RFR: 8036698: Add trace event for updates to metaspace gc threshold

[Mikael Gerdin]

Erik,

On 03/06/2014 09:14 AM, Erik Helin wrote:
> Hi all,
>
> this change adds a new trace event, vm/gc/metaspace/gc_threshold that
> tracks updates to the metaspace GC threshold
> (MetaspaceGC::_capacity_until_gc).
>
> The event contains the old value of MetaspaceGC::_capacity_until_gc, the
> new value and also the named of the method that changed the value.
>
> Enhancement:
> https://bugs.openjdk.java.net/browse/JDK-8036698
>
> Webrev:
> http://cr.openjdk.java.net/~ehelin/8036698/webrev.00/

   61 size_t Metaspace::_compressed_class_space_size;
   62 const MetaspaceTracer* const Metaspace::_tracer = new 
MetaspaceTracer();
   63

I think that in general we try to avoid using static initializers.
MetaspaceTracer is a CHeapObj<mtTracing> so on allocation I believe it 
will attempt to interact with NMT (if enabled).
Can you please move the initialization of _tracer to 
Metaspace::global_initialize
I realize this means _tracer will no longer be a *const but I think 
that's fine compared to static initializer unpredictability.


3230   size_t after_inc = MetaspaceGC::inc_capacity_until_GC(delta_bytes);
3231   size_t before_inc = after_inc - delta_bytes;
3232
3233   tracer()->report_gc_threshold(before_inc, after_inc,
3234        MetaspaceGCThresholdUpdater::ExpandAndAllocate);

Did you consider changing this to get capacity_until_GC() before 
incrementing instead of calculating what it was before based on the delta?
If we ever change inc_capacity_until_GC to not just accept delta_bytes 
as-is this code will break.


I don't really understand the trace*xml changes so I hope someone else 
has a look at them :)

/Mikael

>
> Testing:
> - JFR JTREG tests
> - JPRT
>
> Thanks,
> Erik