Uwe Schindler created LUCENE-10019:
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Summary: Align file starts in CFS files to have proper alignment
(8 bytes)
Key: LUCENE-10019
URL: https://issues.apache.org/jira/browse/LUCENE-10019
Project: Lucene - Core
Issue Type: Improvement
Components: core/codecs, core/store
Affects Versions: main (9.0)
Reporter: Uwe Schindler
Assignee: Uwe Schindler
While discussing about MMapDirectory and fast access to file contents thorigh
MMap, I figured out that for most Lucene files, the data inside is not alig.ed
at all.
We can't fix this easily and it's also not always important, but some files
should really have a CPU fieldly alignment from beginning! This is escpecially
important when we use slices().
I got many tests with aligned VarHandles to pass, but it broke ASAP, if the
file was inside a Compound CFS file.
CompoundFormat.write() just appends all data to the IndexOutput and writes the
offset to the entries file. The fix to make at least file starts aligned is to
just write some null-bytes between the files, so startOffset is aligned to
multiples of 8 bytes.
At a later stage we could also think of aligning to LBA blocks/sectors/whatever
to make OS paging work better. But for performance of index access, slices of
compound files when memory mapped should at least align to 8 bytes.
Fix is easy: Just add some modulo on startOffset and write some extra bytes
before the next file is serialized. The change is only 2 lines. It does not
even change index format!
I'd like to get this in for 9.0 so we can at least say: our CFS files are
aligned. Aligning other files like docvalues to better help CPU is then
possible.
I will provide a simple pull request for Lucene90CompoundFormat soon. If you
don't see any problems, this is a no-brainer.
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