RFR: [XS] 8228658: test GetTotalSafepointTime.java fails on fast Linux machines with Total safepoint time 0 ms

[Jean Christophe Beyler]

Hi Matthias,

I wonder if you should not do what David is suggesting and then put that
whole code (the while loop) in a helper method. Below you have a
calculation again using value2 (which I wonder what the added value of it
is though) but anyway, that value2 could also be 0 at some point, no?

So would it not be best to just refactor the getAllStackTraces and
calculate safepoint time in a helper method for both value / value2
variables?

Thanks,
Jc

On Mon, Jul 29, 2019 at 7:50 PM David Holmes <david.holmes at oracle.com>
wrote:

> Hi Matthias,
>
> On 29/07/2019 8:20 pm, Baesken, Matthias wrote:
> > Hello , please review this small test fix .
> >
> > The test
> test/jdk/sun/management/HotspotRuntimeMBean/GetTotalSafepointTime.java
> fails sometimes on fast Linux machines with this error message :
> >
> > java.lang.RuntimeException: Total safepoint time illegal value: 0 ms
> (MIN = 1; MAX = 9223372036854775807)
> >
> > looks like the total safepoint time is too low currently on these
> machines, it is < 1 ms.
> >
> > There might be several ways to handle this :
> >
> >    *   Change the test  in a way that it might generate nigher safepoint
> times
> >    *   Allow  safepoint time  == 0 ms
> >    *   Offer an additional interface that gives  safepoint times  with
> finer granularity ( currently the HS has safepoint time values in ns , see
> jdk/src/hotspot/share/runtime/safepoint.cpp   SafepointTracing::end
> >
> > But it is converted on ms in this code
> >
> > 114jlong RuntimeService::safepoint_time_ms() {
> > 115  return UsePerfData ?
> > 116    Management::ticks_to_ms(_safepoint_time_ticks->get_value()) : -1;
> > 117}
> >
> > 064jlong Management::ticks_to_ms(jlong ticks) {
> > 2065  assert(os::elapsed_frequency() > 0, "Must be non-zero");
> > 2066  return (jlong)(((double)ticks / (double)os::elapsed_frequency())
> > 2067                 * (double)1000.0);
> > 2068}
> >
> >
> >
> > Currently I go for  the first attempt (and try to generate  higher
> safepoint times in my patch) .
>
> Yes that's probably best. Coarse-grained timing on very fast machines
> was bound to eventually lead to problems.
>
> But perhaps a more future-proof approach is to just add a do-while loop
> around the stack dumps and only exit when we have a non-zero safepoint
> time?
>
> Thanks,
> David
> -----
>
> > Bug/webrev :
> >
> > https://bugs.openjdk.java.net/browse/JDK-8228658
> >
> > http://cr.openjdk.java.net/~mbaesken/webrevs/8228658.0/
> >
> >
> > Thanks, Matthias
> >
>


-- 

Thanks,
Jc
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