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

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mcimadamore commented on pull request #518:
URL: https://github.com/apache/lucene/pull/518#issuecomment-1005995125


   > From what I have learned, copy operations have high overhead because:
   > 
   >     * they are not hot, so aren't optimized so fast
   > 
   >     * when not optimized, the setup cost is high (lots of class checks to get array type, decision for swapping bytes). This is especially heavy for small arrays.
   
   Hi, I'm not sure as to why copy operations should be slower in the memory access API then with the ByteBuffer API. I would expect most of the checks to be similar (except for the liveness tests of the segment involved). I do recall that the ByteBuffer API does optimize bulk copy for very small buffers (I don't recall what the limit is, but it was very very low, like 4 elements or something).
   
   In principle, this JVM fix (as per 18) should help too:
   https://bugs.openjdk.java.net/browse/JDK-8269119
   


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