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https://issues.apache.org/jira/browse/LUCENE-8635?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16755344#comment-16755344
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Michael McCandless commented on LUCENE-8635:
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OK net/net it looks like there is a small performance impact for some queries,
and biggish (-7-8%) impact for {{PKLookup.}}
But this is a nice option to have for users who are heap constrained by the
FSTs, so I wonder how we could add this option off by default? E.g. users
might want their {{id}} field to store the FST in heap (like today), but all
other fields off-heap.
There is no index format change required here, which is nice, but Lucene
doesn't make it easy to have read-time codec behavior changes, so maybe the
solution is that at write-time we add an option e.g. to
{{BlockTreeTermsWriter}} and it stores this in the index and then at read-time
{{BlockTreeTermsReader}} checks that option and loads the FST accordingly?
Then users could customize their codecs to achieve this.
Or I suppose we could add a global system property, e.g. our default stored
fields writer has a property to turn on/off bulk merge, but I think we are
trying not to use Java properties going forward?
Can anyone think of any other approaches to make this option possible?
> 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,
> optional_offheap_ra.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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