Friday, 2 November 2012

Power of Java MemoryMapped File

Power of Java MemoryMapped File


In JDK 1.4 interesting feature of Memory mapped file was added to java, which allow to map any file to OS memory for efficient reading. Memory mapped file can be used to developed  IPC type of solution. This article is experiment with memory mapped file to create IPC.

Some details about Memory Mapped File, definition from WIKI


A memory-mapped file is a segment of virtual memory which has been assigned a direct byte-for-byte correlation with some portion of a file or file-like resource. This resource is typically a file that is physically present on-disk, but can also be a device, shared memory object, or other resource that the operating system can reference through a file descriptor. Once present, this correlation between the file and the memory space permits applications to treat the mapped portion as if it were primary memory.


Sample Program

There are two java program one is writer and other is reader. Writer is producer and tries to write to Memory Mapped file , reader is consumer and it reads message from memory mapped file. This is just a sample program to show to idea, it does't handle many edge case but good enough to build something on top of memory mapped file.

MemoryMapWriter


import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;

public class MemoryMapWriter {

public static void main(String[] args) throws FileNotFoundException, IOException, InterruptedException {
File f = new File("c:/tmp/mapped.txt");
f.delete();
FileChannel fc = new RandomAccessFile(f, "rw").getChannel();
long bufferSize=8*1000;
MappedByteBuffer mem =fc.map(FileChannel.MapMode.READ_WRITE, 0, bufferSize);
int start = 0;
long counter=1;
long HUNDREDK=100000;
long startT = System.currentTimeMillis();
long noOfMessage = HUNDREDK * 10 * 10;
for(;;)
{
if(!mem.hasRemaining())
{
start+=mem.position();
mem =fc.map(FileChannel.MapMode.READ_WRITE, start, bufferSize);
}
mem.putLong(counter);
counter++;
if(counter > noOfMessage )
break;
}
long endT = System.currentTimeMillis();
long tot = endT - startT;
System.out.println(String.format("No Of Message %s , Time(ms) %s ",noOfMessage, tot)) ;
}

}

MemoryMapReader

import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;

public class MemoryMapReader {

/**
* @param args
* @throws IOException 
* @throws FileNotFoundException 
* @throws InterruptedException 
*/
public static void main(String[] args) throws FileNotFoundException, IOException, InterruptedException {
FileChannel fc = new RandomAccessFile(new File("c:/tmp/mapped.txt"), "rw").getChannel();
long bufferSize=8*1000;
MappedByteBuffer mem = fc.map(FileChannel.MapMode.READ_ONLY, 0, bufferSize);
long oldSize=fc.size();
long currentPos = 0;
long xx=currentPos;
long startTime = System.currentTimeMillis();
long lastValue=-1;
for(;;)
{
while(mem.hasRemaining())
{
lastValue=mem.getLong();
currentPos +=8;
}
if(currentPos < oldSize)
{
xx = xx + mem.position();
mem = fc.map(FileChannel.MapMode.READ_ONLY,xx, bufferSize);
continue;
}
else
{
long end = System.currentTimeMillis();
long tot = end-startTime;
System.out.println(String.format("Last Value Read %s , Time(ms) %s ",lastValue, tot));
System.out.println("Waiting for message");
while(true)
{
long newSize=fc.size();
if(newSize>oldSize)
{
oldSize = newSize;
xx = xx + mem.position();
mem = fc.map(FileChannel.MapMode.READ_ONLY,xx , oldSize-xx);
System.out.println("Got some data");
break;
}
}
}
}
}

}

Observation

Memory mapped can be very good option for developing Inter Process communication, throughput is also reasonably well for both produce & consumer.

Performance stats by run producer and consumer together 

Each message is one long number
Produce - 10 Million message - 16(s)
Consumer - 10 Million message 0.6(s)

Very simple message is used to show the idea, but it can any type of complex message, but when there is complex data structure then serialization can add to overhead. There are many technique to get over that overhead. More in next blog.

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