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Subject: Re: hash table size - is a power of 2 still an advantage these days?

Author: Robert Hyatt

Date: 08:49:58 09/24/03

On September 24, 2003 at 11:20:13, William Penn wrote:

>For example, is 512MB hash table size really better than 513MB (512+1), or
>better than 528MB (512+16)?
>
>Another example, is 768MB (3x256) better than 784MB (3x256+16)?
>
>I'm also wondering if these old rules (truisms) regarding optimum numbers for
>hash table size only apply to the Windows 9x/Me op systems which had problems
>with allocation of resources? I'm doubtful that they still apply to 2000/XP op
>systems.
>
>Does anyone really know?

The point is a bit of speed.   You have to convert a hash signature into a
hash table index.  For a tablesize that is a power of 2, you can simply
AND (mask) off the upper bits leaving a power-of-2 table index.  For other
sizes, you will end up doing a divide (mod) to get the remainder.  The divide
is not fast.

How significant this is is debatable, but for some of us, "every cycle counts."


>
>I'm not interested in old truisms, or hand-me-down story tales, or second had
>advice. I mean... Does anyone who writes this kind of code know whether it
>really makes a difference anymore exactly what hash table size is allocated?
>
>My only knowledge here is that I once wrote some code to proofread chess game
>scores, requiring me to define legal chess moves. There was some advantage to
>indirect addressing of memory, it used a cycle less per instruction, if it was a
>particular bank size such as exactly 256 bytes (or whatever), i.e. a power of 2.
>But that was with the old Commodore 64's processor, and I don't know how the
>current processor instruction sets work.
>
>Thanks,
>WP

Re: hash table size - is a power of 2 still an advantage these days? Leen Ammeraal 10:25:49 09/24/03
- Re: hash table size - is a power of 2 still an advantage these days? Robert Hyatt 13:28:57 09/24/03
  - Re: hash table size - is a power of 2 still an advantage these days? Tord Romstad 06:48:33 09/25/03
    - Re: hash table size - is a power of 2 still an advantage these days? Robert Hyatt 08:28:55 09/25/03
      - Re: hash table size - is a power of 2 still an advantage these days? Francesco Di Tolla 07:42:12 09/26/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Robert Hyatt 19:03:45 09/27/03
      - Re: hash table size - is a power of 2 still an advantage these days? Tord Romstad 10:02:22 09/25/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Ricardo Gibert 04:01:49 09/26/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Tord Romstad 07:15:44 09/26/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Gerd Isenberg 06:49:49 09/26/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Uri Blass 08:59:10 09/26/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Tony Werten 22:03:33 09/25/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Uri Blass 01:41:33 09/26/03
    - Re: hash table size - is a power of 2 still an advantage these days? Uri Blass 07:43:56 09/25/03
  - Re: hash table size - is a power of 2 still an advantage these days? Pham Hong Nguyen 18:18:02 09/24/03
    - Re: hash table size - is a power of 2 still an advantage these days? Robert Hyatt 08:30:06 09/25/03
      - Re: hash table size - is a power of 2 still an advantage these days? Pham Hong Nguyen 19:12:24 09/25/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Robert Hyatt 19:30:03 09/27/03
- Re: hash table size - is a power of 2 still an advantage these days? Sune Fischer 12:04:36 09/24/03
  - Re: hash table size - is a power of 2 still an advantage these days? Koundinya Veluri 00:03:14 09/26/03
    - Re: hash table size - is a power of 2 still an advantage these days? Sune Fischer 03:58:07 09/26/03
      - Re: hash table size - is a power of 2 still an advantage these days? Koundinya Veluri 04:41:50 09/26/03
    - Re: hash table size - is a power of 2 still an advantage these days? Koundinya Veluri 00:05:04 09/26/03
  - Re: hash table size - is a power of 2 still an advantage these days? Gerd Isenberg 13:58:20 09/24/03
    - Re: hash table size - is a power of 2 still an advantage these days? Dieter Buerssner 14:32:16 09/24/03
      - Re: hash table size - is a power of 2 still an advantage these days? Vincent Diepeveen 19:55:56 09/24/03
      - Re: hash table size - is a power of 2 still an advantage these days? Dieter Buerssner 15:14:46 09/24/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Johan de Koning 18:03:47 09/26/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Dieter Buerssner 13:40:43 09/27/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Johan de Koning 17:22:21 09/27/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Dieter Buerssner 13:39:58 09/28/03
    - Re: hash table size - is a power of 2 still an advantage these days? Ricardo Gibert 14:17:26 09/24/03
      - Re: hash table size - is a power of 2 still an advantage these days? Gerd Isenberg 14:41:15 09/24/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Ricardo Gibert 16:00:25 09/24/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Dieter Buerssner 16:10:46 09/24/03
      - Re: hash table size - is a power of 2 still an advantage these days? Dieter Buerssner 14:38:13 09/24/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Robert Hyatt 14:52:35 09/24/03
    - Re: hash table size - is a power of 2 still an advantage these days? Sune Fischer 14:11:14 09/24/03
      - Re: hash table size - is a power of 2 still an advantage these days? Gerd Isenberg 14:32:14 09/24/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Tony Werten 22:10:55 09/25/03
  - Re: hash table size - is a power of 2 still an advantage these days? Robert Hyatt 13:31:37 09/24/03
    - Re: hash table size - is a power of 2 still an advantage these days? Sune Fischer 13:45:20 09/24/03
      - Re: hash table size - is a power of 2 still an advantage these days? Robert Hyatt 16:57:57 09/24/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Russell Reagan 20:39:44 09/24/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Robert Hyatt 08:34:25 09/25/03
      - Re: hash table size - is a power of 2 still an advantage these days? Jeremiah Penery 16:00:20 09/24/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Sune Fischer 16:28:58 09/24/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Jeremiah Penery 18:50:31 09/24/03
      - Re: hash table size - is a power of 2 still an advantage these days? Ricardo Gibert 14:26:48 09/24/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Sune Fischer 14:48:45 09/24/03
  - Re: hash table size - is a power of 2 still an advantage these days? Uri Blass 12:49:15 09/24/03
    - Re: hash table size - is a power of 2 still an advantage these days? Sune Fischer 13:38:16 09/24/03
      - Re: hash table size - is a power of 2 still an advantage these days? Uri Blass 14:37:06 09/24/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Sune Fischer 15:47:34 09/24/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Sune Fischer 15:00:09 09/24/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Uri Blass 15:52:10 09/24/03
        
        Re: hash table size - is a power of 2 still an advantage these days? Sune Fischer 16:15:33 09/24/03

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