jpountz commented on code in PR #14510:
URL: https://github.com/apache/lucene/pull/14510#discussion_r2072468622
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lucene/core/src/java/org/apache/lucene/store/MMapDirectory.java:
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@@ -277,6 +277,32 @@ public final long getMaxChunkSize() {
return 1L << chunkSizePower;
}
+ public static ReadAdvice toReadAdvice(IOContext context) {
+ if (context.context() == IOContext.Context.MERGE
+ || context.context() == IOContext.Context.FLUSH) {
+ return ReadAdvice.SEQUENTIAL;
+ }
+
+ if (context.hints().contains(DataAccessHint.RANDOM)) {
+ return ReadAdvice.RANDOM;
+ }
+ if (context.hints().contains(DataAccessHint.SEQUENTIAL)) {
+ return ReadAdvice.SEQUENTIAL;
+ }
+
+ if (context.hints().contains(FileTypeHint.DATA)
+ || context.hints().contains(FileTypeHint.INDEX)) {
+ return ReadAdvice.NORMAL;
+ }
+ // Postings have a forward-only access pattern, so pass ReadAdvice.NORMAL
to perform
+ // readahead.
+ if (context.hints().contains(FileDataHint.POSTINGS)) {
+ return ReadAdvice.NORMAL;
+ }
Review Comment:
I think it's natural for `DataAccessHint` to be closely related to
`ReadAdvice`.
My suggestion would be to do the following:
- DataAccessHint has a single enum constant: RANDOM
- the lack of a DataAccessHint means that the access pattern is Lucene's
standard access pattern: a mix of forward seeks and reads of data in the order
of 1kB-1MB (what terms, postings, points and doc values typically do)
- Vectors pass a DataAccessHint.RANDOM at open time, they are the only ones
to pass a DataAccessHint to the IOContext, because they're the only one to seek
backwards to evaluate a single query
- To mimic today's behavior MMapDirectory's default implementation uses
`Constants.DEFAULT_READADVICE` on all files that have `FileTypeHint.DATA`.
- In the future (different PR), we can discuss whether MMapDirectory's
default implementation should force `ReadAdvice.RANDOM` for
`DataAccessHint.RANDOM`, and whether `Constants.DEFAULT_READADVICE` should move
back to `NORMAL`.
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