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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