Dawid Weiss created LUCENE-8406:
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Summary: Make ByteBufferIndexInput public
Key: LUCENE-8406
URL: https://issues.apache.org/jira/browse/LUCENE-8406
Project: Lucene - Core
Issue Type: Improvement
Reporter: Dawid Weiss
Assignee: Dawid Weiss
Fix For: 6.7
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.
The specific rationale I'm aiming at 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:
{code}
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.
{code}
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).
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