ADDITIONAL SYSTEM INFORMATION :
Linux x64
OpenJDK Runtime Environment (build 21-ea+23-1988)
OpenJDK 64-Bit Server VM (build 21-ea+23-1988, mixed mode, sharing)

A DESCRIPTION OF THE PROBLEM :
Enabling class load or similar events slows down vthread creation by factor 10 compared to just enabling JVMTI, even when no classes are loaded anymore. Enabling JVMTI alone is only a factor two compared to running without JVMTI. Adding or not adding the can_support_virtual_threads capability does not matter.

Some timing with the attached sample:

$JAVA_HOME/bin/java VThreadClassLoad
duration: 1205 ms

LD_LIBRARY_PATH=. $JAVA_HOME/bin/java -agentlib:VThreadClassLoad=noevent VThreadClassLoad
vthread capability added
class load event not enabled
duration: 2506 ms

LD_LIBRARY_PATH=. $JAVA_HOME/bin/java -agentlib:VThreadClassLoad VThreadClassLoad
vthread capability added
class load event enabled
duration: 36212 ms

LD_LIBRARY_PATH=. $JAVA_HOME/bin/java -agentlib:VThreadClassLoad=nocap VThreadClassLoad
vthread capability not added
class load event enabled
duration: 36043 ms

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
With the attached sample and JAVA_HOME set to a JDK 21:

g++ -shared -I$JAVA_HOME/include -I$JAVA_HOME/include/linux -fPIC VThreadClassLoad.cpp -o libVThreadClassLoad.so
$JAVA_HOME/bin/javac VThreadClassLoad.java

LD_LIBRARY_PATH=. $JAVA_HOME/bin/java -agentlib:VThreadClassLoad VThreadClassLoad




EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Timings roughly similar to just enabling JVMTI
ACTUAL -
Timings more than 10 times slower than just enabling JVMTI

---------- BEGIN SOURCE ----------
VThreadClassLoad.java ------------------------------------------
import java.time.Duration;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class VThreadClassLoad {
    public static void main(String[] args) {
        for (int i=0; i<3; i++) {
            work(); // warmup
        }
        long startTime = System.nanoTime();
        work();
        System.out.println("duration: " + Duration.ofNanos(System.nanoTime() - startTime).toMillis() + " ms");
    }

    private static void work() {
        try (ExecutorService executorService = Executors.newVirtualThreadPerTaskExecutor()) {
            for (int i = 0; i < 10_000_000; i++) {
                executorService.execute(() -> {});
            }
        }
    }
}
------------------------------------------------------
VThreadClassLoad.cpp -----------------------------
#include <jvmti.h>
#include <cstdlib>
#include <cstring>
#include <vector>

namespace {
    jvmtiEnv *jvmti = nullptr;

    void checkJvmti(int code, const char* message) {
        if (code != JVMTI_ERROR_NONE) {
            printf("Error %s: %d\n", message, code);
            abort();
        }
    }

    void JNICALL classLoad(jvmtiEnv *jvmti_env, JNIEnv* jni_env, jthread thread, jclass klass) {
    }
}

extern "C" JNIEXPORT jint JNICALL Agent_OnLoad(JavaVM *vm, char *options, void *reserved) {
    if (vm->GetEnv(reinterpret_cast<void **>(&jvmti), JVMTI_VERSION) != JNI_OK || !jvmti) {
        printf("Could not initialize JVMTI\n");
        abort();
    }
    if (options && strstr(options, "nocap")) {
        printf("vthread capability not added\n");
} else {
jvmtiCapabilities capabilities;
memset(&capabilities, 0, sizeof(capabilities));
capabilities.can_support_virtual_threads = 1;
checkJvmti(jvmti->AddCapabilities(&capabilities), "adding capabilities");
        printf("vthread capability added\n");
}

    if (options && strstr(options, "noevent")) {
        printf("class load event not enabled\n");
    } else {
        jvmtiEventCallbacks callbacks;
        memset(&callbacks, 0, sizeof(callbacks));
        callbacks.ClassLoad = &classLoad;
        checkJvmti(jvmti->SetEventCallbacks(&callbacks, (jint)sizeof(callbacks)), "setting callbacks");
        checkJvmti(jvmti->SetEventNotificationMode(JVMTI_ENABLE, JVMTI_EVENT_CLASS_LOAD, nullptr), "enabling class load event");
        printf("class load event enabled\n");
    }
    fflush(stdout);
    return JVMTI_ERROR_NONE;
}
---------------------------------------

---------- END SOURCE ----------

FREQUENCY : always