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From "Bryan Dotzour" <BDotz...@widen.com>
Subject RE: First search is slow after updating index .. subsequent searches very fast
Date Fri, 22 Dec 2006 15:51:35 GMT
Thanks for that tidbit Mark.  I was just looking through the LIA book
and stumbled across this sentence under the "5.1.9 Performance effect of
sorting" section.  It says: "[When sorting by a String type] each unique
term is also cached for each document. Only the actual fields used for
sorting are cached in this manner."

In the case that I originally described, our default sorting mechanism
is an alphabetical sort on the title of each object returned in the
search.  So I take this excerpt from the book to mean that the
FieldCache has to read each title value from each document in order to
perform the sort.  That pretty much sounds like exactly what you're
saying Mark.

I guess the only question left in my mind is, does the FieldCache have
to read every value for every document in the entire index to perform
the sort, or just the values in the documents returned in the search?
My guess would be the latter although this one index seems much slower
than all of the others and the only difference is the sheer number of
items in the index.

-----Original Message-----
From: Mark Miller [mailto:markrmiller@gmail.com] 
Sent: Thursday, December 21, 2006 2:48 PM
To: java-user@lucene.apache.org
Subject: Re: First search is slow after updating index .. subsequent
searches very fast

Since you say you are sorting on a field the bulk of the time will be 
doing the sort and caching it (FieldCache). Subsequent searches use that

cache to avoid paying the full sort cost again. If you where doing 
relevancy sorting you would not experience such a big delay.

