On 23/07/2013 6:29 PM, Jaroslav Bachorik wrote:
On 07/23/2013 10:19 AM, David Holmes wrote:
Hi Jaroslav,
On 22/07/2013 9:55 PM, Jaroslav Bachorik wrote:
The java/lang/management/ThreadMXBean/ResetPeakThreadCount.java test
seems to be failing intermittently.
The test checks the functionality of the
j.l.m.ThreadMXBean.resetPeakThreadCount() method. It does so by
capturing the current value of "getPeakThreadCount()", starting a
predefined number of the user threads, stopping them and resetting the
stored peak value and making sure the new peak equals to the number of
the actually running threads.
The main problem is that it is not possible to prevent JVM to start/stop
arbitrary system threads while executing the test. This might lead to
small variations of the reported peak (a short-lived system thread is
started while the batch of the user threads is running) or the expected
number of running threads (again, a short-lived system thread is started
at the moment the test asks for the number of running threads).
Do you know what "system threads" these are? I would not expect VM
internal threads to be counted in getPeakThreadCount(), but even if they
are I can't think of any short-lived threads that get created other than
the Signal handling thread.
Unfortunatelly I don't. Capturing the thread dump at the moment of
discovering the discrepancy seems to to be too late. I tried monitoring
the JVM under the test from external tools but it just brings more
entropy to the result.
We'd need to instrument the thread creation logic to keep a separate
record. Dtrace probes could probably do it - but the problem is getting
the test to fail.
I am completely relying on the JVM native thread accounting to be
correct and accurate - that it reports the thread count peak based on
the real data.
The spec isn't clear but I would only expect these counters to apply to
Java threads not VM internal threads (compiler, gc etc). So I'd really
like to know what thread is messing up this count.
David
-JB-
The patch does not fix those shortcomings as it is not really possible
to do given the nature of the JVM threading system. It rather tries to
relax the conditions while still maintaining the ability to detect
functional problems - eg. decreasing peak without explicitly resetting
it and reporting false number of threads.
The webrev is at:
http://cr.openjdk.java.net/~jbachorik/8020875/webrev.00
Seems reasonable.
David
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Thanks,
-JB-
