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https://issues.apache.org/jira/browse/SOLR-11882?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16415849#comment-16415849
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Erick Erickson commented on SOLR-11882:
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[~romseygeek] This has been around since at least 6.4 I believe. Plus, it's
rather obscure. In the normal course of events, we don't close cores _and leave
them closed_.
If we re-open a core, the orphan reference is re-assigned. So if I
open/close/open/close the same core a zillion times, I only have one SolrCore
object.
It manifests itself is if someone is using "transient cores". In that case, if
the transient core cache is capped at, say, 10 and I have 100 transient cores,
after I cycle through them all I'll have 90 "orphan" references. I'll never
have more than that though. And never less.
For anyone running "stock" Solr, it won't show up. People will open all the
cores at startup and keep them open (even if reopened) and won't have any
orphans.
I suppose if people are unloading cores it might occur as well, but I think
that's rare.
All FYI to evaluate whether you want to put it in 7.3. For people affected it
is, indeed serious if they have a lot of cores....
> SolrMetric registries retain references to SolrCores when closed
> ----------------------------------------------------------------
>
> Key: SOLR-11882
> URL: https://issues.apache.org/jira/browse/SOLR-11882
> Project: Solr
> Issue Type: Bug
> Security Level: Public(Default Security Level. Issues are Public)
> Components: metrics, Server
> Affects Versions: 7.1
> Reporter: Eros Taborelli
> Assignee: Erick Erickson
> Priority: Major
> Attachments: SOLR-11882.patch, SOLR-11882.patch, SOLR-11882.patch,
> SOLR-11882.patch, SOLR-11882.patch, create-cores.zip,
> solr-dump-full_Leak_Suspects.zip, solr.config.zip
>
>
> *Description:*
> Our setup involves using a lot of small cores (possibly hundred thousand),
> but working only on a few of them at any given time.
> We already followed all recommendations in this guide:
> [https://wiki.apache.org/solr/LotsOfCores]
> We noticed that after creating/loading around 1000-2000 empty cores, with no
> documents inside, the heap consumption went through the roof despite having
> set transientCacheSize to only 64 (heap size set to 12G).
> All cores are correctly set to loadOnStartup=false and transient=true, and we
> have verified via logs that the cores in excess are actually being closed.
> However, a reference remains in the
> org.apache.solr.metrics.SolrMetricManager#registries that is never removed
> until a core if fully unloaded.
> Restarting the JVM loads all cores in the admin UI, but doesn't populate the
> ConcurrentHashMap until a core is actually fully loaded.
> I reproduced the issue on a smaller scale (transientCacheSize = 5, heap size
> = 512m) and made a report (attached) using eclipse MAT.
> *Desired outcome:*
> When a transient core is closed, the references in the SolrMetricManager
> should be removed, in the same fashion the reporters for the core are also
> closed and removed.
> In alternative, a unloadOnClose=true|false flag could be implemented to fully
> unload a transient core when closed due to the cache size.
> *Note:*
> The documentation mentions everywhere that the unused cores will be unloaded,
> but it's misleading as the cores are never fully unloaded.
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