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From Arjen van der Meijden <>
Subject Re: Improving search performance for forum search
Date Sat, 24 Nov 2012 09:31:30 GMT
Hi Uwe,

I forgot to update on this - and since the thread is now a bit old, I 
won't rake it up again - but there was indeed a nice performance gain 
from the change to DocValues. The total search time went down from the 
mentioned 330ms to about 190ms.

I actually have to look at other performance aspects, outside Lucene's 
core, to optimize it any further.

So, thanks for the tip,
Best regards,


On 13-11-2012 9:37 Uwe Schindler wrote:
> IndexReader.document() is documented to be used only for presenting search results. Fetching
the document for every possible hit while scoring is the performance killer (it is funny that
your query only takes 300 ms, maybe the SSD).
> The correct solution is to use the new field type DocValues, which are similar to stored
fields but are stored column wise (and not document wise) and can be loaded to memory completely.
In your CustomScoreQuery, you can use the DocValues (available on AtmoicReader) to score your
> Uwe
> -----
> Uwe Schindler
> H.-H.-Meier-Allee 63, D-28213 Bremen
> eMail:
>> -----Original Message-----
>> From: Arjen van der Meijden []
>> Sent: Tuesday, November 13, 2012 8:36 AM
>> To:
>> Subject: Improving search performance for forum search
>> Hi List,
>> I'm working on a search engine for our forum using Lucene 4. Since its a
>> brand new search engine, I can change it as I see fit.
>> We have about 1.5M topics in the various subforums and on average 20
>> replies to each topic (i.e. about 33M in total).
>> For now, I've opted to index all replies to topics and group the best reply-
>> matches based on their topic-id and only keep the top X (currently at most 5
>> per topic).
>> This works quite well, but the search time is fairly long. It takes about 330ms
>> to achieve a result with a single word that matches about 45k of the topics.
>> The index is on a ssd in my test-machine and the 330ms is after repeated
>> searches and including several other aspects.
>> Obviously, with an average of 20 replies per topic, that could actually be
>> upwards to about 900k actual Documents being matched (I didn't look at the
>> actual count, but it was probably less).
>> According to yourkit, about 50% of the time is spent in the Scorer and
>> Collector. And it mainly breaks down to two aspects, my custom scoring and
>> the fact that my code is set up to retrieve all results and do further
>> processing. But given the grouping on the topic-id, I doubt I can actually
>> escape that last part...
>> To enable customized scoring of the documents, I need access to per-reply
>> and per-topic meta-data. The per-topic meta-data is stored in in-memory
>> objects accessible via a HashMap based on the topic's id and the per-reply
>> meta-data is simply a unix timestamp stored in a binary field.
>> A fair amount of the time (about 20% is spent in Reader.document(doc,
>> StoredFieldVisitor)) is spent retrieving the topicId, replyId and that
>> timestamp from the Document's. The topicId and replyId are encoded into a
>> single binary field.
>> I already use a specialized StoredFieldVisitor that only retrieves those two
>> binary fields from each document.
>> So now the questions:
>> - Can I reduce the overhead of retrieving the document's fields even
>> further?
>> -- Should I use a different Codec (perhaps Pulsing or one of the "load the
>> fielddata in memory"-codecs) to fetch those binary fields?
>> -- Should I change them to other field types?
>> -- Should I encode all binary data in a single field, rather than two fields (i.e.
>> going from 9+8 bytes to 17)?
>> - Should I use a FieldCache to be able to retrieve the required fields quicker
>> (and how do you even use a FieldCache??) once they've been read?
>> - Is there a way to delay or skip part of the scoring, so I can skip retrieving
>> Documents altogether? This would probably require predicting that the
>> results is intended for a topic which already has 5 very good replies, so that
>> seems a bit far-fetched (although it would yield the most gain).
>> Any other tips?
>> Best regards,
>> Arjen van der Meijden
>> B.V.
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