JVMCI compiler thread idle state is RUNNABLE
Doug Simon
doug.simon at oracle.com
Wed Sep 7 17:32:02 UTC 2016
> On 07 Sep 2016, at 19:19, Christian Thalinger <cthalinger at twitter.com> wrote:
>
>>
>> On Sep 7, 2016, at 12:34 AM, Doug Simon <doug.simon at oracle.com> wrote:
>>
>>
>>> On 06 Sep 2016, at 23:39, Christian Thalinger <cthalinger at twitter.com> wrote:
>>>
>>> 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?
>>
>> Why is this very confusing? All sorts of non-app threads show up in a SIGQUIT thread dump don’t they?
>
> The confusing part is that JVMCI threads show up but C1/C2 threads don’t. Oh, maybe I wasn’t clear enough. I’m not talking about a SIGQUIT thread dump; I’m talking about the thread list you can get in Java code (JVM_GetAllThreads aka. “external view”).
I don’t know what exactly is meant by “external view” but I see that a bunch of JVMTI methods use is_hidden_from_external_view which, as you observed, returns false for JVMCI compiler threads. That seems to imply that changing the returned value for JVMCI threads will probably make such threads disappear in a Java debugger. Maybe all uses of is_hidden_from_external_view need to make their own decision on whether or not to include CompilerThreads that can_call_java.
-Doug
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