RFR (XS): 8211123: GC Metaspace printing after full gc does not reflect recent changes (Was: Re: A Bug Of Metaspace After Full GC)

[Thomas Schatzl]

Hi,

On Tue, 2018-09-25 at 13:22 -0700, JC Beyler wrote:
> I did say I was not a GC expert :-)
> 
> So the question would be: is the patch I showed based on Nijiaben's
> patch worth fixing and is it right? It seems like the right thing to
> do for at least fixing SerialGC printouts, no?

Yes. Serial GC is fully supported.

I am not sure what release this is reported for, probably 8. Not sure
if it is worth backporting (then again, on 8u both CMS and Serial are
supported).

> Ps: here is the patch for simplifying things for everyone then:
> diff -r 8f66a57054b7 src/hotspot/share/gc/shared/genCollectedHeap.cpp
> --- a/src/hotspot/share/gc/shared/genCollectedHeap.cpp  Tue Sep 25
> 10:30:32 2018 -0700
> +++ b/src/hotspot/share/gc/shared/genCollectedHeap.cpp  Tue Sep 25
> 12:50:54 2018 -0700
> @@ -649,7 +649,6 @@
>      complete = complete || collected_old;
>  
>      print_heap_change(young_prev_used, old_prev_used);
> -    MetaspaceUtils::print_metaspace_change(metadata_prev_used);
>  
>      // Adjust generation sizes.
>      if (collected_old) {
> @@ -665,6 +664,7 @@
>        MetaspaceGC::compute_new_size();
>        update_full_collections_completed();
>      }
> +    MetaspaceUtils::print_metaspace_change(metadata_prev_used);
>  
>      // Track memory usage and detect low memory after GC finishes
>      MemoryService::track_memory_usage();
> 

Looks good. Would it be possible to write a small junit test that
checks that metaspace usage decreased after full gc?

That should be true for all collectors assuming they do not have class
unloading disabled.

Thanks,
  Thomas