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https://issues.apache.org/jira/browse/LUCENE-1743?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12731632#action_12731632
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Earwin Burrfoot commented on LUCENE-1743:
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The initial motive for the issue seems wrong to me.
bq. "For most operating systems, mapping a file into memory is more expensive
than reading or writing a few tens of kilobytes of data via the usual read and
write methods. From the standpoint of performance it is generally only worth
mapping relatively large files into memory."
It is probably right if you're doing a single read through the file. If you're
opening/mapping it and do thousands of repeated reads, mmap would be superior,
because after initial mapping it's just a memory access VS system call for
file.read().
> MMapDirectory should only mmap large files, small files should be opened
> using SimpleFS/NIOFS
> ---------------------------------------------------------------------------------------------
>
> Key: LUCENE-1743
> URL: https://issues.apache.org/jira/browse/LUCENE-1743
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Store
> Affects Versions: 2.9
> Reporter: Uwe Schindler
> Assignee: Uwe Schindler
> Fix For: 3.1
>
>
> This is a followup to LUCENE-1741:
> Javadocs state (in FileChannel#map): "For most operating systems, mapping a
> file into memory is more expensive than reading or writing a few tens of
> kilobytes of data via the usual read and write methods. From the standpoint
> of performance it is generally only worth mapping relatively large files into
> memory."
> MMapDirectory should get a user-configureable size parameter that is a lower
> limit for mmapping files. All files with a size<limit should be opened using
> a conventional IndexInput from SimpleFS or NIO (another configuration option
> for the fallback?).
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