Don't preload jmethodIDs when cstack=vm with hotspot JVM by zhengyu123 · Pull Request #549 · DataDog/java-profiler

zhengyu123 · 2026-05-27T20:10:57Z

What does this PR do?:
Reduce memory overhead by storing raw Method pointers instead of pre-allocating jmethodIDs for classes loaded by system class loaders (bootstrap, platform and application)

This feature only applies to hotspot JVM when cstack=vm

Motivation:
Pre-allocating jmethodIDs can consume significant amount of native memory, e.g. 9G in some extreme cases.
By introducing optional lazy jmethodID resolution for system classes, HotSpot JVM profiling avoids loading jmethodIDs for every method, but only the methods on stacks, a significant optimization for native memory usage.

Additional Notes:
This feature is optional and is off by default. To enable the feature, set force_jmethodID=false

How to test the change?:
Running renaissance akka benchmark with NMT, exams jmethodID sites:

Mainline (without this PR)

[0x0000726344c5f43d] InstanceKlass::get_jmethod_id(methodHandle const&)+0x8d
[0x000072634500e78a] Method::jmethod_id()+0x6a
[0x0000726344e78f63] JvmtiEnv::GetClassMethods(oopDesc*, int*, _jmethodID***)+0x2b3
[0x0000726344e2eb86] jvmti_GetClassMethods+0x206
                             (malloc=430672 type=Class #3156) (at peak)

[0x000072634500d4d1] Method::ensure_jmethod_ids(ClassLoaderData*, int)+0xd1
[0x0000726344e78f57] JvmtiEnv::GetClassMethods(oopDesc*, int*, _jmethodID***)+0x2a7
[0x0000726344e2eb86] jvmti_GetClassMethods+0x206
[0x0000726344145374] _jvmtiEnv::GetClassMethods(_jclass*, int*, _jmethodID***)+0x34
                             (malloc=392776 type=Internal #2397) (at peak)

[0x000072634500d48d] Method::ensure_jmethod_ids(ClassLoaderData*, int)+0x8d
[0x0000726344e78f57] JvmtiEnv::GetClassMethods(oopDesc*, int*, _jmethodID***)+0x2a7
[0x0000726344e2eb86] jvmti_GetClassMethods+0x206
[0x0000726344145374] _jvmtiEnv::GetClassMethods(_jclass*, int*, _jmethodID***)+0x34
                             (malloc=57528 type=Class #2397) (at peak)

With force_jmethodID=false

[0x0000762891b93651] Method::ensure_jmethod_ids(ClassLoaderData*, int)+0x251
[0x00007628919fef57] JvmtiEnv::GetClassMethods(oopDesc*, int*, _jmethodID***)+0x2a7
[0x00007628919b4b86] jvmti_GetClassMethods+0x206
[0x0000762890ce9594] _jvmtiEnv::GetClassMethods(_jclass*, int*, _jmethodID***)+0x34
                             (malloc=3920 type=Class #98) (at peak)

[0x0000762891b93850] Method::make_jmethod_id(ClassLoaderData*, Method*)+0x90
[0x00007628917e548d] InstanceKlass::get_jmethod_id(methodHandle const&)+0xdd
[0x0000762891b9478a] Method::jmethod_id()+0x6a
[0x00007628918a0186] get_method_id(JNIEnv_*, _jclass*, char const*, char const*, bool, JavaThread*) [clone .constprop.0]+0x186
                             (malloc=1536 type=Class #64) (at peak)

[0x00007628917e543d] InstanceKlass::get_jmethod_id(methodHandle const&)+0x8d
[0x0000762891b9478a] Method::jmethod_id()+0x6a
[0x00007628919fef63] JvmtiEnv::GetClassMethods(oopDesc*, int*, _jmethodID***)+0x2b3
[0x00007628919b4b86] jvmti_GetClassMethods+0x206
                             (malloc=252656 type=Class #1792) (at peak)

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: PROF-13548

Unsure? Have a question? Request a review!

dd-octo-sts · 2026-05-27T20:32:32Z

CI Test Results

Run: #27226908405 | Commit: 40505c9 | Duration: 12m 39s (longest job)

✅ All 32 test jobs passed

Status Overview

JDK	glibc-aarch64/debug	glibc-amd64/debug	musl-aarch64/debug	musl-amd64/debug
8	-	✅	-	-
8-ibm	-	✅	-	-
8-j9	✅	✅	-	-
8-librca	-	-	✅	✅
8-orcl	-	✅	-	-
11	-	✅	-	-
11-j9	✅	✅	-	-
11-librca	-	-	✅	✅
17	✅	✅	-	-
17-graal	✅	✅	-	-
17-j9	✅	✅	-	-
17-librca	-	-	✅	✅
21	✅	✅	-	-
21-graal	✅	✅	-	-
21-librca	-	-	✅	✅
25	✅	✅	-	-
25-graal	✅	✅	-	-
25-librca	-	-	✅	✅

Legend: ✅ passed | ❌ failed | ⚪ skipped | 🚫 cancelled

Summary: Total: 32 | Passed: 32 | Failed: 0

Updated: 2026-06-09 18:58:57 UTC

dd-octo-sts · 2026-05-31T19:09:17Z

Scan-Build Report

User:	runner@runnervm3jyl0
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:	Tue Jun 2 20:41:00 2026

Bug Summary

Logic error
Bug Type	Quantity	Display?
All Bugs	1
Result of operation is garbage or undefined	1

Reports

Bug Group	Bug Type ▾	File	Function/Method	Line	Path Length
Logic error	Result of operation is garbage or undefined	hotspotSupport.cpp	loadMethodIDsImpl	1309	2

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rkennke

Thanks for making the changes. Some old comments are still un-addressed, and I also have some new comments.

rkennke · 2026-06-10T11:44:50Z

+}
+
+// This method only resolves methods that are loaded by system class loaders
+jmethodID HotspotSupport::resolve(const void* method) {


This must only ever be called outside a signal-handler-context. If possible, this should be asserted, or otherwise at least documented.

