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https://issues.apache.org/jira/browse/SOLR-13492?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17069290#comment-17069290
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Munendra S N commented on SOLR-13492:
-------------------------------------
I had suggested [~kgsdora] to pick this up. So, trying to address/answer some
of the above concerns
I have tried with jcmd, jconsole, visualvm and
[jmxterm|https://github.com/jiaqi/jmxterm] and with only master branch (java 11)
In case of troubleshooting memory issues, it could be either local debugging or
remote debugging. For production system, usually only remote debugging be
possible.
h2. Local Debugging
h3. jcmd
* Force triggering GC works even with {{DisableExplicitGC}}. We could run
{{GC.run}} (with disable works only with jdk > 10
https://bugs.openjdk.java.net/browse/JDK-8186902) to force gc. There are other
commands to force gc which works in java 8 too {{GC.class_histogram}} and
{{GC.class_stats}}
h3. jconsole and visualvm
* Both comes with GUI. These identify local processes by checking
hsperfdata_{{yourusername}} in tmp directory for pids. At present, GC config
contains {{-XX:+PerfDisableSharedMem}} due to which pids [won't be present|
http://jtuts.com/2017/02/04/jconsole-not-showing-local-processes/] in above
folder. So, with default settings shipped with solr jconsole and visualvm can't
identify local processes
* I tested with removing the above flag and adding {{-XX:+DisableExplicitGC}},
gc now button in jconsole and visualvm doesn't work
h3. jmxterm
* This needs to jmx enabled
h2. Remote debugging
h3. jcmd
* jcmd needs process id. Not sure if remote debugging is possible
h3. jconsole and visualvm
* If the process has jmx monitoring enabled then, remote debugging is
possible. Solr ships with jmx disabled by default
* I tried enabling and with {{-XX:+DisableExplicitGC}} and gc now button won't
trigger gc
* For visualvm, there is option to connect using jstatd but I haven't tried it
h4. jmxterm (terminal tool)
* This needs jmx monitoring enabled
* This would behave similar to jcmd for local even with
{{-XX:+DisableExplicitGC}}, gc can forced
I checked the usage of {{System.gc()}} in lucene/solr. It is used in 1/2 lucene
tests and lucene benchmark. Also, checked potential problems for disabling
explicit gc and found
[this|https://stackoverflow.com/questions/32912702/impact-of-setting-xxdisableexplicitgc-when-nio-direct-buffers-are-used?rq=1].
With the current default which Solr is shipped with both local or remote
debugging is not possible via jconsole. With all things considered, I still
think, shipping with {{-XX:+DisableExplicitGC}} is good choice and there are
ways to force gc even with the above JVM flag but I haven't yet found GUI tool
for this.
[~erickerickson]
If there are still concerns or objections I would be happy to answer them.
Alternative solution is to add {{-XX:+ExplicitGCInvokesConcurrent}} so that
any force gc is triggered concurrently
> Disallow explicit GC by default during Solr startup
> ---------------------------------------------------
>
> Key: SOLR-13492
> URL: https://issues.apache.org/jira/browse/SOLR-13492
> Project: Solr
> Issue Type: Improvement
> Components: scripts and tools
> Reporter: Shawn Heisey
> Assignee: Shawn Heisey
> Priority: Major
> Time Spent: 20m
> Remaining Estimate: 0h
>
> Solr should use the -XX:+DisableExplicitGC option as part of its default GC
> tuning.
> None of Solr's stock code uses explicit GCs, so that option will have no
> effect on most installs. The effective result of this is that if somebody
> adds custom code to Solr and THAT code does an explicit GC, it won't be
> allowed to function.
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