[jira] [Created] (LUCENE-8438) RAMDirectory speed improvements and cleanup

["Dawid Weiss (JIRA)" <ji...@apache.org>]

Dawid Weiss created LUCENE-8438:
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             Summary: RAMDirectory speed improvements and cleanup
                 Key: LUCENE-8438
                 URL: https://issues.apache.org/jira/browse/LUCENE-8438
             Project: Lucene - Core
          Issue Type: Improvement
            Reporter: Dawid Weiss
            Assignee: Dawid Weiss
         Attachments: capture-4.png

RAMDirectory screams for a cleanup. It is used and abused in many places and even if we discourage its use in favor of native (mmapped) buffers, there seem to be benefits of keeping RAMDirectory available (quick throw-away indexes without the need to setup external tmpfs, for example).

Currently RAMDirectory performs very poorly under concurrent loads. The implementation is also open for all sorts of abuses – the streams can be reset and are used all around the place as temporary buffers, even without the presence of RAMDirectory itself. This complicates the implementation and is pretty confusing.

An example of how dramatically slow RAMDirectory is under concurrent load, consider this PoC pseudo-benchmark. It creates a single monolithic segment with 500K very short documents (single field, with norms). The index is ~60MB once created. We then run semi-complex Boolean queries on top of that index from N concurrent threads. The attached capture-4 shows the result (queries per second over 5-second spans) for a varying number of concurrent threads on an AWS machine with 32 CPUs available (of which it seems 16 seem to be real, 16 hyper-threaded). That red line at the bottom (which drops compared to a single-threaded performance) is the current RAMDirectory. RAMDirectory2 is an alternative implementation I wrote that uses ByteBuffers. Yes, it's slower than the native mmapped implementation, but a *lot* faster then the current RAMDirectory (and more GC-friendly because it uses dynamic progressive block scaling internally).

I'll clean it all up and prepare a patch this week. The PoC code discussed above is at [1] but I wouldn't spend any time on this yet.

[1] https://github.com/dweiss/ramdir2




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