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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Hi Igor,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">we have seen crashes while testing our hotspot 23 based SAPJVM with G1.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">However, there’s no evidence that these crashes are caused by this problem.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">We basically found it by reading code.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Best regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Martin<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> Igor Veresov [mailto:iggy.veresov@gmail.com]
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<b>Sent:</b> Donnerstag, 27. Juni 2013 08:15<br>
<b>To:</b> Doerr, Martin<br>
<b>Cc:</b> John Cuthbertson; hotspot-gc-dev@openjdk.java.net; Braun, Matthias<br>
<b>Subject:</b> Re: G1 question: concurrent cleaning of dirty cards<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Oh, re-read your letter, yup, there seems to be a problem. Have you observed that in practice?<o:p></o:p></p>
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<p class="MsoNormal">igor<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Jun 26, 2013, at 9:27 PM, Igor Veresov <<a href="mailto:iggy.veresov@gmail.com">iggy.veresov@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">The cards that are stored in the buffers are not available for concurrent processing right when they are enqueued. Instead they are passed to the processing threads when the buffer fills up. This passing of the buffer involves signaling
of a condition (like pthread_cond_signal(), literally) that has a write barrier for sure, which would guarantee that the cards in the buffer, and contents of the card table, and the contents of the object are "in sync".<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">The only place in the generated code where there has to be a store-store barrier (for non-TSO architectures) is between the actual field store and the dirtying of the card.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Does this make sense?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">igor<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On May 23, 2013, at 6:12 AM, "Doerr, Martin" <<a href="mailto:martin.doerr@sap.com">martin.doerr@sap.com</a>> wrote:<o:p></o:p></p>
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<o:p></o:p></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Hi John,</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"> </span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">thank you very much for your comments. Your last line explains exactly what we are concerned about.</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Does anybody plan to prevent this situation?</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">I don’t want to propose adding StoreLoad barriers in all G1 post barriers because I’d expect undesired performance impact.</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Would it be feasible to rescan all cards which have been dirtied (at least once) during the next stop-the-world phase?</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Maybe anybody has a better idea.</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"> </span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Kind regards,</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">Martin</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri","sans-serif""> </span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">From:</span></b><span class="apple-converted-space"><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif""> </span></span><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif"">John
Cuthbertson [mailto:john.cuthbertson@<a href="http://oracle.com/">oracle.com</a>]<span class="apple-converted-space"> </span><br>
<b>Sent:</b><span class="apple-converted-space"> </span>Donnerstag, 23. Mai 2013 02:29<br>
<b>To:</b><span class="apple-converted-space"> </span>Doerr, Martin<br>
<b>Cc:</b><span class="apple-converted-space"> </span><a href="mailto:hotspot-gc-dev@openjdk.java.net">hotspot-gc-dev@openjdk.java.net</a>; Mikael Gerdin; Braun, Matthias<br>
<b>Subject:</b><span class="apple-converted-space"> </span>Re: G1 question: concurrent cleaning of dirty cards</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">Hi Martin,<br>
<br>
An enqueued card let's the refinment threads know that the oops spanned by that card need to be walked but we're only interested in the latest contents of the fields in those oops. IOW the oop in (3') doesn't need to be the oop stored in (1). If there's a subsequent
store (3) to the same location then we want the load at (3') to see the lastest contents. For example suppose we have:<br>
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x.f = a;<br>
x.f = b;<br>
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If the application thread sees the card spanning x.f is dirty at the second store then we won't enqueue the card after the second store. As long as the refinement thread sees 'b' when the card is 'refined' then we're OK since we no longer need to add an entry
into the RSet for the region containing a - we do need an entry in the RSet for the region containing b.<br>
<br>
If the application thread sees the card as clean at the second store before the refinement thread loads x.f we have just needlessly enqueued the card again.<br>
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<b>It is only if the application thread sees the card as dirty but the refinement thread reads 'a' then there could be a problem. We have a missing RSet entry for 'b'.</b><br>
<br>
JohnC<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif"">On 5/17/2013 1:29 AM, Doerr, Martin wrote:<o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">Hi all,</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> </span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">we have a question about the interaction between G1 post barriers and the refinement thread's concurrent dirty card cleaning.</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">The case in which the G1 post barrier sees a clean card is obviously not problematic, because it will add an entry in a dirty card queue.</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">However, in case in which the Java thread (mutator thread) sees the card already dirtied, it won’t enqueue the card again. Which is safe as long as its stored oop (1)
is seen and processed (3’) by the parallel refinement after having cleaned the card (1’):</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> </span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">Java Thread (mutator) Refinement Thread (G1RemSet::concurrentRefineOneCard_impl calls oops_on_card_seq_iterate_careful)</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> </span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">(1) store(oop)</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">( StoreLoad required here ?)</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">(2) load(card==dirty)</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> </span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> (1’) store(card==clean)</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> (2’) StoreLoad barrier</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> (3’) load(oop)</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> </span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">So the refinement thread seems to rely on getting the oop which was written BEFORE the (2) load(card==dirty) was observed.</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">We wonder how this ordering is guaranteed? There are no StoreLoad barriers in the Java Thread's path. (StoreLoad ordering needs explicit barriers even on TSO platforms.)</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New""> </span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span lang="EN-US" style="font-size:11.0pt;font-family:"Courier New"">Kind regards,</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Courier New"">Martin</span><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif""> <o:p></o:p></span></p>
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