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From "Nader, John P" <john.na...@cengage.com>
Subject RE: Term browsing much slower in Lucene 3.x.x
Date Thu, 29 Jul 2010 21:48:32 GMT
Thanks much for your response.  Yes, our terms are sorted in index-sort order.  I think you
have a good suggestion, which is to get the term docs once and then seek to each term.  I
will try that approach and report back to the forum on the results.

Like you I am surprised by the overhead of the added synchronization.  I don't think is waiting
on locks, but rather the memory flush and loading that goes on.

-John

-----Original Message-----
From: Michael McCandless [mailto:lucene@mikemccandless.com] 
Sent: Thursday, July 29, 2010 5:55 AM
To: java-user@lucene.apache.org
Subject: Re: Term browsing much slower in Lucene 3.x.x

On Wed, Jul 28, 2010 at 2:39 PM, Nader, John P <john.nader@cengage.com> wrote:
> We recently upgraded from lucene 2.4.0 to lucene 3.0.2.  Our load testing revealed a
serious performance drop specific to traversing the list of terms and their associated documents
for a given indexed field.  Our code looks something like this:
>
> for(Term term : terms) {
> TermDocs termDocs = indexReader.termDocs(term);
> while(termDocs.next()) {   //  much slower here
>    int doc = termDocs.doc();
>    ...do something with each doc...
> }

Is that IndexReader reading multiple segments or single segment?

> The slowness is all on the first call to TermDocs.next() for each term.  Further investigation
comparing 2.4.0 and 3.0.2 revealed that there is some new synchronization on the SegmentTermDocs
constructor and the SegmentReader.getTermsReader().  The first call to next() hits this synchronization,
causing a 4x slowdown on an 8 CPU machine.

There was some added sync, however, the code within those sync blocks
is minuscule (looking up a field).  It's weird that you're seeing a 4X
hit because of this.  We could conceivably optimize this code to avoid
the sync blocks if the reader is readOnly.

> My first question is should we be using a different approach to process each term's doc
list that would be more efficient?  The synchronization appears to be on aspects of these
classes that the next() operation is not concerned with.

Are you sorting your terms in index-sort order (UTF16, ie
String.compareTo)?  This can be an important gain especially if you
have many terms.

Also, if you are working with your top reader, you should see some
perf gain by instead working w/ the sub readers directly, ie:

  for(IndexReader subReader : indexReader.getSequentialSubReaders()) {
     ...
  }

Also, instead of getting a new TermDocs every time, you should get a
single TermDocs up front (IndexReader.termDocs()), and then seek it to
your term (termDocs.seek(term)), validate the term in seek'd to in
fact matches what you asked for, then iterate its docs.

> My other question is whether there are planned performance enhancements to address this
loss of performance?

These APIs are very different in the next major release (4.0) of
Lucene, so except for problems spotted by users like you, there's not
much more dev happening against them.

Mike

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