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https://issues.apache.org/jira/browse/LUCENE-8635?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16753468#comment-16753468
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Mike Sokolov commented on LUCENE-8635:
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I tried that [~akjain] and strangely got a big drop in performance! I just used
a wrapper around {{IndexInput}} rather than the random access approach (using
{{randomAccessSlice}}) and implemented {{skipBytes}} in the obvious way: by
calling the delegate's {{skipBytes}}. But this is bad. The default
implementation of that method comes from {{DataInput}} and that actually reads
bytes into a buffer rather than simply updating a pointer. I'm not sure I
understand the rationale for that - it seems to have to do with checksumming?
Possibly {{ByteBuffer(s)IndexInput}} could (should?) implement this more
efficiently, or maybe it's required to do this reading -- not sure. At any rate
I think in this case we really just want to seek the pointer, so we can have
our {{FST.BytesReader.skipBytes}} call {{IndexInput.seek}} instead of
{{IndexInput.skipBytes}}.
> 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: fst-offheap-ra-rev.patch, 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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