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Windows reads regressed about 20 percent after version packing #62

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@martian56

Both bench runs of the packing change (#54) show the same thing on windows-x64-full and nowhere else:

  • reads 28309 to 22073 and 22362, about minus 21 percent
  • mixed 7448 to 5785 and 5960, about minus 20 percent
  • writes flat at 633 to 635 and 637

Linux profiles show no read regression at all, so this is not the packed page walk itself being slow. Suspects worth measuring: the extra relaxed commit that blanks a page at rollover, the record cache being invalidated on every append so index path reads decode a page more often, or Windows specific cost in reading a page that is still open and therefore only in the dirty table.

The read path normally answers from the in memory version map, so a 20 percent drop suggests reads are falling through to the page more often than they used to.

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