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From Chris McGee <cbmc...@ca.ibm.com>
Subject Re: How to improve performance of large numbers of successive searches?
Date Mon, 14 Apr 2008 17:46:52 GMT
Hi Erick,

Here is a quick overview of what I hope to accomplish with lucene. I am 
using a lucene database to store condensed information about a collection 
of data that I have. The data has to be constantly updated for correctness 
so that when one part changes certain other parts can be changed. Also, 
various queries will be performed on this data but in all cases the total 
result set must be retrieved and not just a select few hits. The results 
are used to manage the overall correctness of my data store and not to 
present to the user in some filtered way (by rank and only the top 100 
hits for example). Also, there could be cases where there will be a large 
set of terms to search for. To load all of this data into RAM is not 
feasible in most cases because there is too much data even if it was 
compressed.

So, I hope to be able to minimize the time to update the lucene database 
from my data store. I have already upgraded to Lucene 2.3.1 and performed 
a number of the suggestions on the lucene wiki with some success. As well, 
I want to help speed up the time it takes to query for a large number of 
terms (in most cases the terms have the same field name but different 
values).

In all cases I want to retrieve all matching documents at once. Because 
all matching documents must be retrieved I have no need for scoring, 
weights, boosts or any ranking of the results. Is there a way to strip 
away any of these pieces for better querying and directory building 
performance?

Thanks for your help,
Chris




"Erick Erickson" <erickerickson@gmail.com> 
14/04/2008 10:36 AM
Please respond to
java-user@lucene.apache.org


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Subject
Re: How to improve performance of large numbers of successive searches?






As I stated in my original reply, a Hits object re-executes the
search every 100 or so objects you examine. So some loop like
Hits hits = search....
for (int idx = 0; idx < hits.length; ++idx ) {
    Document doc = hits.get(idx);
}

really does something like

for (int idx = 0; idx < hits.length; ++idx ) {
    if (idx > 99 && (idx % 100) == 0) {
        re-execute the search and throw away entries 0-idx);
    }
    Document doc = hits.get(idx);
}

So the farther you get into the process, the more you throw away.

About collecting all the documents.... I wouldn't bother putting your
index in RAM until you've fully explored the alternatives. The first
of which is to determine what you really mean by "gather all result
documents"
If you have to return the entire contents of each document, you may have
to rethink your problem. If you're returning some subset of the data (say
some summary information), then you may get significant improvements
by indexing (perhaps UN_TOKENIZED) the data you need. That way, using
FieldSelector will grab things from the index rather than the stored data.
And, assuming your returned data is a small portion of your total 
document,
that should fix you up.

But a higher-level statement of the problem you're trying to resolve would
sure be helpful in terms of making reasonable suggestions. You haven't
characterized the problem you're trying to solve at all. As in *why* you
need
to return all the documents, the characteristics of the docs you're trying
to fetch. How big your data set is (as in # of docs). etc. etc. Unless and
until you
provide some of those details, all the advice in the world is just a shot
in the dark.

Shy do you think that " To perform 1000 term query searches each
with around 2000 hits" taking "well over a minute" is unacceptable?
After all, that's 2,000,000 documents you're analyzing. A minute
seems reasonable. What problem are you *really* trying to solve? or
is this just a load test?

Best
Erick


On Mon, Apr 14, 2008 at 10:17 AM, Chris McGee <cbmcgee@ca.ibm.com> wrote:

> Hi Erick,
>
> Thanks for the information. I tried using a HitCollector and a
> FieldSelector. I'm getting some dramatic improvements gathering large
> result sets using the FieldSelector. As it turned out I was able to 
assume
> in many cases that I could break out after a specific field in each
> document.
>
> Assuming that I need to gather all result documents each time, what are
> the advantages of using a HitCollector over Hits?
>
> Is there some way that I can load the index portion of the lucene data
> storage into RAM without loading everything into a RAMDirectory?
>
> Thanks,
> Chris McGee
>
>
>
>
> "Erick Erickson" <erickerickson@gmail.com>
> 10/04/2008 04:18 PM
> Please respond to
> java-user@lucene.apache.org
>
>
> To
> java-user@lucene.apache.org
> cc
>
> Subject
> Re: How to improve performance of large numbers of successive searches?
>
>
>
>
>
>
> From this <<< iterate over all of the hits>>> I infer that you're
> using a Hits object. This is a no-no when getting more than 100
> or so objects. In a nutshell, the query gets re-executed every 100
> fetches. So your 2,000 hits are executing the query 20 times.
>
> The Hits object is optimized for returning the top few scoring
> documents rather than get the entire result set.
>
> See HitCollector/TopDocs/TopDocCollector etc. for better ways
> of doing this.
>
> Also, if you're calling IndexReader.document(i) for each document
> you'll inevitably take a lot of time as you're loading all of each
> document.
> Think about lazy field loading (see FieldSelector).
>
> Best
> Erick
>
> P.S. If this is totally off base, perhaps you could post some of the
> code you think is slow....
>
> On Thu, Apr 10, 2008 at 2:34 PM, Chris McGee <cbmcgee@ca.ibm.com> wrote:
>
> > Hello,
> >
> > I am building fairly large directories (200-500 MB of disk space) 
using
> > lucene-java. Sometimes it can take upwards of 10-15 mins to create the
> > documents and write them to disk using my current configuration. I 
have
> > upgraded to the latest 2.3.1 version and followed many of the
> > recommendations offered on the wiki:
> >
> > http://wiki.apache.org/lucene-java/ImproveIndexingSpeed
> >
> > These tips have significantly improved the time to build the directory
> and
> > search it. However, I have noticed that when I perform term queries
> using
> > a searcher many times in rapid succession and iterate over all of the
> hits
> > it can take a significant time. To perform 1000 term query searches 
each
> > with around 2000 hits it takes well over a minute. The time seems to
> vary
> > linearly based on the number of searches (ie. 10 times more searches
> take
> > 10 times longer). I tried combining the searches into a BooleanQuery 
but
> > it only shaves off a small percentage (5-10%) of the total time.
> >
> > I was wondering if there is a faster way to retrieve all of the 
results
> > for my large collections of terms without using more memory and 
without
> > taking more time to build the directory? I already looked at bypassing
> the
> > searcher and using the IndexReader.termDocs() method directly to
> retrieve
> > the documents but there did not seem to be much performance 
improvement.
> > In the majority of my cases I am simplying looking for a large number 
of
> > values to the same field. Also, I'm not interested in scoring results
> > based on frequency or weights I need to retrieve all of the results
> > anyway.
> >
> > Any help with this would be great.
> >
> > Thanks,
> > Chris McGee
>
>


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