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From "Uwe Schindler (JIRA)" <j...@apache.org>
Subject [jira] [Created] (LUCENE-3200) Cleanup MMapDirectory to use only one MMapIndexInput impl with mapping sized of powers of 2
Date Mon, 13 Jun 2011 22:39:47 GMT
Cleanup MMapDirectory to use only one MMapIndexInput impl with mapping sized of powers of 2
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                 Key: LUCENE-3200
                 URL: https://issues.apache.org/jira/browse/LUCENE-3200
             Project: Lucene - Java
          Issue Type: Improvement
            Reporter: Uwe Schindler


Robert and me discussed a little bit after Mike's investigations, that using SingleMMapIndexinput
together with MultiMMapIndexInput leads to hotspot slowdowns sometimes.

We had the following ideas:
- MultiMMapIndexInput is almost as fast as SingleMMapIndexInput, as the switching between
buffer boundaries is done in exception catch blocks. So normal code path is always the same
like for Single*
- Only the seek method uses strange calculations (the modulo is totally bogus, it could be
simply: int bufOffset = (int) (pos % maxBufSize); - very strange way of calculating modulo
in the original code)
- Because of speed we suggest to no longer use arbitrary buffer sizes. We should pass only
the power of 2 to the indexinput as size. All calculations in seek and anywhere else would
be simple bit shifts and AND operations (the and masks for the modulo can be calculated in
the ctor like NumericUtils does when calculating precisionSteps).
- the maximum buffer size will now be 2^30, not 2^31-1. But thats not an issue at all. In
my opinion, a buffer size of 2^31-1 is stupid in all cases, as it will no longer fit page
boundaries and mmapping gets harder for the O/S.

We will provide a patch with those cleanups.

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