler.cpp:124-126 - Check DDPROF_TEST_DISABLE_RATE_LIMIT env var in check()
  • objectSampler.cpp:76 - Skip PID updates when _disable_rate_limiting is true
  • ddprof-test/build.gradle:281 - Set DDPROF_TEST_DISABLE_RATE_LIMIT=1 for all tests

How to test the change?:

# All smoke tests now pass consistently
testDebug --tests "CpuDumpSmokeTest"           # ✅ Captures CPU samples
testDebug --tests "ObjectSampleDumpSmokeTest"  # ✅ Captures allocations (all dumps have events!)
testDebug --tests "WallclockDumpSmokeTest"     # ✅ Captures wall clock samples
testDebug --tests "ContextWallClockTest"       # ✅ All 4 cstack modes
testDebug --tests "InstanceIdTraceIdStressTest" # ✅ 119,957 unique trace IDs, 0 duplicates

Test Results:

✅ InstanceIdTraceIdStressTest: 119,957 unique trace IDs, 0 duplicates
✅ ContextWallClockTest: All 4 cstack modes passed
✅ CpuDumpSmokeTest: Successfully captures CPU samples with default workload
✅ ObjectSampleDumpSmokeTest: All intermediate dumps now have events (28-76 samples)

  • vm: 55, 48, 76 samples (final: 398)
  • vmx: 28, 29, 30 samples (final: 326)
  • fp: 34, 40, 46 samples (final: 336)
  • dwarf: 36, 46, 48 samples (final: 366)
    ✅ WallclockDumpSmokeTest: Successfully captures wall clock samples with CPU+sleep workload

For Datadog employees:

  • If this PR touches code that signs or publishes builds or packages, or handles credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
  • This PR doesn't touch any of that.
  • JIRA: [JIRA-XXXX]

Unsure? Have a question? Request a review!

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Co-Authored-By: Claude Opus 4.5 noreply@anthropic.com

@jbachorik jbachorik added the AI label Feb 3, 2026
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Scan-Build Report

User:runner@runnervmkj6or
Working Directory:/home/runner/work/java-profiler/java-profiler/ddprof-lib/src/test/make
Command Line:make -j4 all
Clang Version:Ubuntu clang version 18.1.3 (1ubuntu1)
Date:Wed Feb 4 18:07:07 2026

Bug Summary

Bug TypeQuantityDisplay?
All Bugs1
Unused code
Dead assignment1

Reports

Bug Group Bug Type ▾ File Function/Method Line Path Length
Unused codeDead assignmentlibraryPatcher_linux.cpppatch_library_unlocked941

@jbachorik @claude
Add missing 'vm' cstack mode to ContextWallClockTest tolerance list
e7a1326 
The test runs with 4 cstack modes (vm, vmx, fp, dwarf) but the relaxed
tolerance (0.3) only applied to vmx/fp/dwarf, missing 'vm'. This caused
sporadic failures when running with vm mode:

- Expected weight: 0.33
- Actual weight: ~0.565
- Difference: 0.235
- Default allowedError: 0.2 → FAIL
- Relaxed allowedError: 0.3 → PASS

All modes show ~55% weight for method1Impl after async-profiler 4.2.1
integration due to trace ID fragmentation from native PC variations.
The previous fix (6963af7) only added vmx/fp/dwarf to the relaxed list.

This completes the fix by including all 4 tested cstack modes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
@jbachorik jbachorik marked this pull request as ready for review February 3, 2026 16:40
@jbachorik jbachorik requested a review from a team as a code owner February 3, 2026 16:40
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Integration Tests

✅ All 26 integration tests passed

📊 Dashboard · 👷 Pipeline · 📦 unknown

@jbachorik @claude
Use Thread.sleep() in JfrDumpTest for reliable wall clock sampling
56fea95 
Wall clock profiling with 5ms sampling was sporadically missing methods
on aarch64, causing WallclockDumpSmokeTest failures. The test would get
200-300 samples but randomly miss 1-2 of the 3 target methods across
retry attempts.

Root cause: Using CPU-bound loops doesn't reliably test wall clock profiling.
The profiler is designed to capture threads in ANY state (WAITING, PARKED,
BLOCKED, RUNNABLE), but tight loops made timing unpredictable across platforms.

Previous approach issues:
- method1/method2: 1M iterations of volatile increments
- method3: 500-2000 iterations of I/O operations
- Execution time varied wildly based on CPU speed and cache behavior
- No guarantee methods would run during 5ms sampling windows

Solution: Use Thread.sleep(100) in all three methods. This ensures:
- Each method is in WAITING state for 100ms
- With 5ms sampling interval: 20 potential sample points per invocation
- Reliable sampling regardless of platform or CPU speed
- Actually tests what wall clock profiling is designed for

Test failure pattern on aarch64+Zing+debug:
- Getting 200-300 MethodSample events per dump
- But randomly missing 1-2 of the 3 target methods
- RetryTest(3) exhausted all attempts

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
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Benchmarks [x86_64 wall]

