RFR (XS) 8220228: Improve Shenandoah pacing histogram message

[Aleksey Shipilev]

RFE:
  https://bugs.openjdk.java.net/browse/JDK-8220228

Patch:

diff -r f88fbb5fc085 src/hotspot/share/gc/shenandoah/shenandoahPacer.cpp
> --- a/src/hotspot/share/gc/shenandoah/shenandoahPacer.cpp       Wed Mar 06 12:01:55 2019 +0100
> +++ b/src/hotspot/share/gc/shenandoah/shenandoahPacer.cpp       Wed Mar 06 12:26:06 2019 +0100
> @@ -297,6 +297,12 @@
>  
>      out->print_cr("%7d ms - %7d ms: " SIZE_FORMAT_W(12) SIZE_FORMAT_W(12) " ms", l, r, count, sum);
>    }
>    out->print_cr("%23s: " SIZE_FORMAT_W(12) SIZE_FORMAT_W(12) " ms", "Total", total_count, total_sum);
>    out->cr();
> +  out->print_cr("Pacing delays are measured from entering the pacing code till exiting it. Therefore,");
> +  out->print_cr("observed pacing delays may be higher than the threshold when paced thread spent more");
> +  out->print_cr("time in the pacing code. It usually happens when thread is de-scheduled while paced,");
> +  out->print_cr("OS takes longer to unblock the thread, or JVM experiences the STW pause.");
> +  out->cr();
> +
>  }

Testing: Linux x86_64 build, eyeballing -Xlog:gc+stats

Thanks,
-Aleksey