I saw Daniels comment too late, but using THREAD instead of JavaThread::current() indeed makes the new method again a bit simpler and faster (now 33 ns instead of 35 ns).
JVM_ENTRY(jlong, jmm_GetCurrentThreadAllocatedMemory(JNIEnv *env)) return THREAD->cooked_allocated_bytes(); JVM_END On Mon, Jul 16, 2018 at 9:42 PM Markus Gaisbauer <markus.gaisba...@gmail.com> wrote: > Hi, > > Thank you for all the help. > > I added the code suggested by Paul and ran my small microbenchmark. The > optimized getThreadAllocatedBytes now takes 57 ns if fed with the ID of the > current thread. All calls that aren't for the current thread will be a bit > slower. As far as I am concerned, adding this optimization probably doesn't > hurt. In fact it would be awesome if this could be backported to Java 8. > > But I am still strongly in favor of adding a special method just for the > current thread: > * It is still faster (35 ns vs 57 ns) > * No heap memory is allocated by this method > * If this method is available, callers can always expect good (constant > time) performance. On the other hand two versions would exist for > getThreadAllocatedBytes. > Library code would have to figure out somehow if this particular JVM has > the slow or fast version. In my own use case, I would only want to get the > metric if it is extremely fast. It's not worth the overhead, if it takes > thousands of nanoseconds. > > I wasn't aware of the existence of JavaThread::current() before. My new > method in management.cpp could be simplified to this: > > JVM_ENTRY(jlong, jmm_GetCurrentThreadAllocatedMemory(JNIEnv *env)) > return JavaThread::current()->cooked_allocated_bytes(); > JVM_END > > This code is not only simpler but also a bit faster. Each call now takes > 35 ns instead of 37 ns. > > Is there a technical reason why no new native methods should be added to > ThreadImpl.java? I saw that getCurrentThreadCpuTime also uses this magic > number 0 to indicate the current thread. Wouldn't it be cleaner to have > two methods instead of using and checking a special number in Java/native > code? > > Best regards, > Markus >
