jimczi commented on code in PR #16705:
URL: https://github.com/apache/lucene/pull/16705#discussion_r4141847904
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lucene/core/src/java/org/apache/lucene/search/RescoreTopNQuery.java:
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@@ -70,25 +92,71 @@ public Query rewrite(IndexSearcher indexSearcher) throws
IOException {
}
DoubleValues rescores = rewrittenValueSource.getValues(leaf,
getDoubleValues(innerScorer));
DocIdSetIterator iterator = innerScorer.iterator();
+ while (iterator.nextDoc() != DocIdSetIterator.NO_MORE_DOCS) {
+ rescoreInto(queue, rescores, leaf.docBase, iterator.docID());
+ originalCount++;
+ }
+ }
+ return originalCount;
+ }
+
+ /**
+ * Starts the loads for every candidate before scoring any of them, so that
more than one read is
+ * in flight when the values live on slow storage. A rerank shortlist is
spread over every
+ * segment, so the prefetches for all segments are issued before any scoring
rather than a segment
+ * at a time. Only doc ids are buffered, never values.
+ */
+ private int rescoreWithPrefetch(
+ IndexReader reader, Weight weight, DoubleValuesSource
rewrittenValueSource, HitQueue queue)
+ throws IOException {
+ final List<LeafReaderContext> leaves = reader.leaves();
+ final DoubleValues[] leafValues = new DoubleValues[leaves.size()];
+ final int[][] leafDocs = new int[leaves.size()][];
Review Comment:
That could be a large array here, if the rescoring set is large.
Could we do a fixed-size window rather than "all of them":
- keep a window of, say, 128 (leaf, doc) entries
- fill it by pulling from the leaf iterators in order, calling prefetch(doc)
as each entry goes in
- when it's full, score the entries in order, clear, continue
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