<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<meta name="Generator" content="Microsoft Word 15 (filtered medium)">
<!--[if !mso]><style>v\:* {behavior:url(#default#VML);}
o\:* {behavior:url(#default#VML);}
w\:* {behavior:url(#default#VML);}
.shape {behavior:url(#default#VML);}
</style><![endif]--><style><!--
/* Font Definitions */
@font-face
{font-family:Helvetica;
panose-1:0 0 0 0 0 0 0 0 0 0;}
@font-face
{font-family:"Cambria Math";
panose-1:2 4 5 3 5 4 6 3 2 4;}
@font-face
{font-family:Aptos;
panose-1:2 11 0 4 2 2 2 2 2 4;}
/* Style Definitions */
p.MsoNormal, li.MsoNormal, div.MsoNormal
{margin:0in;
font-size:12.0pt;
font-family:"Aptos",sans-serif;}
h3
{mso-style-priority:9;
mso-style-link:"Heading 3 Char";
mso-margin-top-alt:auto;
margin-right:0in;
mso-margin-bottom-alt:auto;
margin-left:0in;
font-size:13.5pt;
font-family:"Aptos",sans-serif;}
a:link, span.MsoHyperlink
{mso-style-priority:99;
color:blue;
text-decoration:underline;}
span.Heading3Char
{mso-style-name:"Heading 3 Char";
mso-style-priority:9;
mso-style-link:"Heading 3";
font-family:"Aptos",sans-serif;
color:#0F4761;}
.MsoChpDefault
{mso-style-type:export-only;
font-size:10.0pt;
mso-ligatures:none;}
@page WordSection1
{size:8.5in 11.0in;
margin:1.0in 1.0in 1.0in 1.0in;}
div.WordSection1
{page:WordSection1;}
--></style><!--[if gte mso 9]><xml>
<o:shapedefaults v:ext="edit" spidmax="1026" />
</xml><![endif]--><!--[if gte mso 9]><xml>
<o:shapelayout v:ext="edit">
<o:idmap v:ext="edit" data="1" />
</o:shapelayout></xml><![endif]-->
</head>
<body lang="EN-US" link="blue" vlink="purple" style="word-wrap:break-word">
<div class="WordSection1">
<p class="MsoNormal"><span style="font-size:11.0pt">Hi Chen,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Thanks for the insights.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">We did refactor our code to avoid using LambdaMetaFactory,metafactory() directly.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">With increasing use of Lambdas, in our applications and libraries, the metaspace impact becomes a concern. If current implementation (not able to unload unused Lambda classes) here to stay, we must come up
with a coding guideline to avoid excessive creation of Lambda classes, any pointers or suggestions would be greatly appreciated.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">Best,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt">-Zhengyu<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt"><o:p> </o:p></span></p>
<div id="mail-editor-reference-message-container">
<div>
<div style="border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in">
<p class="MsoNormal" style="margin-bottom:12.0pt"><b><span style="color:black">From:
</span></b><span style="color:black">Chen Liang <liangchenblue@gmail.com><br>
<b>Date: </b>Wednesday, May 29, 2024 at 2:43</span><span style="font-family:"Arial",sans-serif;color:black"> </span><span style="color:black">PM<br>
<b>To: </b>Zhengyu Gu <zhengyu.gu@servicenow.com><br>
<b>Cc: </b>core-libs-dev@openjdk.org <core-libs-dev@openjdk.org><br>
<b>Subject: </b>Re: Question on Lambda function<o:p></o:p></span></p>
</div>
<div>
<div>
<h3><strong><span style="font-family:Helvetica;color:black;background:yellow">[External Email]</span></strong><span style="font-family:Helvetica"><o:p></o:p></span></h3>
<p class="MsoNormal"><span style="font-family:Helvetica"><o:p> </o:p></span></p>
<div class="MsoNormal" align="center" style="text-align:center"><span style="font-family:Helvetica">
<hr size="0" width="100%" align="center">
</span></div>
</div>
<p class="MsoNormal">Hi Gu,</p>
<div>
<p class="MsoNormal">CallSite is specific to each invokedynamic instruction instead of each InvokeDynamic constant pool entry:
<a href="https://docs.oracle.com/javase/specs/jvms/se22/html/jvms-6.html#jvms-6.5.invokedynamic">
https://docs.oracle.com/javase/specs/jvms/se22/html/jvms-6.html#jvms-6.5.invokedynamic</a></p>
</div>
<div>
<p class="MsoNormal">And the linking is done by MethodHandleNatives.linkCallSite if you want to follow the Java implementation code.</p>
</div>
<div>
<p class="MsoNormal">For why the lambda in the loop is constant, it's a feature from InnerClassLambdaMetafactory: <a href="https://github.com/openjdk/jdk/blob/c8eea59f508158075382079316cf0990116ff98e/src/java.base/share/classes/java/lang/invoke/InnerClassLambdaMetafactory.java#L236">https://github.com/openjdk/jdk/blob/c8eea59f508158075382079316cf0990116ff98e/src/java.base/share/classes/java/lang/invoke/InnerClassLambdaMetafactory.java#L236</a></p>
</div>
<div>
<p class="MsoNormal">When the lambda is non-capturing, the bootstrap method LambdaMetafactory.metafactory will eagerly create a singleton instance and return this singleton in the indy instruction.</p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Also, your metaspace pressure might be caused by the fact that Lambda classes (not instances) are no longer eagerly unloaded; see <a href="https://github.com/openjdk/jdk/pull/12493">https://github.com/openjdk/jdk/pull/12493</a> and <a href="https://bugs.openjdk.org/browse/JDK-8302154">https://bugs.openjdk.org/browse/JDK-8302154</a>.
