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

zhengyu123 · May 27, 2026

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!

datadog-official · May 27, 2026

✨ Fix all issues with BitsAI

⚠️ Warnings

🚦 14 Pipeline jobs failed

DataDog/java-profiler | benchmarks-candidate-aarch64: [alloc]

DataDog/java-profiler | benchmarks-candidate-aarch64: [cpu,wall,alloc,memleak]

DataDog/java-profiler | benchmarks-candidate-aarch64: [cpu,wall]

View all 14 failed jobs.

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dd-octo-sts · May 27, 2026

CI Test Results

Run: #28370463075 | Commit: 3143794 | Duration: 13m 46s (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: 2026-06-29 12:17:55 UTC

dd-octo-sts · May 31, 2026

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

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dd-octo-sts · Jun 23, 2026

Reliability & Chaos Results

❌ 1 failure(s) detected Pipeline: https://gitlab.ddbuild.io/DataDog/java-profiler/-/pipelines/120816936

❌ chaos: profiler tcmalloc amd64 21 0 3 temXchaos

Chaos harness crashed (RC=124)
siginfo: si_signo: 11 (SIGSEGV), si_code: 2 (SEGV_ACCERR), si_addr: 0x0000753510090e20
# C  [libjavaProfiler-dd-tmp10927298120611365694.so+0x56390]  LinuxThreadList::next()+0x0

Co-authored-by: Roman Kennke <roman.kennke@datadoghq.com>

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… hs_jmethod_id

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chatgpt-codex-connector · Jun 29, 2026

+        const char* slash = strrchr(signature, '/');
+        hidden = slash != nullptr && slash[1] >= '0' && slash[1] <= '9';


Detect hidden classes from JVMTI signatures

With cstack=vm,fjmethodid=false, this check still misses ordinary Lookup.defineHiddenClass classes because JVMTI GetClassSignature exposes hidden names with a dot before the hidden suffix (for example Lp/Foo.0x...;), not a slash followed by a digit. Fresh evidence in this revision is the new slash-based signature check here; for a non-lambda hidden class loaded by the system/application loader, loadMethodIDsIfNeededImpl() can return without GetClassMethods, leaving unloadable raw Method* frames that may be stale or resolve as unknown at dump time.

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chatgpt-codex-connector · Jun 29, 2026

+        method_id = jni->GetMethodID(clz, method_name, method_signature);
+        if (method_id == nullptr) {
+          jni->ExceptionClear();
+          method_id = jni->GetStaticMethodID(clz, method_name, method_signature);


Bypass JNI lookups for raw frames

When a raw <clinit> frame is dumped while that class initializer is still running on another thread, these JNI lookups run before the <clinit> fallback even though JNI cannot return a class-initializer method ID. Fresh evidence in this revision is that the strcmp(method_name, "<clinit>") check remains nested after GetMethodID/GetStaticMethodID; JNI method-ID lookup can trigger or wait for class initialization, so the dump path can block behind the initializer it is trying to resolve instead of going straight to the JVMTI GetClassMethods fallback.

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zhengyu123 added 2 commits May 27, 2026 18:20

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

5f5fa35

v1

4e3bf98

zhengyu123 added 2 commits May 28, 2026 13:29

v2

957d27c

v3

14728a7

zhengyu123 added 11 commits June 1, 2026 12:49

Clean up

64e183b

Fix

c782c3c

Add application/platform class loader support

c462d85

Fix

7710c6f

Fix

6c6f1a7

Harden symbolAt()

07c3a4c

Merge branch 'main' into hs_jmethod_id

4827c68

force_jmethodID argument

3dda4e3

Consolidate JVM initialization

a826e7e

Cleanup: fixed typo and etc.

9d1818a

Remove dead code

13349c3

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

zhengyu123 requested a review from a team as a code owner 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

b95c88c

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

Fix

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

zhengyu123 requested a review from jbachorik June 2, 2026 20:03

Fix

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Comment thread ddprof-lib/src/main/cpp/arguments.h Outdated

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

Fix initExecution order

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Update ddprof-lib/src/main/cpp/hotspot/hotspotSupport.cpp

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Co-authored-by: Roman Kennke <roman.kennke@datadoghq.com>

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zhengyu123 added 2 commits June 24, 2026 15:48

Fix and comments

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Merge branch 'hs_jmethod_id' of github.com:DataDog/java-profiler into…

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… hs_jmethod_id

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Merge branch 'main' into hs_jmethod_id

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		const char* slash = strrchr(signature, '/');
		hidden = slash != nullptr && slash[1] >= '0' && slash[1] <= '9';

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