Jvm options - PrintNMTStatistics save info to file

Question

I use -XX:+PrintNMTStatistics to get Memory Status.

but it can't be store in file

so how can i save those info to a file ?

Answer 1

-XX:+PrintNMTStatistics prints to JVM console, and this can't be changed.

However, it is possible to create your own utility that will dump NMT statistics to a file before JVM exits.

The idea is similar to this answer: create a JVMTI agent that intercepts VMDeath event and then uses JVM Management Interface to call VM.native_memory diagnostic command.

#include <jvmti.h>
#include <stdio.h>
#include <string.h>

static const char* filename;

extern void* JNICALL JVM_GetManagement(jint version);

static void* get_management_func() {
    void** jmm;
    if ((jmm = JVM_GetManagement(0x20010000)) != NULL) return jmm[39];
    if ((jmm = JVM_GetManagement(0x20020000)) != NULL) return jmm[37];
    if ((jmm = JVM_GetManagement(0x20030000)) != NULL) return jmm[38];
    return NULL;
}

static void save_to_file(const char* result_str) {
    FILE* f = fopen(filename, "w");
    if (f != NULL) {
        fprintf(f, "%s", result_str);
        fclose(f);
    } else {
        fprintf(stderr, "%s", result_str);
    }
}

void JNICALL VMDeath(jvmtiEnv* jvmti, JNIEnv* env) {
    jstring (JNICALL *func)(JNIEnv*, jstring) = get_management_func();
    if (func == NULL) {
        fprintf(stderr, "JMM interface is not supported on this JVM\n");
        return;
    }

    jstring cmd = (*env)->NewStringUTF(env, "VM.native_memory summary");
    jstring result = func(env, cmd);
    if (result != NULL) {
        const char* result_str = (*env)->GetStringUTFChars(env, result, NULL);
        save_to_file(result_str);
        (*env)->ReleaseStringUTFChars(env, result, result_str);
    }
}

JNIEXPORT jint JNICALL Agent_OnLoad(JavaVM* vm, char* options, void* reserved) {
    filename = options != NULL ? strdup(options) : "";

    jvmtiEnv* jvmti;
    (*vm)->GetEnv(vm, (void**)&jvmti, JVMTI_VERSION_1_0);

    jvmtiEventCallbacks callbacks = {0};
    callbacks.VMDeath = VMDeath;
    (*jvmti)->SetEventCallbacks(jvmti, &callbacks, sizeof(callbacks));
    (*jvmti)->SetEventNotificationMode(jvmti, JVMTI_ENABLE, JVMTI_EVENT_VM_DEATH, NULL);

    return 0;
}

Save the above code to nmtdump.c and compile it:

gcc -I $JAVA_HOME/include -I $JAVA_HOME/include/linux -shared -fPIC -olibnmtdump.so nmtdump.c

Then run Java with our agent:

java -XX:NativeMemoryTracking=summary -agentpath:/path/to/libnmtdump.so=dump.txt ...

Now, when JVM exits, it will save NMT report to dump.txt.