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<p class="MsoNormal">Hello all,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I believe the calculation of max_eden_size <span style="color:black">
which is </span>needed to check the GC overhead in CMS is incorrect. Namely in concurrentMarkSweepGeneration.cpp the following expression is used:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="text-indent:36.0pt">size_t max_eden_size = _young_gen->max_capacity() - _young_gen->to()->capacity() - _young_gen->from()->capacity();<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">According to the implementation of DefNewGeneration::max_capacity() the expression can be unfolded as:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> size_t max_eden_size = (“reserved size of young gen” – “size of survivor space”) – “size of survivor space” – “size of survivor space”;<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">So the value becomes too small (survival spaces are accounted 3 times!), which can lead to the following problems:<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo2"><![if !supportLists]><span style="mso-list:Ignore">1.<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]>max_eden_size is too small and GC overhead is wrongfully recognized too early<o:p></o:p></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo2"><![if !supportLists]><span style="mso-list:Ignore">2.<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]>max_eden_size == 0 (all young spaces have the same size; -XX:SurvivorRatio=1) and GC overhead is never
<span style="color:black">happens</span> (see implementation of <span style="color:black">
AdaptiveSizePolicy::</span>check_gc_overhead_limit: <o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:72.0pt;text-indent:-18.0pt;mso-list:l0 level2 lfo2">
<![if !supportLists]><span style="mso-list:Ignore">a.<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]>max_eden_size == 0 leads to mem_free_eden_limit == 0; <o:p>
</o:p></p>
<p class="MsoListParagraph" style="margin-left:72.0pt;text-indent:-18.0pt;mso-list:l0 level2 lfo2">
<![if !supportLists]><span style="mso-list:Ignore">b.<span style="font:7.0pt "Times New Roman"">
</span></span><![endif]>free_in_eden < mem_free_eden_limit is always false, since both are unsigned integers and mem_free_eden_limit is 0)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I would <span style="color:black">therefore </span>suggest the following fix
<span style="color:black">(</span>DefNewGeneration:: max_eden_size() <span style="color:black">
already </span>contains the correctly calculated capacity of eden<span style="color:black">):</span><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">diff -r f74b3ce62e1f src/share/vm/gc/cms/concurrentMarkSweepGeneration.cpp<o:p></o:p></p>
<p class="MsoNormal">--- a/src/share/vm/gc/cms/concurrentMarkSweepGeneration.cpp Fri Sep 04 17:33:56 2015 -0700<o:p></o:p></p>
<p class="MsoNormal">+++ b/src/share/vm/gc/cms/concurrentMarkSweepGeneration.cpp Mon Sep 07 18:08:39 2015 +0200<o:p></o:p></p>
<p class="MsoNormal">@@ -1563,9 +1563,7 @@<o:p></o:p></p>
<p class="MsoNormal"> do_compaction_work(clear_all_soft_refs);<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"> // Has the GC time limit been exceeded?<o:p></o:p></p>
<p class="MsoNormal">- size_t max_eden_size = _young_gen->max_capacity() -<o:p></o:p></p>
<p class="MsoNormal">- _young_gen->to()->capacity() -<o:p></o:p></p>
<p class="MsoNormal">- _young_gen->from()->capacity();<o:p></o:p></p>
<p class="MsoNormal">+ size_t max_eden_size = _young_gen->max_eden_size();<o:p></o:p></p>
<p class="MsoNormal"> GCCause::Cause gc_cause = gch->gc_cause();<o:p></o:p></p>
<p class="MsoNormal"> size_policy()->check_gc_overhead_limit(_young_gen->used(),<o:p></o:p></p>
<p class="MsoNormal"> _young_gen->eden()->used(),<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Could anybody please review and sponsor the change?<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thank you in advance,<o:p></o:p></p>
<p class="MsoNormal">Ivan<o:p></o:p></p>
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