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<p class="MsoNormal"><span style="color:#1F497D">Hello Jon,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">thank you for your review and your support.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Besides CMS the function check_gc_overhead_limit() is called only in PSMarkSweep, PSParallelCompact and PSScavenge:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">* PSMarkSweep, PSParallelCompact use the formula “young_gen->max_size() - young_gen->from_space()->capacity_in_bytes() - young_gen->to_space()->capacity_in_bytes()”, which is correct because PSYoungGen::max_size()
== “size in bytes reserved for young generation”<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">* PSScavenge use a similar calculation, however both max_young_size and survivor_limit are recalculated because of MinHeapFreeRation & MaxHeapFreeRatio<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">// START SNIPPET FROM PSScavenge::invoke_no_policy(), see hotspot/src/share/vm/gc/parallel/psScavenge.cpp<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> size_t max_young_size = young_gen->max_size();<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> ...<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> if (MinHeapFreeRatio != 0 || MaxHeapFreeRatio != 100) {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> max_young_size = MIN2(old_gen->capacity_in_bytes() / NewRatio, young_gen->max_size());<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> }<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> ...<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> size_t <b>survivor_limit</b> =<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> size_policy->max_survivor_size(max_young_size);<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> ...<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> assert(<b>young_gen->max_size()</b> ><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> young_gen->from_space()->capacity_in_bytes() +<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> young_gen->to_space()->capacity_in_bytes(),<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> "Sizes of space in young gen are out-of-bounds");<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> ...<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> size_t max_eden_size =
<b>max_young_size</b> -<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> young_gen->from_space()->capacity_in_bytes() -<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> young_gen->to_space()->capacity_in_bytes();<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">// END SNIPPET FROM PSScavenge::invoke_no_policy()<o:p></o:p></span></p>
<p class="MsoNormal" style="text-indent:36.0pt"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal" style="text-indent:36.0pt"><span style="color:#1F497D">I’m not ably to say if there is an error in the calculation of max_eden_size, however I’m wondering a) why recalculated survivor_limit is not used and b) what is the marked assertion
good for?<o:p></o:p></span></p>
<p class="MsoNormal" style="text-indent:36.0pt"><span style="color:#1F497D">In my opinion the calculation should look as following. I didn’t test the change. It’s only deduced, so every comment would be welcome.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">diff -r f74b3ce62e1f src/share/vm/gc/parallel/psScavenge.cpp<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">--- a/src/share/vm/gc/parallel/psScavenge.cpp Fri Sep 04 17:33:56 2015 -0700<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">+++ b/src/share/vm/gc/parallel/psScavenge.cpp Thu Sep 10 12:30:19 2015 +0200<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">@@ -560,18 +560,14 @@<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> if (UseAdaptiveGenerationSizePolicyAtMinorCollection &&<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> (AdaptiveSizePolicy::should_update_eden_stats(gc_cause))) {<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> // Calculate optimal free space amounts<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">- assert(young_gen->max_size() ><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">- young_gen->from_space()->capacity_in_bytes() +<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">- young_gen->to_space()->capacity_in_bytes(),<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">+ assert(max_young_size > 2 * survivor_limit,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> "Sizes of space in young gen are out-of-bounds");<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> size_t young_live = young_gen->used_in_bytes();<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> size_t eden_live = young_gen->eden_space()->used_in_bytes();<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> size_t cur_eden = young_gen->eden_space()->capacity_in_bytes();<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> size_t max_old_gen_size = old_gen->max_gen_size();<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">- size_t max_eden_size = max_young_size -<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">- young_gen->from_space()->capacity_in_bytes() -<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">- young_gen->to_space()->capacity_in_bytes();<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">+ size_t max_eden_size = max_young_size - 2 * survivor_limit;<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> // Used for diagnostics<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> size_policy->clear_generation_free_space_flags();<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Best regard,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">Ivan<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D">P.S. Your questions about contributor agreement and a bug report were answered by my colleague Volker Simonis<o:p></o:p></span></p>
<p class="MsoNormal"><span style="color:#1F497D"> <o:p>
</o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:windowtext;mso-fareast-language:EN-GB">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:windowtext;mso-fareast-language:EN-GB">
hotspot-gc-dev [mailto:hotspot-gc-dev-bounces@openjdk.java.net] <b>On Behalf Of </b>
Jon Masamitsu<br>
<b>Sent:</b> Mittwoch, 9. September 2015 18:22<br>
<b>To:</b> hotspot-gc-dev@openjdk.java.net<br>
<b>Subject:</b> Re: CMS: wrong max_eden_size for check_gc_overhead_limit<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-bottom:12.0pt">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>
<o:p></o:p></p>
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<p class="MsoNormal">On 9/8/2015 2:21 AM, Galkin, Ivan wrote:<o:p></o:p></p>
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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 which is 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:l2 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:l2 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 happens (see implementation of AdaptiveSizePolicy::check_gc_overhead_limit:
<o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:72.0pt;text-indent:-18.0pt;mso-list:l2 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:l2 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 therefore suggest the following fix (DefNewGeneration:: max_eden_size() already contains the correctly calculated capacity of eden):<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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<p class="MsoNormal"><span style="font-size:12.0pt;font-family:"Times New Roman","serif";mso-fareast-language:EN-GB"><o:p> </o:p></span></p>
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