6866591: G1: print update buffer processing stats more often

Summary: It adds parameter -XX:+G1SummarizeRSetStatsPeriod that causes update buffer processing information to be printed periodically. It also includes two small formatting changes.
Reviewed-by: jmasa, jcoomes, ysr

src/share/vm/gc_implementation/g1/g1CollectedHeap.cpp

@@ -2845,6 +2845,11 @@ G1CollectedHeap::do_collection_pause_at_safepoint() {
   if (PrintHeapAtGC) {
     Universe::print_heap_after_gc();
   }
+  if (G1SummarizeRSetStats &&
+      (G1SummarizeRSetStatsPeriod > 0) &&
+      (total_collections() % G1SummarizeRSetStatsPeriod == 0)) {
+    g1_rem_set()->print_summary_info();
+  }
 }
 
 void G1CollectedHeap::set_gc_alloc_region(int purpose, HeapRegion* r) {

src/share/vm/gc_implementation/g1/g1_globals.hpp

@@ -74,6 +74,12 @@
   diagnostic(bool, G1SummarizeRSetStats, false,                             \
           "Summarize remembered set processing info")                       \
                                                                             \
+  diagnostic(intx, G1SummarizeRSetStatsPeriod, 0,                           \
+          "The period (in number of GCs) at which we will generate "        \
+          "update buffer processing info "                                  \
+          "(0 means do not periodically generate this info); "              \
+          "it also requires -XX:+G1SummarizeRSetStats")                     \
+                                                                            \
   diagnostic(bool, G1SummarizeZFStats, false,                               \
           "Summarize zero-filling info")                                    \
                                                                             \