В чем особая разница между FileChannel и MappedByteBuffer

Question

MacOS 10,14 2,2 ГГц Intel Core i7

Когда я читаю исходный код RocketMQ, есть два способа записи данных в MappedFile:

1. Записать данные в writeBuffer, зафиксировать в FileChannel, а затем вызвать fileChannel.force() для сброса данных на диск;
2. Записать данные в буферный буфер, созданный mappedByteBuffer.slice(), а затем вызвать force () для сброса данных на диск;

Я сомневаюсь, почему бы не записать данные непосредственно в mappedByteBuffer?

Поэтому я проверяю производительность с помощью своего кода.

import java.io.*;
import java.nio.ByteBuffer;
import java.nio.MappedByteBuffer;
import java.nio.channels.FileChannel;
public class MMapTest {

    static File file= new File("./test.txt");
    static ByteBuffer buffer;

    static int fileSize = 8 * 1024 * 1024;
    static boolean del = true;

    public static void main(String[] args) {
        init(1);
        deleteFile();
        int[] sizes = {128,256,512,4096,8192,1024*16,1024*32,1024*128,1024*512};
        try {
            for (int size : sizes) {
                testDBChannel(size);
                testMappedByteBuffer(size);

                System.out.println();
            }

        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static void init(int size) {
        buffer = ByteBuffer.allocateDirect(size);
    }

    private static void deleteFile() {
        file.delete();
    }

    private static void testDBChannel(int size) throws IOException {
        init(size);
        RandomAccessFile rw = new RandomAccessFile(file, "rw");
        FileChannel channel = rw.getChannel();
        int writeSize = 0;
        Long start = System.currentTimeMillis();
            while (writeSize < fileSize) {
            buffer.clear();
            buffer.put(new byte[size]);
            buffer.flip();
            channel.position(writeSize);
            channel.write(buffer);
            channel.force(false);
            writeSize += size;
        }
        //channel.force(false);
        System.out.println("DirectBuffer + FileChannel write " + size + " bytes every time cost: " + (System.currentTimeMillis() - start) + "ms");
        if(del)
        deleteFile();

    }

    private static void testMappedByteBuffer(int size) throws IOException {
        init(size);
        RandomAccessFile rw = new RandomAccessFile(file, "rw");
        FileChannel channel = rw.getChannel();
        MappedByteBuffer map = channel.map(FileChannel.MapMode.READ_WRITE, 0, fileSize);
        int writeSize = 0;
        Long start = System.currentTimeMillis();
        while (writeSize < fileSize) {
            map.put(new byte[size]);
            map.force();
            writeSize += size;
        }
        //map.force();
        System.out.println("MappedByteBuffer write " + size + " bytes every time cost: " + (System.currentTimeMillis() - start) + "ms");
        if(del)
            deleteFile();
    }
}

Выход:

DirectBuffer + FileChannel write 128 bytes every time cost: 3577ms
MappedByteBuffer write 128 bytes every time cost: 13518ms

DirectBuffer + FileChannel write 256 bytes every time cost: 1968ms
MappedByteBuffer write 256 bytes every time cost: 7044ms

DirectBuffer + FileChannel write 512 bytes every time cost: 1001ms
MappedByteBuffer write 512 bytes every time cost: 3037ms

DirectBuffer + FileChannel write 1024 bytes every time cost: 659ms
MappedByteBuffer write 1024 bytes every time cost: 1274ms

DirectBuffer + FileChannel write 4096 bytes every time cost: 214ms
MappedByteBuffer write 4096 bytes every time cost: 331ms

DirectBuffer + FileChannel write 8192 bytes every time cost: 137ms
MappedByteBuffer write 8192 bytes every time cost: 168ms

DirectBuffer + FileChannel write 16384 bytes every time cost: 77ms
MappedByteBuffer write 16384 bytes every time cost: 86ms

DirectBuffer + FileChannel write 32768 bytes every time cost: 44ms
MappedByteBuffer write 32768 bytes every time cost: 58ms

DirectBuffer + FileChannel write 131072 bytes every time cost: 16ms
MappedByteBuffer write 131072 bytes every time cost: 25ms

DirectBuffer + FileChannel write 524288 bytes every time cost: 10ms
MappedByteBuffer write 524288 bytes every time cost: 21ms

Почему MappedByteBuffer такой медленный? В чем разница между базовой реализацией?