关于lucene的RAMDirectory和FSDirectory的性能问题的困惑

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关于lucene的RAMDirectory和FSDirectory的性能问题的困惑5

在lucene in Action书中说RAMDirectory的性能总是比FSDirectory优越（书中2.7.2章节）
并附了测试用例
我根据测试用例去实际测试了一下，结果是相反的
这让我很困惑，内存没道理比文件系统慢啊。。
附上执行结果:
RAMDirectory Time: 500 ms
FSDirectory Time: 266 ms

以下是我的代码（基本照搬书中例子，只更改了for循环写法和使用了2.9的推荐方法取代了老版本的方法）

import java.io.File;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Collection;

import org.apache.lucene.analysis.SimpleAnalyzer;
import org.apache.lucene.document.Document;
import org.apache.lucene.document.Field;
import org.apache.lucene.index.IndexWriter;
import org.apache.lucene.store.Directory;
import org.apache.lucene.store.FSDirectory;
import org.apache.lucene.store.RAMDirectory;

import junit.framework.TestCase;

/**
 * 测试FSDirectory和RAMDirectory之间的性能差异
 * 理论上来说,后者应大于前者,但是实际测试值相反,why? lucifer 2010-5-10
 * @author lucifer
 *
 */
public class FSversusRAMDirectoryTest extends TestCase
{
	private Directory fsDir;
	private Directory ramDir;
	private Collection<String> docs = loadDocuments(3000,5);
	
	protected void setUp() throws Exception
	{
		String fsIndexDir = System.getProperty("java.io.tmpdir","tmp")+File.separator+"fs-index";
		ramDir = new RAMDirectory();
		fsDir = FSDirectory.open(new File(fsIndexDir));
	}
	
	public void testTiming() throws IOException
	{
		long ramTiming = timeIndexWriter(ramDir);
		long fsTiming = timeIndexWriter(fsDir);
		
//		assertTrue(fsTiming>ramTiming);
		
		System.out.println("RAMDirectory Time: "+ ramTiming +" ms");
		System.out.println("FSDirectory Time: "+ fsTiming +" ms");
	}
	
	private long timeIndexWriter(Directory dir)throws IOException
	{
		long start = System.currentTimeMillis();
		addDocuments(dir);
		long stop = System.currentTimeMillis();
		return (stop - start);
	}
	
	private void addDocuments(Directory dir)throws IOException
	{
		/**
		 * SimpleAnalyzer:把所有字符过滤成小写
		 * 把参数设为false时,使用RAMDirectory出错,报文件找不到
		 */
		IndexWriter writer = new IndexWriter(dir,new SimpleAnalyzer(),true,IndexWriter.MaxFieldLength.UNLIMITED);
		
		/**
		 * 以下参数影响FSDirectory性能
		 * MergeFactor的值不能小于2
		 * MaxMergeDocs的值可以设置的比MergeFactor小,未见异常抛出
		 */
		writer.setMergeFactor(10);
		writer.setMaxMergeDocs(10000);
		
		for(Object obj:docs)
		{
			Document doc = new Document();
			String word = (String)obj;
			doc.add(new Field("keyword",word,Field.Store.YES,Field.Index.NOT_ANALYZED));
			doc.add(new Field("unindexed",word,Field.Store.YES,Field.Index.NO));
			doc.add(new Field("unstored",word,Field.Store.NO,Field.Index.NOT_ANALYZED));
			doc.add(new Field("text",word,Field.Store.NO,Field.Index.ANALYZED));
			writer.addDocument(doc);
		}
		writer.optimize();
		writer.close();
	}
	
	private Collection<String> loadDocuments(int numDocs,int wordsPerDoc)
	{
		Collection<String> docs = new ArrayList<String>(numDocs);
		for(int i=0;i<numDocs;i++)
		{
			StringBuffer doc = new StringBuffer(wordsPerDoc);
			for(int j=0;j<wordsPerDoc;j++)
				doc.append("bibamus ");
			docs.add(doc.toString());
		}
		
		return docs;
	}
}