You are recommended to create your own facility to create hidden classes in Java 17 instead of continue to use LambdaMetafactory explicitly in code.</p>
</div>
<div>
<p class="MsoNormal"><o:p> </o:p></p>
</div>
<div>
<p class="MsoNormal">Regards,</p>
</div>
<div>
<p class="MsoNormal">Chen Liang</p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<div>
<div>
<p class="MsoNormal">On Wed, May 29, 2024 at 12:53<span style="font-family:"Arial",sans-serif"> </span>PM Zhengyu Gu <<a href="mailto:zhengyu.gu@servicenow.com">zhengyu.gu@servicenow.com</a>> wrote:</p>
</div>
<blockquote style="border:none;border-left:solid #CCCCCC 1.0pt;padding:0in 0in 0in 6.0pt;margin-left:4.8pt;margin-top:5.0pt;margin-right:0in;margin-bottom:5.0pt">
<div>
<div>
<div>
<div id="m_-5447452561659455976mail-editor-reference-message-container">
<div>
<div>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222;background:white">Hello Lambda experts,</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222">Since we upgraded JDK from 11 to 17, we’re experiencing metaspace pressure, largely due to Lambda class implementation
changes.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222">There’s a scenario (see attached test case), that is especially puzzled me, hopefully, you can share some insights.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222">In this test case, there is only one Lambda class is created inside the loop, but each one for the same functions outside
loop.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222">Example output:</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222">0: Func = LambdaFunc$$Lambda/0x00001f80000c4a20@4de8b406</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222">testMethod() called</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222">1: Func = LambdaFunc$$Lambda/0x00001f80000c4a20@4de8b406</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222">testMethod() called</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222">2: Func = LambdaFunc$$Lambda/0x00001f80000c4a20@4de8b406</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222">testMethod() called</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222">3: Func = LambdaFunc$$Lambda/0x00001f80000c4a20@4de8b406</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222">testMethod() called</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222">4: Func = LambdaFunc$$Lambda/0x00001f80000c4a20@4de8b406</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222">testMethod() called</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif;color:#222222"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">….<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Outside loop1, Func = LambdaFunc$$Lambda/0x00001f80000c4c58@402f32ff<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">testMethod() called<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Outside loop2 Func = LambdaFunc$$Lambda/0x00001f80000d1000@5ae9a829<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">testMethod() called<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Outside loop3 Func = LambdaFunc$$Lambda/0x00001f80000d1238@548b7f67<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">testMethod() called<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">And jcmd also confirmed there were 4 Lambda classes created:<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> 49: CLD 0x000060000134cb50: "app" instance of jdk.internal.loader.ClassLoaders$AppClassLoader<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> Loaded classes:<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> 1: LambdaFunc$$Lambda/0x00001f80000d1238<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> 2: LambdaFunc$$Lambda/0x00001f80000d1000
<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> 3: LambdaFunc$$Lambda/0x00001f80000c4c58<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> 4: LambdaFunc$$Lambda/0x00001f80000c4a20<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> 5: LambdaFunc<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Looking into bytecode, all four call sites have the same invokedynamic bytecode (invokedynamic #7, 0 // InvokeDynamic #0:run:()Ljava/lang/Runnable; ) and the first
invokedynamic bytecode is inside the loop.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">But when I ran the program with -XX:+TraceBytecodes, it seems that the first invokedynamic was hoisted and result was used in the subsequence loop.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Can anyone explain where this magic happens? If the magic can apply to the instances outside the loop, so that only one Lambda class is created?<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Thank you for your time and expertise,<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">-Zhengyu<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
</div>
</div>
</div>
</div>
</div>
</div>
</blockquote>
</div>
</div>
</div>
</div>
</body>
</html>