Currently the JVMTI specification only supports two events: Garbage Collection Start and
Garbage Collection Finish. These two events are very useful for tracing garbage collection.
However, the major drawback of these events that they are only useful when stop-the-world
garbage collection algorithm is in use. Many of the modern garbage collector implementations
are either completely or partially parallel or have an option which turns on the parallel garbage
collection.
These implementations are impossible to trace using start/finish JVMTI events. It is desirable
that two more events are introduced: GC Start (per thread) and GC Finish (per thread). It is very
important that JVMTI specification provides fine-grain GC tracing capabilities that would allow
profiling modern GC implementations.
For example, the event callbacks may have the following form:
void JNICALL ParallelGarbageCollectionStart(jvmtiEnv *jvmti_env, jthread
thread);
void JNICALL ParallelGarbageCollectionFinish(jvmtiEnv *jvmti_env, jthread
thread);
ParallelGarbageCollectionStart event is sent immediately before garbage collection on the
thread starts (such as root set enumeration, marking objects etc).
ParallelGarbageCollectionFinish event is sent immediately after garbage collection on the
thread finishes and before it resumes execution of the managed code.
Names of the events are, of course, provided here only to illustrate how extension of the spec
may look like.
Unlike sychnronous (or stop-the-world) garbage collection, precise semantics of these event will
be VM-dependent which makes their interpretation different among different VMs. However, it
is desirable that those events are included as part of the specification as opposed to
implementation extension.