[LUCENE-8406] Make ByteBufferIndexInput public

• Type: Improvement
• Status: Resolved
• Priority: Minor
• Resolution: Duplicate
• Lucene Fields: New
• Assignee: Dawid Weiss
• Reporter: Dawid Weiss
• Created: 17/Jul/18 12:57
• Updated: 28/Aug/22 15:33
• Resolved: 28/Jan/19 13:00

Description

The logic of handling byte buffers splits, their proper closing (cleaner) and all the trickery involved in slicing, cloning and proper exception handling is quite daunting.

While ByteBufferIndexInput.newInstance(..) is public, the parent class ByteBufferIndexInput is not. I think we should make the parent class public to allow advanced users to make use of this (complex) piece of code to create IndexInput based on a sequence of ByteBuffers.

One particular example here is RAMDirectory, which currently uses a custom IndexInput implementation, which in turn reaches to RAMFile's synchronized methods. This is the cause of quite dramatic congestions on multithreaded systems. While we clearly discourage RAMDirectory from being used in production environments, there really is no need for it to be slow. If modified only slightly (to use ByteBuffer-based input), the performance is on par with FSDirectory. Here's a sample log comparing FSDirectory with RAMDirectory and the "modified" RAMDirectory making use of the ByteBuffer input:

14:26:40 INFO  console: FSDirectory index.
14:26:41 INFO  console: Opened with 299943 documents.
14:26:50 INFO  console: Finished: 8.820 s, 240000 matches.

14:26:50 INFO  console: RAMDirectory index.
14:26:50 INFO  console: Opened with 299943 documents.
14:28:50 INFO  console: Finished: 2.012 min, 240000 matches.

14:28:50 INFO  console: RAMDirectory2 index (wrapped byte[] buffers).
14:28:50 INFO  console: Opened with 299943 documents.
14:29:00 INFO  console: Finished: 9.215 s, 240000 matches.

14:29:00 INFO  console: RAMDirectory2 index (direct memory buffers).
14:29:00 INFO  console: Opened with 299943 documents.
14:29:08 INFO  console: Finished: 8.817 s, 240000 matches.

Note the performance difference is an order of magnitude on this 32-CPU system (2 minutes vs. 9 seconds). The tiny performance difference between the implementation based on direct memory buffers vs. those acquired via ByteBuffer.wrap(byte[]) is due to the fact that direct buffers access their data via unsafe and the wrapped counterpart uses regular java array access (my best guess).

Issue Links

• is related to: LUCENE-8438 RAMDirectory speed improvements and cleanup
• is related to: LUCENE-8661 Replace ByteBuffersIndexInput with ByteBufferIndexInput (replace and rename)