On 25/07/2013 12:08 AM, Jaroslav Bachorik wrote:
On 07/24/2013 03:17 PM, Chris Hegarty wrote:
On 24/07/2013 13:49, Jaroslav Bachorik wrote:
On 07/24/2013 02:32 PM, Chris Hegarty wrote:
On 24/07/2013 12:21, David Holmes wrote:
On 24/07/2013 7:31 PM, Mandy Chung wrote:
On 7/24/2013 4:50 PM, shanliang wrote:
So we have 2 kinds of issues here:
1) the test related, like Thread state checking, we can fix them in
the test
2) MBean.getThreadCount() issue, we can create a bug to trace it (add
your test case to the bug), and add a workaround (sleep or call 2
times) in the test to make the test pass. Mandy is the expert and
better to get her opinion.
It's probably a race in the VM implementation in determining the
thread
count. You will need to diagnose the VM implementation and compare the
thread list and the implementation of getting the thread count (check
hotspot/src/share/vm/services/threadService.cpp)
There is a considerable code path between the point where a terminating
thread causes Thread.join() to be allowed to return, and the point
where
the live thread count gets decremented. So using join() does not help
here. Arguably JVMTI should have based its counts around the lifecycle
of the Java thread not the underlying native thread.
It appears, from my reading of the code, that this situation ( a thread
exiting ) should be handled. Or maybe I'm looking at the wrong
interface.
JavaThread::exit(...) {
...
ThreadService::current_thread_exiting(this);
...
ensure_join(..)
...
}
So the exiting thread should be removed from the live thread count
before Thread.join returns.
Unfortunately, ensure_join(...) is called on line 1860 but
Threads::remove(this), which does the actual cleanup of the live threads
counter, is called only on line 1919, leaving at least a few ns window
when the thread is reported as terminated in java but the counters
haven't been updated yet.
Again, maybe I'm missing something but,
static jlong get_live_thread_count() { return
_live_threads_count->get_value() - _exiting_threads_count; }
... and current_thread_exiting(..) increments _exiting_threads_count, no?
Well, apparently it does.
Yes. Thanks Chris I completely missed the use of the
_exiting_threads_count to address this very issue.
I am a complete stranger to the concurrency issues in the hotspot -
would it be possible that in ThreadService::remove_thread(..) the
_exiting_threads_count is decremented but _live_threads_count hasn't
been updated yet when someone calls the get_live_thread_count() function?
Yes. Updates are guarded by acquiring the Threads_lock, but reads are
not. So it is indeed possible to request the live count between the
decrement of the exiting count and the decrement of the live count
itself. Mind you that is an extremely small window of opportunity in
terms of this bug manifesting as often as it does.
Because get_live_thread_count returns the sum of two variables it has to
use the same synchronization as is used to update those variables to
ensure it returns a valid value. We can't grab the Threads_lock directly
in get_live_thread_count as it is already called from code that holds
the lock. So we would have to push this out to management.cpp's
get_long_attribute.
David
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-JB-
-Chris.
-JB-
-Chris.
David
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Mandy
