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<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Hi Michihiro,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">I have removed the mailing lists except hotspot-gc-dev which is the one for this review.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">Thank you for taking care of this PPC64 performance problem. I think we shouldn’t ship jdk11 on PPC64 without addressing it.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">I guess handle_evacuation_failure_par is not performance critical, so I wonder if it needs to be part of this change. I haven’t checked if it’s correct.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">Your description and change of copy_to_survivor_space fit to the comments and how the algorithm works. So it looks good to me.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">I couldn’t find any requirement for memory barriers regarding the CAS. But we should have a G1 expert double-check that we haven’t missed anything.<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">Best regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US">Martin<o:p></o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span lang="EN-US" style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> Michihiro Horie [mailto:HORIE@jp.ibm.com]
<br>
<b>Sent:</b> Donnerstag, 7. Juni 2018 08:01<br>
<b>To:</b> hotspot-dev@openjdk.java.net; ppc-aix-port-dev@openjdk.java.net; hotspot-gc-dev@openjdk.java.net<br>
<b>Cc:</b> Kim Barrett <kim.barrett@oracle.com>; Gustavo Bueno Romero <gromero@br.ibm.com>; david.holmes@oracle.com; Erik Osterlund <erik.osterlund@oracle.com>; Doerr, Martin <martin.doerr@sap.com><br>
<b>Subject:</b> RFR(M): 8204524: Unnecessary memory barriers in G1ParScanThreadState::copy_to_survivor_space<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p><span style="font-size:10.0pt;color:#2F2F2F">Dear all,</span><br>
<br>
<span style="font-size:10.0pt;color:#2F2F2F">Would you please review the following change?</span><br>
<br>
<span style="font-size:10.0pt;color:#2F2F2F">Bug: </span><a href="https://bugs.openjdk.java.net/browse/JDK-8204524"><span style="font-size:10.0pt">https://bugs.openjdk.java.net/browse/JDK-8204524</span></a><br>
<span style="font-size:10.0pt;color:#2F2F2F">Webrev: </span><span style="font-size:10.0pt;color:blue"><a href="http://cr.openjdk.java.net/~mhorie/8204524/webrev.00">http://cr.openjdk.java.net/~mhorie/8204524/webrev.00</a></span><br>
<br>
<span style="font-size:10.0pt;color:#2F2F2F">G1ParScanThreadState::copy_to_survivor_space tries to move live objects to a different location. It uses a forwarding technique and allows multiple threads to compete for performing the copy step.</span><span style="font-size:10.0pt;font-family:"MS Gothic";color:#2F2F2F">
</span><br>
<br>
<span style="font-size:10.0pt;color:#2F2F2F">A copy is performed after a thread succeeds in the CAS. CAS-failed threads are not allowed to dereference the forwardee concurrently. Current code is already written so that CAS-failed threads do not dereference
the forwardee. Also, this constraint is documented in a caller function mark_forwarded_object as “the object might be in the process of being copied by another worker so we cannot trust that its to-space image is well-formed”.</span><o:p></o:p></p>
<ul type="disc">
<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo1">
<span style="font-size:10.0pt;color:#2F2F2F">There is no copy that must finish before the CAS.</span><o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo1">
<span style="font-size:10.0pt;color:#2F2F2F">Threads that failed in the CAS must not dereference the forwardee.</span><o:p></o:p></li></ul>
<p class="MsoNormal"><br>
<span style="font-size:10.0pt;color:#2F2F2F">Therefore, no fence is necessary before and after the CAS.</span><br>
<br>
<span style="font-size:10.0pt;color:#2F2F2F">I measured SPECjbb2015 with this change. As a result, critical-jOPS performance improved by 27% on POWER8.</span><br>
<br>
<br>
<span style="font-size:10.0pt">Best regards,</span><br>
<span style="font-size:10.0pt">--</span><br>
<span style="font-size:10.0pt">Michihiro,</span><br>
<span style="font-size:10.0pt">IBM Research - Tokyo</span><o:p></o:p></p>
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