[
https://issues.apache.org/jira/browse/LUCENE-2324?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12857164#action_12857164
]
Michael Busch commented on LUCENE-2324:
---------------------------------------
{quote}
It's for performance. I expect there are apps where a given
thread/pool indexes certain kind of docs, ie, the app threads
themselves have "affinity" for docs with similar term distributions.
In which case, it's best (most RAM efficient) if those docs w/
presumably similar term stats are sent back to the same DW. If you
mix in different term stats into one buffer you get worse RAM
efficiency.
{quote}
I do see your point, but I feel like we shouldn't optimize/make compromises for
this use case. Mainly, because I think apps with such an affinity that you
describe are very rare? The usual design is a queued ingestion pipeline, where
a pool of indexer threads take docs out of a queue and feed them to an
IndexWriter, I think? In such a world the threads wouldn't have an affinity
for similar docs.
And if a user really has so different docs, maybe the right answer would be to
have more than one single index? Even if today an app utilizes the thread
affinity, this only results in maybe somewhat faster indexing performance, but
the benefits would be lost after flusing/merging.
If we assign docs randomly to available DocumentsWriterPerThreads, then we
should on average make good use of the overall memory? Alternatively we could
also select the DWPT from the pool of available DWPTs that has the highest
amount of free memory?
Having a fully decoupled memory management is compelling I think, mainly
because it makes everything so much simpler. A DWPT could decide itself when
it's time to flush, and the other ones can keep going independently.
If you do have a global RAM management, how would the flushing work? E.g. when
a global flush is triggered because all RAM is consumed, and we pick the DWPT
with the highest amount of allocated memory for flushing, what will the other
DWPTs do during that flush? Wouldn't we have to pause the other DWPTs to make
sure we don't exceed the maxRAMBufferSize?
Of course we could say "always flush when 90% of the overall memory is
consumed", but how would we know that the remaining 10% won't fill up during
the time the flush takes?
> Per thread DocumentsWriters that write their own private segments
> -----------------------------------------------------------------
>
> Key: LUCENE-2324
> URL: https://issues.apache.org/jira/browse/LUCENE-2324
> Project: Lucene - Java
> Issue Type: Improvement
> Components: Index
> Reporter: Michael Busch
> Assignee: Michael Busch
> Priority: Minor
> Fix For: 3.1
>
> Attachments: lucene-2324.patch, LUCENE-2324.patch
>
>
> See LUCENE-2293 for motivation and more details.
> I'm copying here Mike's summary he posted on 2293:
> Change the approach for how we buffer in RAM to a more isolated
> approach, whereby IW has N fully independent RAM segments
> in-process and when a doc needs to be indexed it's added to one of
> them. Each segment would also write its own doc stores and
> "normal" segment merging (not the inefficient merge we now do on
> flush) would merge them. This should be a good simplification in
> the chain (eg maybe we can remove the *PerThread classes). The
> segments can flush independently, letting us make much better
> concurrent use of IO & CPU.
--
This message is automatically generated by JIRA.
-
If you think it was sent incorrectly contact one of the administrators:
https://issues.apache.org/jira/secure/Administrators.jspa
-
For more information on JIRA, see: http://www.atlassian.com/software/jira
---------------------------------------------------------------------
To unsubscribe, e-mail: java-dev-unsubscr...@lucene.apache.org
For additional commands, e-mail: java-dev-h...@lucene.apache.org
