On Tue, Feb 23, 2021 at 4:07 PM <baris.ka...@oracle.com> wrote: > What i want to achieve: Problem statement: > > base case is disk based Lucene index with FSDirectory > > speedup case was supposed to be in memory Lucene index with MMapDirectory > On 64-bit systems, FSDirectory just invokes MMapDirectory already. So you don't need to do anything.
Either way MMapDirectory or NIOFSDirectory are doing the same thing: reading your index as a normal file and letting the operating system cache it. The MMapDirectory is just better because it avoids some overheads, such as read() system call, copying and buffering into java memory space, etc etc. Some of these overheads are only getting worse, e.g. spectre/meltdown-type fixes make syscalls 8x slower on my computer. So it is good that MMapDirectory avoids it. So I suggest just stop fighting the operating system, don't give your J2EE container huge amounts of ram, let the kernel do its job. If you want to "warm" a cold system because nothing is in kernel's cache, then look into preload and so on. It is just "reading files" to get them cached.
