CMS: wrong max_eden_size for check_gc_overhead_limit

[Galkin, Ivan]

Hello all,

I believe the calculation of max_eden_size which is needed to check the GC overhead in CMS is incorrect. Namely in concurrentMarkSweepGeneration.cpp the following expression is used:

size_t max_eden_size = _young_gen->max_capacity() - _young_gen->to()->capacity() - _young_gen->from()->capacity();

According to the implementation of DefNewGeneration::max_capacity() the expression can be unfolded as:

                size_t max_eden_size = ("reserved size of young gen" - "size of survivor space") - "size of survivor space" - "size of survivor space";

So the value becomes too small (survival spaces are accounted 3 times!), which can lead to the following problems:


1.       max_eden_size is too small and GC overhead is wrongfully recognized too early

2.       max_eden_size == 0 (all young spaces have the same size; -XX:SurvivorRatio=1) and GC overhead is never happens (see implementation of AdaptiveSizePolicy::check_gc_overhead_limit:

a.       max_eden_size == 0 leads to mem_free_eden_limit == 0;

b.      free_in_eden < mem_free_eden_limit is always false, since both are unsigned integers and mem_free_eden_limit is 0)

I would therefore suggest the following fix (DefNewGeneration:: max_eden_size() already contains the correctly calculated capacity of eden):

diff -r f74b3ce62e1f src/share/vm/gc/cms/concurrentMarkSweepGeneration.cpp
--- a/src/share/vm/gc/cms/concurrentMarkSweepGeneration.cpp         Fri Sep 04 17:33:56 2015 -0700
+++ b/src/share/vm/gc/cms/concurrentMarkSweepGeneration.cpp      Mon Sep 07 18:08:39 2015 +0200
@@ -1563,9 +1563,7 @@
   do_compaction_work(clear_all_soft_refs);

   // Has the GC time limit been exceeded?
-  size_t max_eden_size = _young_gen->max_capacity() -
-                         _young_gen->to()->capacity() -
-                         _young_gen->from()->capacity();
+  size_t max_eden_size = _young_gen->max_eden_size();
   GCCause::Cause gc_cause = gch->gc_cause();
   size_policy()->check_gc_overhead_limit(_young_gen->used(),
                                          _young_gen->eden()->used(),

Could anybody please review and sponsor the change?

Thank you in advance,
Ivan
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