ThreadMXBean::getCurrentThreadAllocatedBytes

[Daniel D. Daugherty]

On 7/13/18 12:35 PM, Markus Gaisbauer wrote:
> Hello,
>
> I am trying to use ThreadMXBean::getThreadAllocatedBytes 
> (com.sun.management) to get the amount of allocated memory of the 
> current thread in some performance critical code.
>
> Unfortunately, the current implementation can be rather slow and the 
> duration of each call unpredictable. I ran a test in a JVM with 500 
> threads. Depending on which thread was queried, 
> getThreadAllocatedBytes took between 100 ns and 2500 ns.
>
> The root cause of the problem is 
> ThreadsList::find_JavaThread_from_java_tid which performs a linear 
> scan through all Java threads in the current process. The more threads 
> a JVM has, the slower it gets. In the worst case, the thread with the 
> given TID is found as the last entry in the list.
>
> Before Java 10, the oldest thread is the slowest one to query.
> Since Java 10, the youngest thread is the slowest one to query. I 
> think this was a side effect of introducing "Thread Safe Memory 
> Reclamation (Thread-SMR) support".
>
>              Oldest Thread   Youngest Thread
> Java 8             8740 ns             76 ns
> Java 10             109 ns           2485 ns

It is good to see that longest search is much faster. Erik and Robbin
will be pleased since speeding up traversal of the ThreadsList was one
of the things that we tried to do during the Thread-SMR project.

A first step is get a new bug filed that documents the issue with
ThreadMXBean::getThreadAllocatedBytes(). Perhaps Gary or Serguei
will take care of that.

Dan


> A common use case is to query the metric for the current thread (e.g. 
> before and after performing some operation). This case can be 
> optimized by introducing a new method: getCurrentThreadAllocatedBytes.
>
> I created a patch for http://hg.openjdk.java.net/jdk/jdk/ and by using 
> the new method I saw the following improvements in my test:
>              Oldest Thread   Youngest Thread
> Proposal             37 ns             37 ns
>
> This is a 60x improvement over the worst case of the current API. In 
> the best case of the current API, the new method is still 3 times faster.
>
> // based on JVM_SetNativeThreadName in jvm.cpp.
> JVM_ENTRY(jlong, jmm_GetCurrentThreadAllocatedMemory(JNIEnv *env, 
> jobject currentThread))
>   // We don't use a ThreadsListHandle here because the current thread
>   // must be alive.
>   oop java_thread = JNIHandles::resolve_non_null(currentThread);
>   JavaThread* thr = java_lang_Thread::thread(java_thread);
>   if (thread == thr) {
>     // only supported for the current thread
>     return thr->cooked_allocated_bytes();
>   }
>   return -1;
> JVM_END
>
> The proposed method also fixes the problem, that 
> getThreadAllocatedBytes itself allocates some memory on the current 
> thread (two long arrays, 24 bytes) and therefore can slightly skew 
> measurements. The new method, getCurrentThreadAllocatedBytes, returns 
> exactly the same value if it is called twice without allocating any 
> memory between those calls.
>
> I also built a variation of this method that could be used to query 
> allocated memory more efficiently for anyone who already has a 
> java.lang.Thread object:
>
> JVM_ENTRY(jlong, jmm_GetThreadAllocatedMemory(JNIEnv *env, jobject 
> threadObj))
>   // based on code proposedin threadSMR.hpp
> ThreadsListHandle tlh;
>   JavaThread* thr = NULL;
>   bool is_alive = tlh.cv_internal_thread_to_JavaThread(threadObj, 
> &thr, NULL);
>   if (is_alive) {
>     return thr->cooked_allocated_bytes();
>   }
>   return -1;
> JVM_END
>
> This method took 70 ns in my test, which is 85% slower 
> than GetCurrentThreadAllocatedMemory but still 30% faster than the 
> best case of the current API. I currently have no immediate need for 
> this second method, but I think it would also be a valueable addition 
> to the API.
>
> I attached a patch for getCurrentThreadAllocatedBytes. I can create a 
> second patch for also adding 
> getThreadAllocatedMemory(java.lang.Thread) to the API.
>
> I am a first time contributor and I am not 100% sure what process I 
> must follow to get a change like this into OpenJDK. Can someone have a 
> look at my proposal and help me through the process?
>
> Best regards,
> Markus
>

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