[LUCENE-9480] investigate slow DataInput.skipBytes

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 9.0
• Lucene Fields: New
• Assignee: Unassigned
• Reporter: Robert Muir
• Created: 25/Aug/20 02:18
• Updated: 28/Aug/22 16:06
• Resolved: 20/Feb/21 17:22

Description

Currently DataInput has skipBytes(), but IndexInput also adds seek().

There isn't a clear reason about the differences in the two methods: why would you choose one over the other?

It causes some performance issues: for example the default implementation actually reads bytes into a byte array and throws everything away. This is really silly for MMapDirectory: skipping bytes should only be a glorified +=.

So when I look at latest LUCENE-9447 patch, I can't help but think a ton of waste is happening:

• Maybe skipBytes() is only used because the stored fields compressor interface happens to take DataInput? Should it take IndexInput instead?
• Should skipBytes() be overridden by MMapDirectory rather than delegating to super? doing real reads and byte array copies isn't free. It should be a += with single bounds check.
• Should we revisit having DataInput vs IndexInput at all? Maybe they should be collapsed into one thing?