why python is slower than java?

[Kent Johnson]

I rarely find myself acting as an apologist for Java, and I understand 
that the point Alex is making is that Python's performance for this 
operation is quite good, and that the OP should post some code, but this 
is really too unfair a comparison for me not to say something.

There are two major differences between these two programs:
- The Java version is doing a character by character copy; the Python 
program reads the entire file into a buffer in one operation.
- The Java program is converting the entire file to and from Unicode; 
the Python program is copying the literal bytes.

Here is a much more comparable Java program (that will fail if the file 
size is over 2^31-1):

import java.io.*;

public class Copy {
     public static void main(String[] args) throws IOException {
         File inputFile = new File("/usr/share/dict/web2");
         int bufferSize = (int)inputFile.length();
         File outputFile = new File("/tmp/acopy");

         FileInputStream in = new FileInputStream(inputFile);
         FileOutputStream out = new FileOutputStream(outputFile);

         byte buffer[] = new byte[bufferSize];
         int len=bufferSize;

         while (true)
         {
             len=in.read(buffer,0,bufferSize);
             if (len<0 )
                 break;
             out.write(buffer,0,len);
         }

         in.close();
         out.close();
     }
}

Here are the results I get with this program and Alex's Python program 
on my G4-400 Mac:
kent% time java Copy
0.440u 0.320s 0:00.96 79.1%     0+0k 9+3io 0pf+0w
kent% time python Copy.py
0.100u 0.120s 0:00.31 70.9%     0+0k 2+4io 0pf+0w

The Python program is still substantially faster (3x), but with nowhere 
near the margin Alex saw.

Kent

Alex Martelli wrote:
> OK, could you provide a simple toy example that meets these conditions 
> -- does lot of identical disk-intensive I/O "in batch" -- and the 
> execution speed measured (and on what platform) for what Python and Java 
> implementations, please?
> 
> For example, taking a trivial Copy.java from somewhere on the net:
> 
> import java.io.*;
> 
> public class Copy {
>     public static void main(String[] args) throws IOException {
>         File inputFile = new File("/usr/share/dict/web2");
>         File outputFile = new File("/tmp/acopy");
> 
>         FileReader in = new FileReader(inputFile);
>         FileWriter out = new FileWriter(outputFile);
>         int c;
> 
>         while ((c = in.read()) != -1)
>            out.write(c);
> 
>         in.close();
>         out.close();
>     }
> }
> 
> and I observe (on an iBook 800, MacOSX 10.3.5):
> 
> kallisti:~ alex$ java -version
> java version "1.4.2_05"
> Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2_05-141.3)
> Java HotSpot(TM) Client VM (build 1.4.2-38, mixed mode)
> 
> -r--r--r--  1 root  wheel  2486825 12 Sep  2003 /usr/share/dict/web2
> 
> kallisti:~ alex$ time java Copy
> 
> real    0m7.058s
> user    0m5.820s
> sys     0m0.390s
> 
> versus:
> 
> kallisti:~ alex$ time python2.4 Copy.py
> 
> real    0m0.296s
> user    0m0.080s
> sys     0m0.170s
> 
> with Python 2.4 beta 1 for the roughly equivalent:
> 
> inputFile = file("/usr/share/dict/web2", 'r')
> outputFile = file("/tmp/acopy", 'w')
> 
> outputFile.write(inputFile.read())
> 
> inputFile.close()
> outputFile.close()
> 
> which isn't all that far from highly optimized system commands:
> 
> kallisti:~ alex$ time cp /usr/share/dict/web2 /tmp/acopy
> 
> real    0m0.167s
> user    0m0.000s
> sys     0m0.040s
> 
> kallisti:~ alex$ time cat /usr/share/dict/web2 >/tmp/acopy
> 
> real    0m0.149s
> user    0m0.000s
> sys     0m0.090s
> 
> 
> I'm sure the Java version can be optimized easily, too -- I just grabbed 
> the first thing I saw off the net.  But surely this example doesn't 
> point to any big performance issue with Python disk I/O wrt Java.  So, 
> unless you post concrete examples yourself, the smallest the better, 
> it's going to be pretty difficult to understand where your doubts are 
> coming from!
> 
> 
> Alex
>