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<p>Hi Jungwoo, <br>
<br>
The situation I described only affects concurrent collections with CMS.<br>
If the explicit GC is STW (and thereby recorded as a Full GC in the log)
then you should not be seeing any issues. <br>
<br>
Please make sure that you don't leak classes as it is very easy to miss
due to the implicit reference from Java object->Class->ClassLoader. <br>
<br>
/Mikael<br>
</p>
<br><p>
<p>On 26 August 2013 23:26:12 Jungwoo Ha <jwha@google.com> wrote:</p>
<blockquote type="cite">
<div dir="ltr">Hi Mikael, <div><br></div><div>Does this also happen if the
Explicit GC is STW?</div><div>I have been reported that it also happens in
STW Explicit GC.</div><div>Much rarely though.</div><div><br></div><div>
Jungwoo</div></div><div class="gmail_extra"><br><br><div
class="gmail_quote">On Wed, Aug 14, 2013 at 11:43 PM, Mikael Gerdin <span
dir="ltr"><<a href="mailto:mikael.gerdin@oracle.com"
target="_blank">mikael.gerdin@oracle.com</a>></span> wrote:<br>
<blockquote class="gmail_quote"
style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Hi,<div><div
class="h5"><br>
<br>
On 08/15/2013 02:37 AM, Jungwoo Ha wrote:<br>
<blockquote class="gmail_quote"
style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Hi,<br>
<br>
Is ExplicitGCInvokesConcurrentAnd<u></u>UnloadsClasses guaranteed to unload
all<br>
unused classes?<br>
We installed weak reference on the class objects to see none of the<br>
class objects are leaked.<br>
When using STW Explict GC all of the unused classes are unloaded,<br>
but when using above flag, only some are get unloaded.<br>
I double-checked the GC log and waited until the concurrent GC to finish<br>
its job.<br>
Is this an expected behavior or a bug?<br>
BTW, The leak detecting code using weak reference is used for a long time,<br>
which I think is quite stable.<br>
Any comments are appreciated!<br>
</blockquote>
<br></div></div>
A caveat with CMS and class unloading is that CMS treats all objects in the
young generation as strong roots. You need to make sure that you've
triggered enough young GCs so that your weak references are promoted out of
the young gen before triggering the explicit CMS cycle.<br>
<br>
If you have a small test case you can try reducing the young gen size and
setting MaxTenuringThreshold=1 and see if all the classes get unloaded as
you expected.<br>
<br>
/Mikael<br>
<br>
<blockquote class="gmail_quote"
style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<br>
Thanks,<br>
Jungwoo Ha<br>
<br>
</blockquote>
<br>
</blockquote></div><br></div>
</blockquote>
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