JVMCI compiler thread idle state is RUNNABLE

[Christian Thalinger]

One thing we noticed here at Twitter is that JVMCI threads are not hidden (is_hidden_from_external_view) but at the same time they show up as always active.

I don’t know the history here but I’m speculating that since compiler threads were always hidden no-one bothered.  In the SIGQUIT thread dump compiler threads show up as RUNNABLE:

"C1 CompilerThread3" #8 daemon prio=9 os_prio=31 tid=0x00007fdcc2016800 nid=0x5103 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

"C2 CompilerThread2" #7 daemon prio=9 os_prio=31 tid=0x00007fdcc2821800 nid=0x4f03 waiting on condition [0x0000000000000000]
   java.lang.Thread.State: RUNNABLE

The specification of RUNNABLE is:

        /**
         * Thread state for a runnable thread.  A thread in the runnable
         * state is executing in the Java virtual machine but it may
         * be waiting for other resources from the operating system
         * such as processor.
         */
        RUNNABLE,

and that makes sense.  But this is very confusing to the user (as one of our internal users reported to me).  Maybe JVMCI threads should just be hidden, too?
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