Pushkar Raste commented on SOLR-9524:

I checked logs on one of our biggest collection with following stats
{{num docs: 1591412892}}
{{shards: 12}} spread across two solr instances (each process hosts 6 shards 
and each instance runs on separate physical box).
{{auto soft commit : 1 secs}} (not sure this matters as computing fingerprint 
opens a new NR searcher anyway)
There is replication too but I don't think that matters a lot here.

It is typically taking  ~2 seconds on avg.

This definitely requires caching. 

I am planning to take on 
[SOLR-9506|https://issues.apache.org/jira/browse/SOLR-9506] this week. 

> SolrIndexSearcher.getIndexFingerprint uses dubious sunchronization
> ------------------------------------------------------------------
>                 Key: SOLR-9524
>                 URL: https://issues.apache.org/jira/browse/SOLR-9524
>             Project: Solr
>          Issue Type: Bug
>      Security Level: Public(Default Security Level. Issues are Public) 
>    Affects Versions: 6.3
>            Reporter: Mike Drob
>         Attachments: SOLR-9524.patch
> In SOLR-9310 we added more code that does some fingerprint caching in 
> SolrIndexSearcher. However, the synchronization looks like it could be made 
> more efficient and may have issues with correctness.
> https://github.com/apache/lucene-solr/blob/branch_6x/solr/core/src/java/org/apache/solr/search/SolrIndexSearcher.java#L2371-L2385
> Some of the issues:
> * Double checked locking needs use of volatile variables to ensure proper 
> memory semantics.
> * sync on a ConcurrentHashMap is usually a code smell

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