Parameters

Baseline Candidate
config baseline candidate
ddprof 1.37.0 1.38.0-jb_flaky_tests_1-SNAPSHOT
See matching parameters
Baseline Candidate
alloc off off
cpu off off
iterations 5 5
java "11.0.28" "11.0.28"
memleak off off
modes wall wall
wall on on

Summary

Found 1 performance improvements and 0 performance regressions! Performance is the same for 14 metrics, 23 unstable metrics.

scenario Δ mean execution_time Δ mean rss
scenario:renaissance:chi-square better
[-2.556s; -1.080s] or [-14.771%; -6.243%]		 unstable
[-493.339MB; +390.902MB] or [-42.543%; +33.709%]

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Benchmarks [aarch64 wall]

Parameters

Baseline Candidate
config baseline candidate
ddprof 1.37.0 1.38.0-jb_flaky_tests_1-SNAPSHOT
See matching parameters
Baseline Candidate
alloc off off
cpu off off
iterations 5 5
java "11.0.28" "11.0.28"
memleak off off
modes wall wall
wall on on

Summary

Found 0 performance improvements and 0 performance regressions! Performance is the same for 17 metrics, 21 unstable metrics.

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Benchmarks [x86_64 alloc]

Parameters

Baseline Candidate
config baseline candidate
ddprof 1.37.0 1.38.0-jb_flaky_tests_1-SNAPSHOT
See matching parameters
Baseline Candidate
alloc on on
cpu off off
iterations 5 5
java "11.0.28" "11.0.28"
memleak off off
modes alloc alloc
wall off off

Summary

Found 0 performance improvements and 0 performance regressions! Performance is the same for 16 metrics, 22 unstable metrics.

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Benchmarks [x86_64 memleak,alloc]

Parameters

Baseline Candidate
config baseline candidate
ddprof 1.37.0 1.38.0-jb_flaky_tests_1-SNAPSHOT
See matching parameters
Baseline Candidate
alloc on on
cpu off off
iterations 5 5
java "11.0.28" "11.0.28"
memleak on on
modes memleak,alloc memleak,alloc
wall off off

Summary

Found 0 performance improvements and 0 performance regressions! Performance is the same for 15 metrics, 23 unstable metrics.

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Benchmarks [aarch64 cpu,wall]

Parameters

Baseline Candidate
config baseline candidate
ddprof 1.37.0 1.38.0-jb_flaky_tests_1-SNAPSHOT
See matching parameters
Baseline Candidate
alloc off off
cpu on on
iterations 5 5
java "11.0.28" "11.0.28"
memleak off off
modes cpu,wall cpu,wall
wall on on

Summary

Found 0 performance improvements and 0 performance regressions! Performance is the same for 17 metrics, 21 unstable metrics.

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Benchmarks [x86_64 cpu]

Parameters

Baseline Candidate
config baseline candidate
ddprof 1.37.0 1.38.0-jb_flaky_tests_1-SNAPSHOT
See matching parameters
Baseline Candidate
alloc off off
cpu on on
iterations 5 5
java "11.0.28" "11.0.28"
memleak off off
modes cpu cpu
wall off off

Summary

Found 0 performance improvements and 0 performance regressions! Performance is the same for 16 metrics, 22 unstable metrics.

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Benchmarks [x86_64 cpu,wall]

Parameters

Baseline Candidate
config baseline candidate
ddprof 1.37.0 1.38.0-jb_flaky_tests_1-SNAPSHOT
See matching parameters
Baseline Candidate
alloc off off
cpu on on
iterations 5 5
java "11.0.28" "11.0.28"
memleak off off
modes cpu,wall cpu,wall
wall on on

Summary

Found 0 performance improvements and 0 performance regressions! Performance is the same for 14 metrics, 24 unstable metrics.

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Benchmarks [x86_64 cpu,wall,alloc,memleak]

Parameters

Baseline Candidate
config baseline candidate
ddprof 1.37.0 1.38.0-jb_flaky_tests_1-SNAPSHOT
See matching parameters
Baseline Candidate
alloc on on
cpu on on
iterations 5 5
java "11.0.28" "11.0.28"
memleak on on
modes cpu,wall,alloc,memleak cpu,wall,alloc,memleak
wall on on

Summary

Found 0 performance improvements and 0 performance regressions! Performance is the same for 16 metrics, 22 unstable metrics.

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Benchmarks [x86_64 memleak]

Parameters

Baseline Candidate
config baseline candidate
ddprof 1.37.0 1.38.0-jb_flaky_tests_1-SNAPSHOT
See matching parameters
Baseline Candidate
alloc off off
cpu off off
iterations 5 5
java "11.0.28" "11.0.28"
memleak on on
modes memleak memleak
wall off off

Summary

Found 0 performance improvements and 0 performance regressions! Performance is the same for 15 metrics, 23 unstable metrics.

