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https://issues.apache.org/jira/browse/LUCENE-8406?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16546729#comment-16546729
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Uwe Schindler commented on LUCENE-8406:
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I have no problem in making the class public, if the constructors are correctly
documented and take an array of ByteBuffer. The unmapping an mmap logic is part
of MMapDirectory and only available internally, so it's not risky to do this.
But we should not make the BBGuard public, I think that can be hidden inside?
BufferCleaner interface is fine, so somebody can hook in his own impl.
> Make ByteBufferIndexInput public
> --------------------------------
>
> Key: LUCENE-8406
> URL: https://issues.apache.org/jira/browse/LUCENE-8406
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Dawid Weiss
> Assignee: Dawid Weiss
> Priority: Minor
> Fix For: 6.7
>
>
> The logic of handling byte buffers splits, their proper closing (cleaner) and
> all the trickery involved in slicing, cloning and proper exception handling
> is quite daunting.
> While ByteBufferIndexInput.newInstance(..) is public, the parent class
> ByteBufferIndexInput is not. I think we should make the parent class public
> to allow advanced users to make use of this (complex) piece of code to create
> IndexInput based on a sequence of ByteBuffers.
> One particular example here is RAMDirectory, which currently uses a custom
> IndexInput implementation, which in turn reaches to RAMFile's synchronized
> methods. This is the cause of quite dramatic congestions on multithreaded
> systems. While we clearly discourage RAMDirectory from being used in
> production environments, there really is no need for it to be slow. If
> modified only slightly (to use ByteBuffer-based input), the performance is on
> par with FSDirectory. Here's a sample log comparing FSDirectory with
> RAMDirectory and the "modified" RAMDirectory making use of the ByteBuffer
> input:
> {code}
> 14:26:40 INFO console: FSDirectory index.
> 14:26:41 INFO console: Opened with 299943 documents.
> 14:26:50 INFO console: Finished: 8.820 s, 240000 matches.
> 14:26:50 INFO console: RAMDirectory index.
> 14:26:50 INFO console: Opened with 299943 documents.
> 14:28:50 INFO console: Finished: 2.012 min, 240000 matches.
> 14:28:50 INFO console: RAMDirectory2 index (wrapped byte[] buffers).
> 14:28:50 INFO console: Opened with 299943 documents.
> 14:29:00 INFO console: Finished: 9.215 s, 240000 matches.
> 14:29:00 INFO console: RAMDirectory2 index (direct memory buffers).
> 14:29:00 INFO console: Opened with 299943 documents.
> 14:29:08 INFO console: Finished: 8.817 s, 240000 matches.
> {code}
> Note the performance difference is an order of magnitude on this 32-CPU
> system (2 minutes vs. 9 seconds). The tiny performance difference between the
> implementation based on direct memory buffers vs. those acquired via
> ByteBuffer.wrap(byte[]) is due to the fact that direct buffers access their
> data via unsafe and the wrapped counterpart uses regular java array access
> (my best guess).
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