如何排查JNI调用和内存分配问题

最近在工作中遇到GC问题，发现GC日志中存在很多[GC pause (GCLocker Initiated GC) (young)的日志，另外还发现特别多的大对象分配:

 56123.146: [G1Ergonomics (Concurrent Cycles) request concurrent cycle initiation, reason: occupancy higher than threshold, occupancy: 2415919104 bytes, allocation request: 3718600 bytes, threshold: 2415919095 bytes (45.00 %), source: concurrent humongous allocation]

在排查过程中用到了些排查工具，特记录下备忘；

snappy

项目中使用了snappy做压缩和解压缩，系统的负载很高：

JNIEXPORT jint JNICALL Java_org_xerial_snappy_SnappyNative_rawCompress__Ljava_lang_Object_2IILjava_lang_Object_2I
  (JNIEnv * env, jobject self, jobject input, jint inputOffset, jint inputLen, jobject output, jint outputOffset)
{
    char* in = (char*) env->GetPrimitiveArrayCritical((jarray) input, 0);
    char* out = (char*) env->GetPrimitiveArrayCritical((jarray) output, 0);
    if(in == 0 || out == 0) {
        // out of memory
        if(in != 0) {
            env->ReleasePrimitiveArrayCritical((jarray) input, in, 0);
        }
        if(out != 0) {
            env->ReleasePrimitiveArrayCritical((jarray) output, out, 0);
        }
        throw_exception(env, self, 4);
        return 0;
    }

    size_t compressedLength;
    snappy::RawCompress(in + inputOffset, (size_t) inputLen, out + outputOffset, &compressedLength);

    env->ReleasePrimitiveArrayCritical((jarray) input, in, 0);
    env->ReleasePrimitiveArrayCritical((jarray) output, out, 0);

    return (jint) compressedLength;
}

JNI_ENTRY(void*, jni_GetPrimitiveArrayCritical(JNIEnv *env, jarray array, jboolean *isCopy))
  JNIWrapper("GetPrimitiveArrayCritical");
  GC_locker::lock_critical(thread);
  if (isCopy != NULL) {
    *isCopy = JNI_FALSE;
  }
  oop a = JNIHandles::resolve_non_null(array);
  BasicType type;
  if (a->is_objArray()) {
    type = T_OBJECT;
  } else {
    type = TypeArrayKlass::cast(a->klass())->element_type();
  }
  void* ret = arrayOop(a)->base(type);
  return ret;
JNI_END

JNI_ENTRY(void, jni_ReleasePrimitiveArrayCritical(JNIEnv *env, jarray array, void *carray, jint mode))
  JNIWrapper("ReleasePrimitiveArrayCritical");
  // The array, carray and mode arguments are ignored
  GC_locker::unlock_critical(thread);

JNI_END

可以看到snappy最终使用了GC_locker::lock_critical(thread)和GC_locker::unlock_critical(thread)来进出JNI临界区;

JNI相关日志参数

1. verbose:jni 打印JNI调用日志
2. PrintJNIGCStalls: 打印线程进出JNI临界区日志

async-profiler

项目地址：https://github.com/jvm-profiling-tools/async-profiler

常用命令:

1. ./profiler.sh -d 20 -f 2449-alloc.svg -e alloc 2449
2. ./profiler.sh -d 10 -f humongous.jfr -e alloc 2449

对于jfr文件，可以用java自带的jfr命令来进行分析

jfr print humongous.jfr
jfr summary humongous.jfr
jfr print --events jdk.ObjectAllocationOutsideTLAB humongous.jfr