Added comment in .h file.

Where? I don't see it.

In header file:

// Resolve a method to a jmethodID at dumping time static jmethodID resolve(const void* method);

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chatgpt-codex-connector · 2026-06-12T20:40:45Z

+  // Prepare JVMSupport for execution
+  JVMSupport::initExecution(VM::jvmti(), VM::jni());


Initialize jmethodID policy after cstack fallback

When a start request uses cstack=vm on a HotSpot where VM stack walking is not supported, this call computes the partial-preload policy before _cstack is normalized and later downgraded to DWARF/no at lines 1317-1337. The recording then samples through the ASGCT path, which still needs preallocated jmethodIDs, but loadAllMethodIDsIfNeeded() has already skipped normal system/application classes, so Java frames in that fallback recording can be serialized as unknown; move this initialization until after the final cstack mode is known or pass the resolved mode into the policy.

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rkennke · 2026-06-17T10:02:24Z

+}
+
+// This method only resolves methods that are loaded by system class loaders
+jmethodID HotspotSupport::resolve(const void* method) {


Where? I don't see it.

In header file:

// Resolve a method to a jmethodID at dumping time static jmethodID resolve(const void* method);

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This reverts commit b41f1b3.

rkennke

Some things you said you fixed are still not fixed, and I also found a new thing.

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chatgpt-codex-connector · 2026-06-17T14:55:57Z

+    static constexpr jint JVM_ACC_HIDDEN = 0x800;
+    static constexpr jint hidden_mask = (ACC_SYNTHETIC | ACC_BRIDGE | JVM_ACC_HIDDEN);
+    return modifiers != 0 && // JVMTI GetClassModifiers returns 0 for ordinary package-private classes
+           ((modifiers & hidden_mask) != 0);


Detect hidden classes with the real HotSpot flag

Fresh evidence in this revision is that the replacement helper still keys hidden-class detection off 0x800/synthetic/bridge bits. In the partial-preload path this is the only guard before system/application-loader classes skip GetClassMethods, but arbitrary Lookup.defineHiddenClass classes are unloadable and need not be synthetic or lambda-shaped; those classes can therefore be sampled as raw Method* frames and unload before JFR serialization, leaving the resolver with stale VM metadata and producing missing/unknown Java frames.

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zhengyu123 added 2 commits May 27, 2026 18:20

Don't preload jmethodIDs when cstack=vm with hotspot JVM

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zhengyu123 added 2 commits May 28, 2026 13:29

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zhengyu123 added 11 commits June 1, 2026 12:49

Clean up

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Fix

c782c3c

Add application/platform class loader support

c462d85

Fix

7710c6f

Fix

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Harden symbolAt()

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Merge branch 'main' into hs_jmethod_id

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force_jmethodID argument

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Consolidate JVM initialization

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Cleanup: fixed typo and etc.

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Remove dead code

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zhengyu123 marked this pull request as ready for review June 2, 2026 15:26

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Comment thread ddprof-lib/src/main/cpp/hotspot/hotspotSupport.cpp

Comment thread ddprof-lib/src/main/cpp/hotspot/hotspotSupport.cpp Outdated

zhengyu123 marked this pull request as draft June 2, 2026 15:45

zhengyu123 added 2 commits June 2, 2026 16:24

Fix

06b1a92

Fix

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zhengyu123 changed the title ~~WIP: Don't preload jmethodIDs when cstack=vm~~ Don't preload jmethodIDs when cstack=vm with hotspot JVM Jun 2, 2026

zhengyu123 marked this pull request as ready for review June 2, 2026 17:13

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Comment thread ddprof-lib/src/main/cpp/hotspot/hotspotSupport.cpp

Comment thread ddprof-lib/src/main/cpp/hotspot/hotspotSupport.cpp

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Comment thread ddprof-lib/src/main/cpp/hotspot/hotspotSupport.cpp

Fix default value

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Comment thread ddprof-lib/src/main/cpp/jvmSupport.inline.h

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Comment thread ddprof-lib/src/main/cpp/jvmSupport.cpp Outdated

zhengyu123 added 3 commits June 11, 2026 00:42

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Deal with <cinit>

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Comment thread ddprof-lib/src/main/cpp/hotspot/hotspotSupport.h

Fix

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Comment thread ddprof-lib/src/main/cpp/jvmSupport.cpp

Postponding jmethodID loading decision till execution

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Fix jdk8 patching

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Revert "Fix"

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This reverts commit b41f1b3.

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		// Prepare JVMSupport for execution
		JVMSupport::initExecution(VM::jvmti(), VM::jni());

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