DO NOT REPLY [Bug 34930] New: - IndexWriter.maybeMergeSegments() takes lots of CPU resources

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http://issues.apache.org/bugzilla/show_bug.cgi?id=34930

           Summary: IndexWriter.maybeMergeSegments() takes lots of CPU
                    resources
           Product: Lucene
           Version: CVS Nightly - Specify date in submission
          Platform: Macintosh
        OS/Version: Mac OS X 10.3
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Index
        AssignedTo: lucene-dev@jakarta.apache.org
        ReportedBy: psmith@apache.org


Note: I believe this to be the same situation with 1.4.3 as with SVN HEAD.

Analysis using hprof utility shows that during index creation with many
documents highlights that the CPU spends a large portion of it's time in
IndexWriter.maybeMergeSegments(), which seems to be a 'waste' compared with
other valuable CPU intensive operations such as tokenization etc.

Using the following test snippet to retrieve some rows from the db and create an
index:

        Analyzer a = new StandardAnalyzer();
        writer = new IndexWriter(indexDir, a, true);
        writer.setMergeFactor(1000);
        writer.setMaxBufferedDocs(10000);
        writer.setUseCompoundFile(false);
        connection = DriverManager.getConnection(
                "jdbc:inetdae7:tower.aconex.com?database=<somedb>", "secret",
                "squirrel");
        String sql = "select userid, userfirstname, userlastname, email from userx";
        LOG.info("sql=" + sql);
        Statement statement = connection.createStatement();
        statement.setFetchSize(5000);
        LOG.info("Executing sql");
        ResultSet rs = statement.executeQuery(sql);
        LOG.info("ResultSet retrieved");
        int row = 0;

        LOG.info("Indexing users");
        long begin = System.currentTimeMillis();
        while (rs.next()) {
            int userid = rs.getInt(1);
            String firstname = rs.getString(2);
            String lastname = rs.getString(3);
            String email = rs.getString(4);
            String fullName = firstname + " " + lastname;
            Document doc = new Document();
            doc.add(Field.Keyword("userid", userid+""));
            doc.add(Field.Keyword("firstname", firstname.toLowerCase()));
            doc.add(Field.Keyword("lastname", lastname.toLowerCase()));
            doc.add(Field.Text("name", fullName.toLowerCase()));
            doc.add(Field.Keyword("email", email.toLowerCase()));
            writer.addDocument(doc);
            row++;
            if((row % 100)==0){
                LOG.info(row + " indexed");
            }
        }
        double end = System.currentTimeMillis();
        double diff = (end-begin)/1000;
        double rate = row/diff;
        LOG.info("rate:" +rate);

On my 1.5GHz PowerBook with 1.5Gb RAM and a 5400 RPM drive, my CPU is maxed out,
and I end up getting a rate of indexing between 490-515 documents/second run
over 10 times in succession.  

By applying a simple patch to IndexWriter (see attached shortly), which defers
the calling of maybeMergeSegments() so that it is only called every 2000
times(an arbitrary figure), I appear to get a new rate of between 945-970
documents/second.  Using Luke to look inside each index created between these 2
there does not appear to be any difference.  Same number of Documents, same
number of Terms.

I'm not suggesting one should apply this patch, I'm just highlighting the
difference in performance that this sort of change gives you.  

We are about to use Lucene to index 4 million construction document records, and
so speeding up the indexing process is in our best interest! :)  If one
considers the amount of CPU time spent in maybeMergeSegments over the initial
index creation of 4 million documents, I think one could see how it would be
ideal to try to speed this area up (at least move the bottleneck to IO). 

I woul appreciate anyone taking a moment to comment on this.

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