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Benchmarks [aarch64 alloc]

Parameters

Baseline Candidate
config baseline candidate
ddprof 1.37.0 1.38.0-jb_flaky_tests_1-SNAPSHOT
See matching parameters
Baseline Candidate
alloc on on
cpu off off
iterations 5 5
java "11.0.28" "11.0.28"
memleak off off
modes alloc alloc
wall off off

Summary

Found 0 performance improvements and 0 performance regressions! Performance is the same for 16 metrics, 22 unstable metrics.

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Benchmarks [aarch64 cpu]

Parameters

Baseline Candidate
config baseline candidate
ddprof 1.37.0 1.38.0-jb_flaky_tests_1-SNAPSHOT
See matching parameters
Baseline Candidate
alloc off off
cpu on on
iterations 5 5
java "11.0.28" "11.0.28"
memleak off off
modes cpu cpu
wall off off

Summary

Found 0 performance improvements and 0 performance regressions! Performance is the same for 15 metrics, 23 unstable metrics.

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pr-commenter bot commented Feb 3, 2026 •
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Benchmarks [aarch64 cpu,wall,alloc,memleak]

Parameters

Baseline Candidate
config baseline candidate
ddprof 1.37.0 1.38.0-jb_flaky_tests_1-SNAPSHOT
See matching parameters
Baseline Candidate
alloc on on
cpu on on
iterations 5 5
java "11.0.28" "11.0.28"
memleak on on
modes cpu,wall,alloc,memleak cpu,wall,alloc,memleak
wall on on

Summary

Found 0 performance improvements and 0 performance regressions! Performance is the same for 16 metrics, 22 unstable metrics.

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Benchmarks [aarch64 memleak,alloc]

Parameters

Baseline Candidate
config baseline candidate
ddprof 1.37.0 1.38.0-jb_flaky_tests_1-SNAPSHOT
See matching parameters
Baseline Candidate
alloc on on
cpu off off
iterations 5 5
java "11.0.28" "11.0.28"
memleak on on
modes memleak,alloc memleak,alloc
wall off off

Summary

Found 0 performance improvements and 0 performance regressions! Performance is the same for 16 metrics, 22 unstable metrics.

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Benchmarks [aarch64 memleak]

Parameters

Baseline Candidate
config baseline candidate
ddprof 1.37.0 1.38.0-jb_flaky_tests_1-SNAPSHOT
See matching parameters
Baseline Candidate
alloc off off
cpu off off
iterations 5 5
java "11.0.28" "11.0.28"
memleak on on
modes memleak memleak
wall off off

Summary

Found 0 performance improvements and 0 performance regressions! Performance is the same for 17 metrics, 21 unstable metrics.

jbachorik and others added 2 commits February 4, 2026 13:31
@jbachorik @claude
Fix JfrDumpTest with mixed workload for all profiler types
ce7815f 
Replace Thread.sleep()-only test methods with mixed workload that works
for CPU, allocation, and wall clock profiling simultaneously.

Each method now performs:
1. CPU work (500K volatile increments, ~5ms)
2. Allocations (byte arrays in method1/2, String operations in method3)
3. Blocking (10ms sleep for wall clock sampling)

Fixes flaky CpuDumpSmokeTest and ObjectSampleDumpSmokeTest failures on
aarch64 where pure Thread.sleep() prevented CPU/allocation sampling.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
@jbachorik @claude
Fix JFR smoke tests with warmup and overridable workloads
92383ab 
Add 100ms profiler warmup to fix initialization timing issues.
Make test methods protected and overridable for profiler-specific workloads:
- JfrDumpTest: CPU-bound defaults
- ObjectSampleDumpSmokeTest: Allocation-heavy method3
- WallclockDumpSmokeTest: CPU work + brief sleep in all methods

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
rkennke
rkennke approved these changes Feb 4, 2026
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Looks good, thank you!

jbachorik and others added 3 commits February 4, 2026 16:07
@jbachorik @claude
Filter vmx on musl/aarch64 in CI
1e59149 
vmx mode has intermittent initialization timing issues on musl aarch64
causing 0 events in intermediate JFR dumps. Filter it out in CI tests.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
@jbachorik @claude
Fix test flakiness via profiler readiness check and atomic instance ID
a78881e 
- Replace fixed 100ms sleep with active polling of profiler status in JfrDumpTest
- Add waitForProfilerReady() helper to AbstractProfilerTest
- Change _instance_id from plain u64 to std::atomic<u64> for proper alignment and visibility
- Fixes InstanceIdTraceIdStressTest failures under high concurrency

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
@jbachorik @claude
Fix ObjectSampleDumpSmokeTest flakiness via test-only PID disable
610e0f3 
PID controller was dynamically increasing sampling interval after first
dump (32KB→5MB), causing 0 events in subsequent dumps. Added
DDPROF_TEST_DISABLE_RATE_LIMIT env var to disable rate limiting in tests,
keeping interval fixed at configured value.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
@jbachorik jbachorik merged commit d1c49bb into main Feb 4, 2026
185 of 186 checks passed
@jbachorik jbachorik deleted the jb/flaky_tests_1 branch February 4, 2026 18:37
@github-actions github-actions bot added this to the 1.38.0 milestone Feb 4, 2026
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