RFR: 8232588: G1 concurrent System.gc can return early or late

[Kim Barrett]

> On Nov 7, 2019, at 4:02 PM, Kim Barrett <kim.barrett at oracle.com> wrote:
> 
>> On Nov 7, 2019, at 4:58 AM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
>>> New webrevs:
>>> full: https://cr.openjdk.java.net/~kbarrett/8232588/open.01/
>>> incr: https://cr.openjdk.java.net/~kbarrett/8232588/open.01.inc/
>> 
>> Looks good (obviously missing the items discussed here).

After sending out the open.01* set of webrevs I noticed the comment
describing should_do_concurrent_full_gc had fallen out of date a while
ago (should have been updated by JDK-8212657), and could also use a
bit of tidying up.  I'd like to deal with this now, since it's minimal
and I'm waiting on a 2nd reviewer.

Changes are to add missing _g1_periodic_collection to the list and
regularize punctuation there, and improve wording of the description.
Not bothering with a webrev for now; here's the diff:

diff -r f95bdf58fc7e -r a772f9ce0594 src/hotspot/share/gc/g1/g1CollectedHeap.hpp
--- a/src/hotspot/share/gc/g1/g1CollectedHeap.hpp	Wed Nov 06 18:52:16 2019 -0500
+++ b/src/hotspot/share/gc/g1/g1CollectedHeap.hpp	Mon Nov 11 14:36:22 2019 -0500
@@ -256,14 +256,14 @@
 
   G1HRPrinter _hr_printer;
 
-  // It decides whether an explicit GC should start a concurrent cycle
-  // instead of doing a STW GC. Currently, a concurrent cycle is
-  // explicitly started if:
-  // (a) cause == _gc_locker and +GCLockerInvokesConcurrent, or
-  // (b) cause == _g1_humongous_allocation
-  // (c) cause == _java_lang_system_gc and +ExplicitGCInvokesConcurrent.
-  // (d) cause == _dcmd_gc_run and +ExplicitGCInvokesConcurrent.
-  // (e) cause == _wb_conc_mark
+  // Return true if an explicit GC should start a concurrent cycle instead
+  // of doing a STW full GC. A concurrent cycle should be started if:
+  // (a) cause == _gc_locker and +GCLockerInvokesConcurrent,
+  // (b) cause == _g1_humongous_allocation,
+  // (c) cause == _java_lang_system_gc and +ExplicitGCInvokesConcurrent,
+  // (d) cause == _dcmd_gc_run and +ExplicitGCInvokesConcurrent,
+  // (e) cause == _wb_conc_mark,
+  // (f) cause == _g1_periodic_collection and +G1PeriodicGCInvokesConcurrent.
   bool should_do_concurrent_full_gc(GCCause::Cause cause);
 
   // Attempt to start a concurrent cycle with the indicated cause.