- Mark

Bryan Dotzour wrote:
> Otis thanks for your suggestion, it seems to be working pretty well!
> I'm just curious if you (or anyone else) could describe what is
actually
> happening during that initial query that ends up taking so much time.
> We have several different indexes for different types of objects and
> it's only this one index that exhibits this kind of behavior.  Is it
> something related to the size of the index, or the number of fields,
or
> how fragmented the index is?
>
> I'm just trying to get a little better understanding of what is going
on
> under the covers there.  I'll spend some time with the source to see
if
> I can figure it out, but any tips from the experts would be much
> appreciated. =)
>
> Thanks again!
> Bryan
>
> -----Original Message-----
> From: Otis Gospodnetic [mailto:otis_gospodnetic@yahoo.com] 
> Sent: Wednesday, December 20, 2006 4:28 PM
> To: java-user@lucene.apache.org
> Subject: Re: First search is slow after updating index .. subsequent
> searches very fast
>
> To populate FieldCache, the number of matches doesn't matter.  There
is
> no need to be scrimy there - you don't really save anything by running
a
> query that matches only a few docs.  Just run something that looks
like
> a common query.
>
> For warming up new indices, one can also use the `dd' trick under
UNIX.
>
> Otis
>
> ----- Original Message ----
> From: Bryan Dotzour <BDotzour@widen.com>
> To: java-user@lucene.apache.org
> Sent: Wednesday, December 20, 2006 5:23:40 PM
> Subject: RE: First search is slow after updating index .. subsequent
> searches very fast
>
> One question about this, Otis... When "warming up" the new searcher,
> should the query return a lot of results, or does it matter?  Can I
just
> do like an ID = X query and get one document back?  Is that sufficient
> or is it better to run a query that will get lots of hits?
>
> Thanks again,
> Bryan
>
> -----Original Message-----
> From: Otis Gospodnetic [mailto:otis_gospodnetic@yahoo.com] 
> Sent: Wednesday, December 20, 2006 3:28 PM
> To: java-user@lucene.apache.org
> Subject: Re: First search is slow after updating index .. subsequent
> searches very fast
>
> All sounds good.  Opening a new IndexReader can take a bit of time.
If
> you use sorting of any kind other than default sorting by relevance,
> this delay on the first search is also probably caused by the lazy
> FieldCache population.  The cure for that is to open a new
> IndexReader/Searcher before you close the old one, warm it up with a
> query + sort, and then switch IndexReader/Searchers, closing the old
> one.
>
> Otis
>
> ----- Original Message ----
> From: Bryan Dotzour <BDotzour@widen.com>
> To: java-user@lucene.apache.org
> Sent: Wednesday, December 20, 2006 3:59:19 PM
> Subject: First search is slow after updating index .. subsequent
> searches very fast
>
> I'm investigating some performance issues with the way we're using
> Lucene in our web app and am interested if anyone could shed some
light
> on what might be going on.  Hopefully I can provide enough
information,
> please let me know if there's more I can give.
>
>  
>
> We're using Lucene 2.0.0 and I'm currently working with disk-based
> indexing (although in production I'll want to be using RAM indexing).
> In our environment, we build up our Lucene index at application start
up
> time and then we optimize the index.  From then on, updates and
deletes
> to the index occur fairly frequently but we don't optimize until the
> middle of the night when the impact would be at its minimum.  After a
> while, what I see is that searches will be very fast (~400 ms) until I
> make a modification that will require a single document to be
> re-indexed.  Immediately after that has occurred, the next search will
> take substantially longer (sometimes up to ~25s).  After that search
has
> run, the next search will be back at the ~400ms time.
>
>  
>
> Our algorithm for handling the updates is as follows:
>
> 1.       open an IndexReader on the directory
>
> 2.       delete the document using the reader
>
> 3.       close the reader
>
> 4.       open an IndexWriter
>
> 5.       add the new document using the writer
>
> 6.       close the writer
>
>  
>
> For searches:
>
> 1.    We cache off an IndexReader for the index, as well as an
> IndexSearcher, which uses that reader
> 2.    When a search is initiated we check to see if the version of the
> index has changed using getCurrentVersion()
> 3.    If it has changed, we close our IndexSearcher, close the
> IndexReader and re-open them both
>
>  
>
>  
>
> Anything sound non-standard in that workflow?  Does anyone have an
idea
> of what might be happening during that slow down?
>
>  
>
> Thanks for your time,
>
> Bryan
>
>  
>
>  
>
>  
>
> (For a little more info, here is a very common stack trace snippet
that
> I gather when the "slow search" is running.  It seems much of the time
> is spent in MultiReader or MultiTermDocs)
>
>  
>
org.apache.lucene.index.CompoundFileReader$CSIndexInput.readInternal(Com
> poundFileReader.java:214)
>  
>
org.apache.lucene.store.BufferedIndexInput.refill(BufferedIndexInput.jav
> a:64)
>  
>
org.apache.lucene.store.BufferedIndexInput.readByte(BufferedIndexInput.j
> ava:33)
>       org.apache.lucene.store.IndexInput.readVInt(IndexInput.java:56)
>       org.apache.lucene.index.TermBuffer.read(TermBuffer.java:62)
>  
> org.apache.lucene.index.SegmentTermEnum.next(SegmentTermEnum.java:117)
>  
>
org.apache.lucene.index.SegmentTermEnum.scanTo(SegmentTermEnum.java:148)
>  
>
org.apache.lucene.index.TermInfosReader.scanEnum(TermInfosReader.java:15
> 7)
>  
> org.apache.lucene.index.TermInfosReader.get(TermInfosReader.java:151)
>  
> org.apache.lucene.index.SegmentTermDocs.seek(SegmentTermDocs.java:50)
>  
> org.apache.lucene.index.MultiTermDocs.termDocs(MultiReader.java:392)
>       org.apache.lucene.index.MultiTermDocs.next(MultiReader.java:348)
>       org.apache.lucene.index.MultiTermDocs.next(MultiReader.java:349)
>       org.apache.lucene.index.MultiTermDocs.next(MultiReader.java:349)
>       org.apache.lucene.index.MultiTermDocs.next(MultiReader.java:349)
>  
>
org.apache.lucene.search.FieldCacheImpl.getInts(FieldCacheImpl.java:171)
>  
>
org.apache.lucene.search.FieldCacheImpl.getInts(FieldCacheImpl.java:153)
>  
>
org.apache.lucene.search.FieldCacheImpl.getAuto(FieldCacheImpl.java:349)
>  
>
org.apache.lucene.search.FieldSortedHitQueue.comparatorAuto(FieldSortedH
> itQueue.java:346)
>  
>
org.apache.lucene.search.FieldSortedHitQueue.getCachedComparator(FieldSo
> rtedHitQueue.java:189)
>  
>
org.apache.lucene.search.FieldSortedHitQueue.(FieldSortedHitQueue.java:5
> 8)
>  
>
org.apache.lucene.search.TopFieldDocCollector.(TopFieldDocCollector.java
> :40)
>  
> org.apache.lucene.search.IndexSearcher.search(IndexSearcher.java:108)
>       org.apache.lucene.search.Hits.getMoreDocs(Hits.java:65)
>       org.apache.lucene.search.Hits.(Hits.java:52)
>       org.apache.lucene.search.Searcher.search(Searcher.java:53)
>
>  
>
>
>
>
>
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