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<p class="MsoNormal">Hi <span style="font-size:10.0pt">Michihiro,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt">We are testing this patch on aarch64, and are seeing an (inexplicable) performance regression with G1GC on SPECjbb. The major difference in the generated code is removing a barrier as expected.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt">We are investigating further, and will retest to rule out user error, but wanted throw out a caution flag.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt"><o:p> </o:p></span></p>
<ul style="margin-top:0in" type="disc">
<li class="MsoListParagraph" style="margin-left:-15.6pt;mso-list:l0 level1 lfo2">
Derek<o:p></o:p></li></ul>
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<p class="MsoNormal"><b>From:</b> hotspot-gc-dev [mailto:hotspot-gc-dev-bounces@openjdk.java.net]
<b>On Behalf Of </b>Michihiro Horie<br>
<b>Sent:</b> Tuesday, June 12, 2018 8:17 AM<br>
<b>To:</b> Thomas Schatzl <thomas.schatzl@oracle.com><br>
<b>Cc:</b> david.holmes@oracle.com; hotspot-gc-dev@openjdk.java.net<br>
<b>Subject:</b> Re: RFR(M): 8204524: Unnecessary memory barriers in G1ParScanThreadState::copy_to_survivor_space<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p><span style="font-size:10.0pt">Hi Thomas,</span><br>
<br>
<span style="font-size:10.0pt">Thank you for telling that some people get a problem in handle_evacuation_failure.</span><br>
<br>
<span style="font-size:10.0pt">The handle_evacuation_failure is invoked from copy_to_survivor_space and also keeps the following two:</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:l1 level1 lfo1">
<span style="font-size:10.0pt">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:l1 level1 lfo1">
<span style="font-size:10.0pt">Threads that failed in the CAS must not dereference the forwardee.</span><o:p></o:p></li></ul>
<p class="MsoNormal"><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><br>
<br>
<img width="16" height="16" style="width:.1666in;height:.1666in" id="_x0000_i1025" src="cid:image001.gif@01D4030F.89DAFD10" alt="Inactive hide details for Thomas Schatzl ---2018/06/12 20:26:54---Hi, On Tue, 2018-06-12 at 20:18 +0900, Michihiro Horie wrote:"><span style="font-size:10.0pt;color:#424282">Thomas
Schatzl ---2018/06/12 20:26:54---Hi, On Tue, 2018-06-12 at 20:18 +0900, Michihiro Horie wrote:</span><br>
<br>
<span style="font-size:10.0pt;color:#5F5F5F">From: </span><span style="font-size:10.0pt">Thomas Schatzl <<a href="mailto:thomas.schatzl@oracle.com">thomas.schatzl@oracle.com</a>></span><br>
<span style="font-size:10.0pt;color:#5F5F5F">To: </span><span style="font-size:10.0pt">Michihiro Horie <<a href="mailto:HORIE@jp.ibm.com">HORIE@jp.ibm.com</a>>, "Doerr, Martin" <<a href="mailto:martin.doerr@sap.com">martin.doerr@sap.com</a>></span><br>
<span style="font-size:10.0pt;color:#5F5F5F">Cc: </span><span style="font-size:10.0pt">"<a href="mailto:david.holmes@oracle.com">david.holmes@oracle.com</a>" <<a href="mailto:david.holmes@oracle.com">david.holmes@oracle.com</a>>, "<a href="mailto:hotspot-gc-dev@openjdk.java.net">hotspot-gc-dev@openjdk.java.net</a>"
<<a href="mailto:hotspot-gc-dev@openjdk.java.net">hotspot-gc-dev@openjdk.java.net</a>></span><br>
<span style="font-size:10.0pt;color:#5F5F5F">Date: </span><span style="font-size:10.0pt">2018/06/12 20:26</span><br>
<span style="font-size:10.0pt;color:#5F5F5F">Subject: </span><span style="font-size:10.0pt">Re: RFR(M): 8204524: Unnecessary memory barriers in G1ParScanThreadState::copy_to_survivor_space</span><o:p></o:p></p>
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<tt><span style="font-size:10.0pt">Hi,</span></tt><span style="font-size:10.0pt;font-family:"Courier New""><br>
<br>
<tt>On Tue, 2018-06-12 at 20:18 +0900, Michihiro Horie wrote:</tt><br>
<tt>> Hi Martin,</tt><br>
<tt>> </tt><br>
<tt>> Thank you for your comments. Yes, this change is significant on PPC64</tt><br>
<tt>> as I showed a big improvement in SPECjbb2015 (27% better critical-</tt><br>
<tt>> jOPS).</tt><br>
<tt>> </tt><br>
<tt>> Changing the handle_evacuation_failure_par is not necessary. I could</tt><br>
<tt>> not observe the performance bottleneck in</tt><br>
<tt>> handle_evacuation_failure_par from the profiles,</tt><br>
<tt>> </tt><br>
<tt>> New webrev: <a href="http://cr.openjdk.java.net/~mhorie/8204524/webrev.01/">
http://cr.openjdk.java.net/~mhorie/8204524/webrev.01/</a></tt><br>
<tt>> </tt><br>
<br>
<tt> of course handle_evacuation_failure_par() will not show up in the</tt><br>
<tt>profiles if there is no evacuation failure.</tt><br>
<br>
<tt>However during evacuation failure, this method will be stressed a lot.</tt><br>
<br>
<tt>While from the outside it might not look like it is worth optimizing,</tt><br>
<tt>lack of performance in evacuation failure has been a rather often</tt><br>
<tt>voiced complaint (for the users where this occurs).</tt><br>
<br>
<tt>This is only a side remark: I have not looked through the code for</tt><br>
<tt>issues due to relaxing memory order recently, but I remember having</tt><br>
<tt>done so when the similar change for parallel gc had been proposed last</tt><br>
<tt>year.</tt><br>
<tt>I remember that Michihiro's reasoning why this works for G1 the way it</tt><br>
<tt>does is sound, but as mentioned please wait for a proper review.</tt><br>
<br>
<tt>Thanks,</tt><br>
<tt> Thomas</tt><br>
<br>
<br>
<br>
</span><o:p></o:p></p>
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