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List: openjdk-serviceability-dev
Subject: Re: Slow class loading when running JVM in debug mode
From: Chris Plummer <chris.plummer () oracle ! com>
Date: 2019-06-21 18:50:40
Message-ID: ed380e5a-e00e-639b-0ec7-2230016a44e1 () oracle ! com
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<div class="moz-cite-prefix">You might also want to have a look at:<br>
<br>
JDK:8214892: Delayed starting of debugging via jcmd<br>
<br>
This should allow you to defer initialization of the debug agent
(and the creation of the loaded classes cache) until you turn on
debugging using the jcmd (and the presumably follow that with an
attach from a debugger).<br>
<br>
Note there is also an after-the-fact CSR in progress for this
which might result in an eventual name change for debug agent
option and jcmd command.<br>
<br>
Chris<br>
<br>
<br>
On 6/21/19 3:06 AM, Egor Ushakov wrote:<br>
</div>
<blockquote type="cite"
cite="mid:074598ed-c337-a0cd-7c78-10e587de8431@jetbrains.com">
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Will have a look, thanks for advices!<br>
<br>
<div class="moz-cite-prefix">On 21-Jun-19 00:42, Chris Plummer
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:f0c4cc00-f277-46ee-1fe9-538b723c3504@oracle.com">
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<div class="moz-cite-prefix">With respect to the specific issue
brought up in <a
href="https://youtrack.jetbrains.com/issue/JBR-1611"
rel="noreferrer" target="_blank" \
moz-do-not-send="true">https://youtrack.jetbrains.com/issue/JBR-1611</a>:<br> <br>
"If we look into the code, we'll see that whenever a new class
is loaded and an event about it is delivered, when a garbage
collection has occurred, classTrack_processUnloads iterates
over all loaded classes to see if any of them have been
unloaded. This leads to O(classCount * gcCount) performance,
which in case of frequent GCs (and they are frequent,
especially the minor ones) is close to O(classCount^2). In
IDEA, we have quite a lot of classes, especially counting all
lambdas, so this results in quite significant overhead."<br>
<br>
The debug agent calls JVMTI GetLoadedClasses() during
initialization to get a cache of all prepared classes. It
keeps that cache up-to-date by getting JVMTI CLASS_PREPARE
events. When there is a gc, the debug agent basically throws
away the cache and creates a new one by calling
GetLoadedClasses() again. It also compares the old and new
caches to determine which classes where unloaded, and
generates JDWP CLASS_UNLOAD events for them (if there is a
debugger attached that wants them).<br>
<br>
It might be possible to defer initialization of the loaded
classes cache (and any maintenance of it) until there is a
debugger attached. I'm not sure if the only reason for the
cache is for delivery of CLASS_UNLOAD events, but at first
glance that appears to be the case.<br>
<br>
Chris<br>
<br>
On 6/20/19 1:31 PM, Volker Simonis wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CA+3eh108bX-diLxb9nYuczWYJsXLrMUKgyzxqrqzQF-jhDv3iQ@mail.gmail.com">
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<div>
<div dir="auto">Hi Egor,</div>
</div>
<div dir="auto"><br>
</div>
<div dir="auto">yes, I‘d say this is something well known. The
reason for this is that a debugging agent will request some
JVMTI capabilities like <span
style="color:rgb(23,43,77);font-family:"DejaVu
Sans",sans-serif;font-size:19px">"can_access_local_variables"</span> \
which can only be requested at JVM startup. However, once these
capabilities have been taken, certain kinds of optimizations
like for example Escape Analysis, can't be done anymore
which results in a performance degradation even if you don't
ever attach a debugger.</div>
<div dir="auto"><br>
</div>
<div dir="auto">We‘ve always enabled debugging in our
commercial SAP JVM and called it „Debugging on Demand" but
usually didn't observed any performance slowdown of more
than 3-5% at most. We've started contributing some of the
improvements we've done to make this possible to the
OpenJDK.</div>
<div dir="auto"><br>
</div>
<div dir="auto">I've not looked into your concrete class
loading use case. Maybe it's possible to improve that. First
you have to check by which JVMTI capability it is triggered
and if that capability is really necessary for debugging and
can only be requested at startup. If the answer is yes, you
can still check if it's not possible to improve the
implementation of that capability in order to get a better
runtime behavior.</div>
<div dir="auto"><br>
</div>
<div dir="auto">Regards,</div>
<div dir="auto">Volker</div>
<div dir="auto"><br>
</div>
<div><br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">Egor Ushakov <<a
href="mailto:egor.ushakov@jetbrains.com"
moz-do-not-send="true">egor.ushakov@jetbrains.com</a>>
schrieb am Do. 20. Juni 2019 um 11:36:<br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">Hi
everyone!<br>
<br>
we have detected that a process started with the debug
agent (even when <br>
debugger is not actually attached) may run significantly
slower than <br>
without the agent, see:<br>
<a href="https://youtrack.jetbrains.com/issue/JBR-1611"
rel="noreferrer" target="_blank"
moz-do-not-send="true">https://youtrack.jetbrains.com/issue/JBR-1611</a><br>
We all thought that without the debugger attached there
should not be a <br>
difference.<br>
Is that something well known? Should we file a bug?<br>
<br>
Thanks,<br>
Egor<br>
<br>
-- <br>
Egor Ushakov<br>
Software Developer<br>
JetBrains<br>
<a href="http://www.jetbrains.com" rel="noreferrer"
target="_blank" \
moz-do-not-send="true">http://www.jetbrains.com</a><br> The Drive to Develop<br>
<br>
</blockquote>
</div>
</div>
</blockquote>
<p><br>
</p>
</blockquote>
<br>
<pre class="moz-signature" cols="72">--
Egor Ushakov
Software Developer
JetBrains
<a class="moz-txt-link-freetext" href="http://www.jetbrains.com" \
moz-do-not-send="true">http://www.jetbrains.com</a> The Drive to Develop</pre>
</blockquote>
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