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https://issues.apache.org/jira/browse/LUCENE-1717?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12724074#action_12724074
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Simon Willnauer commented on LUCENE-1717:
-----------------------------------------

Regarding buffered delete's RAM usage, accounting an exact number is quite 
difficult in this case as there are many strings involved (Terms with field and 
value) . BufferedDeletes#terms stores <Term, Num> and BufferedDeletes#queries 
stores <Query, Num> in both cases the value part is easy to account while 
especially for query the memory consumption is hard to guess similarly the 
amount of memory a Term takes.

On the other hand I would like to have a notion of memory consumption os 
BufferedDeletes but the IndexWriters#setRAMBufferSizeMB javaDoc clearly says 
that this does not include the memory used by buffered deletes. I would rather 
tend to leave it as it is and make it clear in javadoc / wiki that 
setMaxBufferedDeleteTerms is the way to go if you run into memory problems. 
Feels quite ambiguous to estimate the memory of buffered deletes.

bq. I think is a good default once we fix the accounting in IndexWriter to 
properly account for buffered delete's RAM usage.
is there already an issue to fix the RAM usage? 

> IndexWriter does not properly account for the RAM consumed by pending deletes
> -----------------------------------------------------------------------------
>
>                 Key: LUCENE-1717
>                 URL: https://issues.apache.org/jira/browse/LUCENE-1717
>             Project: Lucene - Java
>          Issue Type: Bug
>    Affects Versions: 2.4, 2.4.1
>            Reporter: Michael McCandless
>             Fix For: 2.9
>
>         Attachments: BufferedDeletes_beautification.patch
>
>
> IndexWriter, with autoCommit false, is able to carry buffered deletes for 
> quite some time before materializing them to docIDs (thus freeing up RAM 
> used).
> It's only on triggering a merge (or, commit/close) that the deletes are 
> materialized and the RAM is freed.
> I expect this in practice is a smallish amount of RAM, but we should still 
> fix it.
> I don't have a patch yet so if someone wants to grab this, feel free!!

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