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<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D">Hi John,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D">thank you very much for your comments. Your last line explains exactly what we are concerned about.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D">Does anybody plan to prevent this situation?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D"><o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D">I don’t want to propose adding StoreLoad barriers in all G1 post barriers because I’d expect undesired performance impact.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D">Would it be feasible to rescan all cards which have been dirtied (at least once) during the next stop-the-world phase?<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D">Maybe anybody has a better idea.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="color:#1F497D">Kind regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="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";color:windowtext;mso-fareast-language:DE">From:</span></b><span lang="EN-US" style="font-size:10.0pt;font-family:"Tahoma","sans-serif";color:windowtext;mso-fareast-language:DE">
John Cuthbertson [mailto:john.cuthbertson@oracle.com] <br>
<b>Sent:</b> Donnerstag, 23. Mai 2013 02:29<br>
<b>To:</b> Doerr, Martin<br>
<b>Cc:</b> hotspot-gc-dev@openjdk.java.net; Mikael Gerdin; 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" style="margin-bottom:12.0pt">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>
<br>
x.f = a;<br>
x.f = b;<br>
<br>
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>
<br>
<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></p>
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<p class="MsoNormal">On 5/17/2013 1:29 AM, Doerr, Martin wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="font-family:"Courier New"">Hi all,</span><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="font-family:"Courier New""> </span><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="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><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="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><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="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><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="font-family:"Courier New""> </span><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="font-family:"Courier New"">Java Thread (mutator) Refinement Thread (G1RemSet::concurrentRefineOneCard_impl calls oops_on_card_seq_iterate_careful)</span><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="font-family:"Courier New""> </span><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="font-family:"Courier New"">(1) store(oop)</span><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="font-family:"Courier New"">( StoreLoad required here ?)</span><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="font-family:"Courier New"">(2) load(card==dirty)</span><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="font-family:"Courier New""> </span><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="font-family:"Courier New""> (1’) store(card==clean)</span><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="font-family:"Courier New""> (2’) StoreLoad barrier</span><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="font-family:"Courier New""> (3’) load(oop)</span><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="font-family:"Courier New""> </span><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="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><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="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><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="font-family:"Courier New""> </span><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span lang="EN-US" style="font-family:"Courier New"">Kind regards,</span><o:p></o:p></p>
<p class="MsoNormal" style="text-autospace:none"><span style="font-family:"Courier New"">Martin</span><o:p></o:p></p>
<p class="MsoNormal"> <o:p></o:p></p>
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