Index: CHANGES.txt
===================================================================
--- CHANGES.txt	(revision 922587)
+++ CHANGES.txt	(working copy)
@@ -167,6 +167,11 @@
 * LUCENE-2247: Added a CharArrayMap<V> for performance improvements
   in some stemmers and synonym filters. (Uwe Schindler)
 
+* LUCENE-2293: Expose control over max number of threads that
+  IndexWriter will allow to run concurrently while indexing
+  documents (previously this was hardwired to 5), using
+  IndexWriterConfig.setMaxThreadStates.  (Mike McCandless)
+
 Optimizations
 
 * LUCENE-2075: Terms dict cache is now shared across threads instead
Index: src/test/org/apache/lucene/index/TestIndexWriterConfig.java
===================================================================
--- src/test/org/apache/lucene/index/TestIndexWriterConfig.java	(revision 922587)
+++ src/test/org/apache/lucene/index/TestIndexWriterConfig.java	(working copy)
@@ -98,6 +98,7 @@
     getters.add("getMaxBufferedDocs");
     getters.add("getIndexingChain");
     getters.add("getMergedSegmentWarmer");
+    getters.add("getMaxThreadStates");
     for (Method m : IndexWriterConfig.class.getDeclaredMethods()) {
       if (m.getDeclaringClass() == IndexWriterConfig.class && m.getName().startsWith("get")) {
         assertTrue("method " + m.getName() + " is not tested for defaults", getters.contains(m.getName()));
Index: src/java/org/apache/lucene/index/DocumentsWriter.java
===================================================================
--- src/java/org/apache/lucene/index/DocumentsWriter.java	(revision 922587)
+++ src/java/org/apache/lucene/index/DocumentsWriter.java	(working copy)
@@ -125,7 +125,6 @@
 
   // Max # ThreadState instances; if there are more threads
   // than this they share ThreadStates
-  private final static int MAX_THREAD_STATE = 5;
   private DocumentsWriterThreadState[] threadStates = new DocumentsWriterThreadState[0];
   private final HashMap<Thread,DocumentsWriterThreadState> threadBindings = new HashMap<Thread,DocumentsWriterThreadState>();
 
@@ -141,6 +140,10 @@
   int maxFieldLength = IndexWriterConfig.UNLIMITED_FIELD_LENGTH;
   Similarity similarity;
 
+  // max # simultaneous threads; if there are more than
+  // this, they wait for others to finish first
+  private final int maxThreadStates;
+
   List<String> newFiles;
 
   static class DocState {
@@ -301,10 +304,11 @@
 
   private boolean closed;
 
-  DocumentsWriter(Directory directory, IndexWriter writer, IndexingChain indexingChain) throws IOException {
+  DocumentsWriter(Directory directory, IndexWriter writer, IndexingChain indexingChain, int maxThreadStates) throws IOException {
     this.directory = directory;
     this.writer = writer;
     this.similarity = writer.getConfig().getSimilarity();
+    this.maxThreadStates = maxThreadStates;
     flushedDocCount = writer.maxDoc();
 
     consumer = indexingChain.getChain(this);
@@ -721,7 +725,7 @@
         if (minThreadState == null || ts.numThreads < minThreadState.numThreads)
           minThreadState = ts;
       }
-      if (minThreadState != null && (minThreadState.numThreads == 0 || threadStates.length >= MAX_THREAD_STATE)) {
+      if (minThreadState != null && (minThreadState.numThreads == 0 || threadStates.length >= maxThreadStates)) {
         state = minThreadState;
         state.numThreads++;
       } else {
Index: src/java/org/apache/lucene/index/IndexWriter.java
===================================================================
--- src/java/org/apache/lucene/index/IndexWriter.java	(revision 922587)
+++ src/java/org/apache/lucene/index/IndexWriter.java	(working copy)
@@ -1153,7 +1153,7 @@
 
       setRollbackSegmentInfos(segmentInfos);
 
-      docWriter = new DocumentsWriter(directory, this, conf.getIndexingChain());
+      docWriter = new DocumentsWriter(directory, this, conf.getIndexingChain(), conf.getMaxThreadStates());
       docWriter.setInfoStream(infoStream);
       docWriter.setMaxFieldLength(maxFieldLength);
 
Index: src/java/org/apache/lucene/index/IndexWriterConfig.java
===================================================================
--- src/java/org/apache/lucene/index/IndexWriterConfig.java	(revision 922587)
+++ src/java/org/apache/lucene/index/IndexWriterConfig.java	(working copy)
@@ -78,6 +78,12 @@
    */
   public static long WRITE_LOCK_TIMEOUT = 1000;
 
+  /** The maximum number of simultaneous threads that may be
+   *  indexing documents at once in IndexWriter; if more
+   *  than this many threads arrive they will wait for
+   *  others to finish. */
+  public final static int DEFAULT_MAX_THREAD_STATES = 8;
+
   /**
    * Sets the default (for any instance) maximum time to wait for a write lock
    * (in milliseconds).
@@ -110,6 +116,7 @@
   private int maxBufferedDocs;
   private IndexingChain indexingChain;
   private IndexReaderWarmer mergedSegmentWarmer;
+  private int maxThreadStates;
   
   // required for clone
   private Version matchVersion;
@@ -137,6 +144,7 @@
     maxBufferedDocs = DEFAULT_MAX_BUFFERED_DOCS;
     indexingChain = DocumentsWriter.defaultIndexingChain;
     mergedSegmentWarmer = null;
+    maxThreadStates = DEFAULT_MAX_THREAD_STATES;
   }
   
   @Override
@@ -483,7 +491,19 @@
     return mergedSegmentWarmer;
   }
 
+  /** Sets the max number of simultaneous threads that may
+   *  be indexing documents at once in IndexWriter. */
+  public IndexWriterConfig setMaxThreadStates(int maxThreadStates) {
+    this.maxThreadStates = maxThreadStates;
+    return this;
+  }
 
+  /** Returns the max number of simultaneous threads that
+   *  may be indexing documents at once in IndexWriter. */
+  public int getMaxThreadStates() {
+    return maxThreadStates;
+  }
+
   /** Expert: sets the {@link DocConsumer} chain to be used to process documents. */
   IndexWriterConfig setIndexingChain(IndexingChain indexingChain) {
     this.indexingChain = indexingChain == null ? DocumentsWriter.defaultIndexingChain : indexingChain;
@@ -513,6 +533,7 @@
     sb.append("ramBufferSizeMB=").append(ramBufferSizeMB).append("\n");
     sb.append("maxBufferedDocs=").append(maxBufferedDocs).append("\n");
     sb.append("mergedSegmentWarmer=").append(mergedSegmentWarmer).append("\n");
+    sb.append("maxThreadStates=").append(maxThreadStates).append("\n");
     return sb.toString();
   }
 }