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https://issues.apache.org/jira/browse/LUCENE-2573?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12914296#action_12914296
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Jason Rutherglen commented on LUCENE-2573:
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I was hoping something clever would come to me about how to unit test this, 
nothing has.  We can do the slowdown of writes to the file(s) via a 
Thread.sleep, however this will only emulate a real file system in RAM, what 
then?  I thought about testing the percentage however is it going to be exact?  
We could test a percentage range of each of the segments flushed?  I guess I 
just need to run the all of the unit tests, however some of those will fail 
because deletes aren't working properly yet.  

> Tiered flushing of DWPTs by RAM with low/high water marks
> ---------------------------------------------------------
>
>                 Key: LUCENE-2573
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2573
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>            Reporter: Michael Busch
>            Assignee: Michael Busch
>            Priority: Minor
>             Fix For: Realtime Branch
>
>         Attachments: LUCENE-2573.patch, LUCENE-2573.patch, LUCENE-2573.patch
>
>
> Now that we have DocumentsWriterPerThreads we need to track total consumed 
> RAM across all DWPTs.
> A flushing strategy idea that was discussed in LUCENE-2324 was to use a 
> tiered approach:  
> - Flush the first DWPT at a low water mark (e.g. at 90% of allowed RAM)
> - Flush all DWPTs at a high water mark (e.g. at 110%)
> - Use linear steps in between high and low watermark:  E.g. when 5 DWPTs are 
> used, flush at 90%, 95%, 100%, 105% and 110%.
> Should we allow the user to configure the low and high water mark values 
> explicitly using total values (e.g. low water mark at 120MB, high water mark 
> at 140MB)?  Or shall we keep for simplicity the single setRAMBufferSizeMB() 
> config method and use something like 90% and 110% for the water marks?

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