Uwe Schindler commented on LUCENE-8438:

Maybe we can then still cleaup the other names. E.g. remove your own 
ByteBuffersIndexInput and rename the one behind MMAPDir. At the moment I see 
too much names in the store folder and it's very hard to figure out whoich one 
is the right one. We should also make everything package private, which is 
internal implementations.

> 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: 1h 40m
>  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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