In Lucene 8.4, we updated postings to work on long[] arrays internally. This
allowed us to workaround the lack of explicit vectorization (auto-vectorization
doesn't detect all the scenarios that we would like to handle) support in the
JVM by summing up two integers in one operation for instance.
With explicit vectorization now available, it looks like we can get more
benefits from the ability to compare multiple intetgers in one operations than
from summing up two integers in one operation. Moving back to ints helps
compare 2x more integers at once vs. longs.
This updates file formats to compute prefix sums by summing up 8 deltas per
long at the same time if the number of bits per value is 4 or less, and 4
deltas per long at the same time if the number of bits per value is between 5
included and 11 included. Otherwise, we keep summing up 2 deltas per long like
today.
The `PostingDecodingUtil` was slightly modified due to the fact that more
numbers of bits per value now need to apply different shifts to the input data.
E.g. now that we store integers that require 5 bits per value as 16-bit
integers under the hood rather than 8, we extract the first values by shifting
by 16-5=11, 16-2*5=6 and 16-3*5=1 and then decode tail values from the
remaining bit per 16-bit integer.
This updates the postings format in order to inline skip data into postings. This format is generally similar to the current `Lucene99PostingsFormat`, e.g. it shares the same block encoding logic, but it has a few differences:
- Skip data is inlined into postings to make the access pattern more sequential.
- There are only 2 levels of skip data: on every block (128 docs) and every 32 blocks (4,096 docs).
In general, I found that the fact that skip data is inlined may slow down a bit queries that don't need skip data at all (e.g. `CountOrXXX` tasks that never advance of consult impacts) and speed up a bit queries that advance by small intervals. The fact that the greatest level only allows skipping 4096 docs at once means that we're slower at advancing by large intervals, but data suggests that it doesn't significantly hurt performance.
* Change Postings back to using FOR in Lucene99PostingsFormat
We are still keeping PFOR for positions only.
This is a partial revert of https://github.com/apache/lucene/pull/69 which brings back ForDeltaUtil.
* fix merge commit
* Add forgotten forDeltaUtil calls to reader
* Addressing comments: adding Lucene90RWPostingsFormat + more
Also:
* Change to Changes.txt
* Removal of dead code which was only used in unit tests
* Removal of test code from PForUtil
* Changes.txt edit in right place now
* Apply suggestions from code review: `90 -> 99 refactoring`
Co-authored-by: gf2121 <52390227+gf2121@users.noreply.github.com>
* Remove decodeTo32 from ForUtil and regenerate
---------
Co-authored-by: gf2121 <52390227+gf2121@users.noreply.github.com>
Adrien Grand
Dawid Weiss
Jakub Slowinski
Uwe Schindler