Michael McCandless commented on LUCENE-7579:

Thanks [~jim.ferenczi], I also see comparable speedups on the taxis benchmark.  
I'll have a look at the change!  It looks like a doozie :)

> Sorting on flushed segment
> --------------------------
>                 Key: LUCENE-7579
>                 URL: https://issues.apache.org/jira/browse/LUCENE-7579
>             Project: Lucene - Core
>          Issue Type: Bug
>            Reporter: Ferenczi Jim
> Today flushed segments built by an index writer with an index sort specified 
> are not sorted. The merge is responsible of sorting these segments 
> potentially with others that are already sorted (resulted from another 
> merge). 
> I'd like to investigate the cost of sorting the segment directly during the 
> flush. This could make the merge faster since they are some cheap 
> optimizations that can be done only if all segments to be merged are sorted.
>  For instance the merge of the points could use the bulk merge instead of 
> rebuilding the points from scratch.
> I made a small prototype which sort the segment on flush here:
> https://github.com/apache/lucene-solr/compare/master...jimczi:flush_sort
> The idea is simple, for points, norms, docvalues and terms I use the 
> SortingLeafReader implementation to translate the values that we have in RAM 
> in a sorted enumeration for the writers.
> For stored fields I use a two pass scheme where the documents are first 
> written to disk unsorted and then copied to another file with the correct 
> sorting. I use the same stored field format for the two steps and just remove 
> the file produced by the first pass at the end of the process.
> This prototype has no implementation for index sorting that use term vectors 
> yet. I'll add this later if the tests are good enough.
> Speaking of testing, I tried this branch on [~mikemccand] benchmark scripts 
> and compared master with index sorting against my branch with index sorting 
> on flush. I tried with sparsetaxis and wikipedia and the first results are 
> weird. When I use the SerialScheduler and only one thread to write the docs,  
> index sorting on flush is slower. But when I use two threads the sorting on 
> flush is much faster even with the SerialScheduler. I'll continue to run the 
> tests in order to be able to share something more meaningful.
> The tests are passing except one about concurrent DV updates. I don't know 
> this part at all so I did not fix the test yet. I don't even know if we can 
> make it work with index sorting ;).
>  [~mikemccand] I would love to have your feedback about the prototype. Could 
> you please take a look ? I am sure there are plenty of bugs, ... but I think 
> it's a good start to evaluate the feasibility of this feature.

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