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https://issues.apache.org/jira/browse/LUCENE-8438?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16572907#comment-16572907
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Uwe Schindler edited comment on LUCENE-8438 at 8/8/18 9:06 AM:
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And yes, I agree with you about bytebuffers: Let's use ByteBuffers and not
byte[]. I know the problems of misuse/incorrect use of the bytebuffer APIs by
beginners, but in general we have an abstraction, so Lucene's codecs or users
of the new directory implementations do not need to take care. We just have to
make the implementations in this issue correct. To me it looks ok, no major
problems.
In addition with bytebuffers, we can (possibly) make a small knob in
ByteBufferDirectory do use ByteBuffer.allocateDirect() instead of allocate().
But then we have to add the usual unmapping code, so when the file is deleted
in the directory, we can unmap all buffers.
was (Author: thetaphi):
And yes, I agree with you about bytebuffers: Let's use ByteBuffers and not
byte[]. I know the problems of misuse/incorrect use of the bytebuffer APIs by
beginners, but in general we have an abstraction, so Lucene's codecs or users
of the new directory implementations do not need to take care. We just have to
make the implementations in this issue correct. To me it looks ok, no major
problems.
> 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
> Priority: Minor
> Attachments: capture-1.png, capture-4.png
>
> Time Spent: 10m
> Remaining Estimate: 0h
>
> 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).
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