Javacore Overhead
By default, when a Java
Dump (javacore*.txt) is requested under normal
conditions (unless something else already has
exclusive access), the Java Dump waits for exclusive
access meaning that all threads have paused themselves. Therefore, the
javacore pauses the JVM for this time plus the time to produce the
javacore itself and thus has a direct impact on performance. In general,
the duration of this pause is less than a few hundred milliseconds.
Starting with OpenJ9 0.36.0 on non-Windows operating systems (IBM Java 8.0.8.0, IBM Semeru Runtimes 11.0.18.0 & 17.0.6.0) and OpenJ9 0.40.0 on Windows (IBM Java 8.0.8.10, IBM Semeru Runtimes 11.0.20.0 & 17.0.8.0), a line at the end of the javacore shows how long the JVM was paused. For example:
1TIDMPDURATION Approximate time to produce this dump: X msThe overhead of javacores may be gauged in a performance test
environment by adding up these 1TIDMPDURATION values and
dividing by the sum of intervals between javacores.
There have been rare observed cases on Linux of javacores causing 20
second pauses with high system/kernel CPU time during the first 10
seconds. As observed with Linux perf On-CPU
sampling with wall-clock times, the Signal Reporter
thread consumes most of that CPU (with stack frames in various places
such as _raw_spin_unlock_irqrestore,
system_call_after_swapgs, __sigqueue,
do_send_sig_info, __sched_yield, and
__GI___getuid). There is a timeout
of 20 seconds waiting for threads to quiesce and observed cases of
unreliable
Linux sigqueue. If you observe this behavior, please
open a support case to help us understand this issue.
There have been similar observed cases of javacores taking about 20 seconds to produce although with low system/kernel CPU time. Again, if you observe this behavior, please open a support case to help us understand this issue.
A workaround for these long pauses may be to remove the request=preempt
option from the javacore agents which will avoid trying to gather native
stack traces where this signal processing is done:
-Xdump:java:defaults:request=exclusive