[GitHub] [lucene] uschindler edited a comment on pull request #518: Initial rewrite of MMapDirectory for JDK-18 preview (incubating) Panama APIs (>= JDK-18-ea-b26)

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uschindler edited a comment on pull request #518:
URL: https://github.com/apache/lucene/pull/518#issuecomment-1084563803


   > I'm working on a similar approach for my data store, but I'm currently not sure if it's a good idea for multiple readers plus a single reader/writer to map a segment for each reader. I guess the OS will then share the mapped regions/the pages between the mapped memory segments? Not sure if it's the same approach in Lucene, so that you'd create multiple `IndexInput`s for multiple index readers, because you also seem to a have a clone method (but it will fail once the segments are closed from one reader).
   
   This PR does not change anything in Lucene's current behaviour. The code using MappedByteBuffer behaves the same way. There are also no multiple mappings. If a user may open several IndexReaders on the same index that's not our fault. Well behaving code of Lucene only opens a single IndexReader.
   
   The clone() method is used for several threads. There is no remapping, we only refcount the ResourceContext with Panama. If you close the main index, the clones used by different threads should really fail then - thats the improvement here.
    
   > On another note, what's your take on this (Andy and Victor are real genioses regarding database systems)? http://cidrdb.org/cidr2022/papers/p13-crotty.pdf
   
   We don't agree with that for Lucene:
   - the model behind Lucene is different: All files are write-once so there are no updates to files which were written before. MMAP only works on files that are never changed anymore. The paging works very well with those.
   - we do not write with mmap, lucene index files are written with standard output streams


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