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Ivan,<br>
<br>
You're correct about this bug.<br>
<br>
Is it correct that you're covered by the SAP contributor<br>
agreement?<br>
<br>
Have you filed a bug report on this?<br>
<br>
Have you checked the other GC's for this problem?<br>
<br>
I can sponsor this fix.<br>
<br>
Thanks.<br>
<br>
Jon<br>
<br>
<br>
<br>
<div class="moz-cite-prefix">On 9/8/2015 2:21 AM, Galkin, Ivan
wrote:<br>
</div>
<blockquote
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<div class="WordSection1">
<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>
</div>
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