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

This is a great improvement!

Here comes my first pass of (manual) review. It's mostly minor stuff.

rkennke · 2026-06-09T11:21:37Z

        msg = "jstackdepth must be > 0";
      }

+      CASE("fjmethodid")


I find it unintuitive/confusing that in order to turn the new feature on, we have to set the flag to false.

I don't like to name, what do you suggest?

Yeah I don't know either, it seems hard. I could only come up with dont-preload-methodIds or no-preload-methodIds, but it doesn't seem very much better. I leave that up to you.

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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 · 2026-06-10T11:38:00Z

+           strstr(signature, "$$Lambda.") != nullptr ||
+           strstr(signature, "$$Lambda$") != nullptr ||
+           strstr(signature, ".lambda$") != nullptr ||
+           strncmp(signature, FFM_PREFIX, strlen(FFM_PREFIX)) == 0;


you can use sizeof(FFM_PREFIX) - 1 to avoid the runtime call, because the string is a constexpr.

rkennke · 2026-06-10T11:40:40Z

+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+ #include "flightRecorder.h"


leading space

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chatgpt-codex-connector · 2026-06-12T18:32:50Z

+    } else {
+        return loadMethodIDsImpl(jvmti, jni, klass);


Preserve the JDK 8 CLD patch on full preloads

When HotSpot still needs full jmethodID preloading (for example Java 8 with a non-VM cstack mode or any explicit full-preload path), this branch now calls JVMSupport::loadMethodIDsImpl() directly, but that implementation only does GetClassMethods and no longer runs the JDK-8062116 patchClassLoaderData() workaround that the old VM::loadMethodIDs() always performed before preloading. On Java 8 HotSpot this reintroduces the class-loader-data ordering bug for the full-preload path; keep the patch in the unconditional HotSpot preload path as well, or route full HotSpot preloads through a helper that applies it before GetClassMethods.

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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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chatgpt-codex-connector · 2026-06-12T21:40:35Z

+void JVMSupport::setLoadState(JMethodIDLoadStats state) {
+    // Volatile store
+    __atomic_store(&jmethodID_load_state, &state, __ATOMIC_RELAXED);


Publish loader globals before enabling partial preload

When a HotSpot start chooses the partial-preload path, initClassloaderInfo() writes JAVA_PLATFORM_CLASSLOADER/JAVA_APPLICATION_CLASSLOADER and then this relaxed store makes ClassPrepare callbacks start using Partial_loaded. On weakly ordered arm64, a callback thread can observe the new state without seeing the loader globals, so isSystemClassLoader() compares against stale null refs and preloads platform/application classes that the new cstack=vm path is supposed to skip; use a release store paired with acquire loads for this cross-thread publication.

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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

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Add application/platform class loader support

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Fix

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Merge branch 'main' into hs_jmethod_id

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force_jmethodID argument

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Cleanup: fixed typo and etc.

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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

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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

Fix

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Merge branch 'main' into hs_jmethod_id

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

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Deal with <cinit>

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Postponding jmethodID loading decision till execution

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

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