Optimizing ‘yes’ UNIX command in Java

the One of Unix core commands is yes which simply prints infinity number of letter y.

I read a blog post (https://matthias-endler.de/2017/yes/) about how people optimize it to output y with a speed of 10 GB/s. I get 7.5 GB/s on my machine using this command.

I tried to achieve similar results with Java but it stopped at 70 MB/s.

My code is:

public class yes {

    static byte[] bb = new byte[]{'y','n'};

    public static void main(String[] args) throws IOException {

        BufferedOutputStream writer = new BufferedOutputStream(System.out);
        for (;;) {
            writer.write(bb);
        }

    }
}

Do you have any idea how to optimize it to get results similar to UNIX version?

Your program calls the write() method of the BufferedOutputStream
with a two byte array. These two bytes are stored in the
stream’s buffer, and the buffer is written to standard output when it is
full.

On macOS 10.13 the fs_usage tool shows that the buffer size is 8 kB, and on my MacBook the

java -cp . yes | pv > /dev/null

benchmark reports that the data is written with about 50 MB/s.

This can be improved by calling write() with a larger byte array:

public class yes {

    static final int BUFFERSIZE = 8 * 1024;
    static byte[] bb = new byte[BUFFERSIZE];

    public static void main(String[] args) throws IOException {

        for (int i = 0; i < BUFFERSIZE; i += 2) {
            bb[i] = 'y';
            bb[i+1] = 'n';
        }

        BufferedOutputStream writer = new BufferedOutputStream(System.out);
        for (;;) {
            writer.write(bb);
        }
    }
}

Now only one write() call is needed to write 8 kB data, instead
of 4096 calls. On my MacBook this increased the speed to
about 2 GB/s, that is a factor of more than 40.

This can possibly be further improved by choosing a larger buffer.

With a byte array larger that the stream buffer size, BufferedOutputStream does
not buffer anymore and writes the data to stdout immediately. Therefore the
“lower-level” unbuffered FileOutputStream would be sufficient:

    FileOutputStream writer = new FileOutputStream(FileDescriptor.out);
    for (;;) {
        writer.write(bb);
    }

However, I could not observe a difference in the performance.