OutOfMemoryError throwed by SimpleMergedSegmentWarmer

[308181687]

Hi, all
   we got an OutOfMemoryError throwed ‍by SimpleMergedSegmentWarmer. We use 
lucene 4.7， and access index file by NRTCachingDirectory/MMapDirectory. Could 
any body give me a hand?  Strack trace is as follows:






org.apache.lucene.index.MergePolicy$MergeException: java.lang.OutOfMemoryError: 
Java heap space‍   
     at 
org.apache.lucene.index.ConcurrentMergeScheduler$MergeThread.run(ConcurrentMergeScheduler.java:518)


Caused by: java.lang.OutOfMemoryError: Java heap space
        at 
org.apache.lucene.codecs.lucene42.Lucene42DocValuesProducer.loadNumeric(Lucene42DocValuesProducer.java:228)
        at 
org.apache.lucene.codecs.lucene42.Lucene42DocValuesProducer.getNumeric(Lucene42DocValuesProducer.java:188)
        at 
org.apache.lucene.index.SegmentCoreReaders.getNormValues(SegmentCoreReaders.java:166)
        at 
org.apache.lucene.index.SegmentReader.getNormValues(SegmentReader.java:519)
        at 
org.apache.lucene.index.SimpleMergedSegmentWarmer.warm(SimpleMergedSegmentWarmer.java:52)
        at 
org.apache.lucene.index.IndexWriter.mergeMiddle(IndexWriter.java:4275)
        at org.apache.lucene.index.IndexWriter.merge(IndexWriter.java:3743)
        at 
org.apache.lucene.index.ConcurrentMergeScheduler.doMerge(ConcurrentMergeScheduler.java:405)
        at 
org.apache.lucene.index.ConcurrentMergeScheduler$MergeThread.run(ConcurrentMergeScheduler.java:482)‍




Thanks & Best Regards!