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https://issues.apache.org/jira/browse/SOLR-14639?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17177826#comment-17177826
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Erick Erickson commented on SOLR-14639:
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I'll add my thanks, this is one of those thankless tasks...
I did a quick grep and there are about 130 instances of "new
ConcurrentHashMap..." in the code base. Changing them all at once is
...daunting... Is there a way to tell when a particular instance is causing the
slowdown?
Even if we implement the method, this means we'd potentially be playing
whack-a-mole due to: "worse with your Zipf-distributed keys".
We'd have to watch that asMap view, particularly any time we catch
ConcurrentModificationExceptions:
{code}
The asMap view (and its collection views) have weakly consistent iterators.
This means that they are safe for concurrent use, but if other threads modify
the cache after the iterator is created, it is undefined which of these
changes, if any, are reflected in that iterator. These iterators never throw
ConcurrentModificationException.
{code}
I don't know whether we actually depend on this exception or not, it just looks
tricky.
> Improve concurrency of SlowCompositeReaderWrapper.terms
> -------------------------------------------------------
>
> Key: SOLR-14639
> URL: https://issues.apache.org/jira/browse/SOLR-14639
> Project: Solr
> Issue Type: Improvement
> Components: search
> Affects Versions: 8.4.1
> Reporter: Shalin Shekhar Mangar
> Priority: Major
> Attachments: Screen Shot 2020-07-09 at 4.38.03 PM.png
>
>
> Under heavy query load, the ConcurrentHashMap.computeIfAbsent method inside
> the SlowCompositeReaderWrapper.terms(String) method blocks searcher threads
> (see attached screenshot of a java flight recording).
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