Re: Function call overhead

I can't get a difference as large as yours, my times converge to 156ms versus 203ms.  My guess is that the cost comes from changing pos to a field.  I guess it's no big surprise that field writes are slower than local variable writes.  However I was surprised by the cost of changing pos to volatile: the time exploded to 4.5sec.

import java.util.Random;
public class F {
  public static void main(String[] args) throws Throwable {
    for (int x = 0; x < 100; x++)
      main2(args);
  }
  public static void main2(String[] args) throws Throwable {
    Random random = new Random();
    byte[] bytes = new byte[64 * 1024 * 1024];
    random.nextBytes(bytes);
    int pos = 0;
    int count = bytes.length;
    long start2 = System.currentTimeMillis();
    while (true) {
      int value = (pos < count) ? (bytes[pos] & 0xff) : -1;
      ++pos;
      if (value == -1)
        break;
    }
    System.out.println("TIME TAKEN A : " + (System.currentTimeMillis() - start2));
    final MyClass myClass = new MyClass(bytes);
    long start = System.currentTimeMillis();
    while (true) {
      int read = myClass.read();
      if (read == -1)
        break;
    }
    System.out.println("TIME TAKEN B : " + (System.currentTimeMillis() - start));
  }
}
class MyClass {
  private final byte[] buf;
  private /* volatile */ int pos;
  private int count;
  public MyClass(byte[] buf) {
    this.buf = buf;
    this.count = buf.length;
  }
  public final int read() {
    if (pos < count) {
      return (buf[pos++] & 0xFF);
    } else
      return -1;
  }
}

> -----Original Message-----
> From: Discussion of advanced Java topics. 
> [mailto:[email protected]] On Behalf Of 
> Avinash Lakshman
> Sent: Friday, December 07, 2007 12:05 AM
> To: [email protected]
> Subject: [ADVANCED-JAVA] Function call overhead
> 
> I am seeing this wierd problem. When I do the following I see 
> the time taken is around 170 ms.
> 
> public static void main(String[] args) throws Throwable
>     {
>         Random random = new Random();
>         byte[] bytes = new byte[64*1024*1024];
>         random.nextBytes(bytes);
>                 
>         int pos = 0;
>         int count = bytes.length;
>         long start2 = System.currentTimeMillis();
>         while ( true )
>         {
>             int value = (pos < count) ? (bytes[pos] & 0xff) : -1;
>             ++pos;
>             if ( value == -1 )
>                 break;
>         }
>         System.out.println("TIME TAKEN : " + 
> (System.currentTimeMillis() - start2)); }
> 
> Now I do the following I see the time taken is 516 ms:
> 
> class MyClass
> {
>     private byte[] buf = new byte[0];
>     private int pos;
>     private int count;
> 
>     public final int read()
>         {      
>             if (pos < count)
>             {
>                 return ( buf[pos++] & 0xFF );
>             }
>             else
>                 return -1;
>         }
> }
> 
> public void main(String[] args)
> {
>      MyClass myClass = new MyClass(bytes);        
>         int read = 0;        
>         long start = System.currentTimeMillis();        
>         while ( true )
>         {
>             read = myClass.read();
>             if ( read == -1 )
>                 break;            
>         }        
>         System.out.println("TIME TAKEN : " + 
> (System.currentTimeMillis() - start)); }
> 
> What is going on?
> 
> Thanks
> A
> 
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