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https://issues.apache.org/jira/browse/LUCENE-8635?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16744538#comment-16744538
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Michael McCandless commented on LUCENE-8635:
--------------------------------------------

Thanks [~sokolov] – those numbers look quite a bit better!  Though, your QPSs 
are kinda high overall – how many Wikipedia docs were in your index?

I do wonder if we simply reversed the FST's byte[] when we create it, what 
impact that'd have on lookup performance.  Hmm even if we did that, we'd still 
have to {{readBytes}} one byte at a time since {{RandomAccessInput}} does not 
have a {{readBytes}} method?  But ... maybe {{IndexInput}} would give good 
performance in that case?  We should probably pursue that separately though...

> Lazy loading Lucene FST offheap using mmap
> ------------------------------------------
>
>                 Key: LUCENE-8635
>                 URL: https://issues.apache.org/jira/browse/LUCENE-8635
>             Project: Lucene - Core
>          Issue Type: New Feature
>          Components: core/FSTs
>         Environment: I used below setup for es_rally tests:
> single node i3.xlarge running ES 6.5
> es_rally was running on another i3.xlarge instance
>            Reporter: Ankit Jain
>            Priority: Major
>         Attachments: offheap.patch, ra.patch, rally_benchmark.xlsx
>
>
> Currently, FST loads all the terms into heap memory during index open. This 
> causes frequent JVM OOM issues if the term size gets big. A better way of 
> doing this will be to lazily load FST using mmap. That ensures only the 
> required terms get loaded into memory.
>  
> Lucene can expose API for providing list of fields to load terms offheap. I'm 
> planning to take following approach for this:
>  # Add a boolean property fstOffHeap in FieldInfo
>  # Pass list of offheap fields to lucene during index open (ALL can be 
> special keyword for loading ALL fields offheap)
>  # Initialize the fstOffHeap property during lucene index open
>  # FieldReader invokes default FST constructor or OffHeap constructor based 
> on fstOffHeap field
>  
> I created a patch (that loads all fields offheap), did some benchmarks using 
> es_rally and results look good.



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