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https://issues.apache.org/jira/browse/LUCENE-10068?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17419202#comment-17419202
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Greg Miller commented on LUCENE-10068:
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[~gworah] would you mind re-running benchmarks now that [~mikemccand] has added 
more faceting tasks in {{luceneutil}}? (see: 
https://github.com/mikemccand/luceneutil/issues/137)

> Switch to a "double barrel" HPPC cache for the taxonomy LRU cache
> -----------------------------------------------------------------
>
>                 Key: LUCENE-10068
>                 URL: https://issues.apache.org/jira/browse/LUCENE-10068
>             Project: Lucene - Core
>          Issue Type: Improvement
>          Components: modules/facet
>    Affects Versions: 8.8.1
>            Reporter: Gautam Worah
>            Priority: Minor
>         Attachments: disable_taxo_category_cache_benchmark
>
>
> While working on an unrelated getBulkPath API 
> [PR|https://github.com/apache/lucene/pull/179], [~mikemccand] and I came 
> across a nice optimization that could be made to the taxonomy cache.
> The taxonomy cache today caches frequently used ordinals and their 
> corresponding FacetLabels. It uses the existing LRUHashMap (backed by a 
> LinkedList) class for its implementation.
> This implementation performs sub optimally when it has a large number of 
> threads accessing it, and consumes a large amount of RAM.
> [~mikemccand] suggested the idea of a two array backed HPPC int->FacetLabel 
> cache. The basic idea behind the cache being:
>  # We use two hashmaps primary and secondary.
>  # In case of a cache miss in the primary and a cache hit in the secondary, 
> we add the key to the primary map as well.
>  # In case of a cache miss in both the maps, we add it to the primary map.
>  # When we reach (make this check each time we insert?) a large number of 
> elements in say the primary cache, (say larger than the existing 
> {color:#871094}DEFAULT_CACHE_VALUE{color}=4000), we dump the secondary map 
> and copy all the values of the primary map into it.
> The idea was originally explained in 
> [this|https://github.com/apache/lucene/pull/179#discussion_r692907559] 
> comment.
>  
>  



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