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https://issues.apache.org/jira/browse/LUCENE-4163?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Uwe Schindler updated LUCENE-4163:
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Attachment: LUCENE-4163.patch
Slightly improved test (null keys and iterator conformance).
I think that's ready to commit and brings a big improvement in concurrency. We
should backport this to 3.6.1!
> Improve concurrency in MMapIndexInput.clone()
> ---------------------------------------------
>
> Key: LUCENE-4163
> URL: https://issues.apache.org/jira/browse/LUCENE-4163
> Project: Lucene - Java
> Issue Type: Improvement
> Components: core/store
> Affects Versions: 3.6, 4.0, 5.0
> Reporter: Uwe Schindler
> Assignee: Uwe Schindler
> Fix For: 4.0, 3.6.1, 5.0
>
> Attachments: LUCENE-4163.patch, LUCENE-4163.patch
>
>
> Followup issue from SOLR-3566:
> Whenever you clone the TermIndex, it also creates a clone of the underlying
> IndexInputs. In high cocurrent environments, the clone method of
> MMapIndexInput is a bottleneck (it has heavy work to do to manage the weak
> references in a synchronized block).
> Everywhere else in Lucene we use my new WeakIdentityMap for managing
> concurrent weak maps. For this case I did not do this, as the WeakIdentityMap
> has no iterators (it doe snot implement Map interface). This issue will add a
> key and values iterator (the key iterator will not return GC'ed keys), so
> MMapIndexInput can use WeakIdentityMap backed by ConcurrentHashMap and needs
> no synchronization. ConcurrentHashMap has better concurrency because it
> distributes the hash keys in different buckets per thread.
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