From: stryder@telenet.be.invalid   
      
   On Saturday 21 July 2012 22:41, Warren Post conveyed the following to   
   alt.os.linux.mandriva...   
      
   > Searching this NG and the web, I am left with the impression that   
   > using a 64 bit distro will only provide a noticeable improvement over   
   > 32 bits if you have lots of RAM and a fast processor.   
      
   The general rule is that x86-64 running in 64-bit long mode has more and   
   bigger registers, so there is a performance benefit.  However, due to   
   the greater pointer sizes, it does require a bit more RAM than native   
   32-bit mode - about 20%.   
      
   Do however bear in mind that you have made a big mistake in the subject   
   line of this thread, because x32 is not the same thing as x86-64, and   
   "x64", although used by some, is not an /official/ name for x86-64.   
   When in doubt, "AMD64" is a much better moniker.   
      
   x86-32 is also known as IA32, but this too raises confusion, because of   
   the existence of IA64, which is _not x86-64 - instead, IA64 is the Intel   
   Itanium architecture - but the moniker x32 is a brandnew software   
   architecture, based upon x86-64 and running in 64-bit long mode, but   
   with 32-bit pointers instead of 64-bit pointers.  It is a memory-   
   optimized version of x68-64.   
      
   > But how much is "lots"?   
      
   x86-64 consumes about 20% more memory than x86-32.  However, x86-32 can   
   address 4 GiB of memory, of which you use up to about 0.7 GiB due to the   
   PCI memory hole.   
      
   PCI devices are accessed by way of a memory address in the 32-bit range,   
   mapped from the top down of the 4GiB barrier, because this is also where   
   kernel memory is mapped, and only the kernel has direct physical access   
   to the hardware.  This means that if you have 4 GiB of RAM, up to 0.7   
   GiB of that RAM will be obscured, because memory addresses cannot be   
   assigned to more than one device at the time, and that address range   
   will have been mapped to your PCI devices.   
      
   On i686-compatible processors (and with an i686-optimized kernel), there   
   is an extra mode available within protected mode, called PAE.  When the   
   processor runs in this mode, it can address up to 64 GiB of physical RAM   
   by way of an extra pagetable, which allows for 36-bit memory addressing,   
   in pages of 3 GiB each per individual process.  With such a kernel, the   
   RAM obscured by the PCI memory hole can be remapped to a location above   
   the 4 GiB barrier.  As such, you could have a machine with, say, 16 GiB   
   of RAM installed, and you could still make use of a 32-bit operating   
   system, provided that the kernel was optimized for i686 and PAE.   
      
   Therefore, it is commonly said that if you don't have more than 4 GiB of   
   RAM in your machine, you will definitely not need to go with a 64-bit   
   system, but this is silly advice.  Some even claim that for 64-bit,   
   you'd need twice the RAM as for 32-bit.  In this machine here I have 4   
   GiB of RAM and I am running the 64-bit version of Mageia 1, without ever   
   even hitting swap.  By contrast, I did regularly hit swap with a 32-bit   
   PCLinuxOS 2009.2 installation on a machine that had 2 GiB of RAM, and   
   that was with KDE 3, which is quite a bit slimmer than the KDE 4 I'm   
   using here and now.   
      
   Going with a 64-bit operating system on 64-bit hardware is the sane   
   thing to do.  It's faster - because it uses more processor registers and   
   those registers are now wider too - and like PAE, it uses an extra   
   pagetable, and it can remap the RAM lost to the PCI memory hole.  The   
   only problem would be if your machine is really low on RAM - say 2 GiB   
   or so - but in that case, you'd be hitting swap with any modern 32-bit   
   operating system as well, but then still I would advise to check out the   
   new and upcoming x32 architecture - which, as stated higher up, is not   
   32-bit but 64-bit with 32-bit pointers.   
      
   As for 32-bit compatibility, very little software still exists in 32-bit   
   form only, but x86-64 is compatible with 32-bit code, and most 64-bit   
   distributions install a so-called /multilib/ system by default -   
   Slackware doesn't do it by default, but it does support it - which   
   includes the necessary shared libraries in 32-bit form, so that 32-bit   
   software can use those.   
      
   > How fast is "fast"?   
      
   Overall, 64-bit code is faster than 32-bit code, so the claim that you   
   would somehow need a faster processor for running 64-bit code is void.   
      
   > Having reached end of life, I'll soon be replacing my 32 bit mdv2010.2   
   > with something newer. While I have no complaint with my current 32   
   > bits, now is a good time for me to consider the alternative.   
   >   
   > Per hardinfo, my hardware is:   
   >   
   > Processor		: 2x Intel(R) Pentium(R) Dual  CPU  E2140  @   
   > 1.60GHz   
      
   Well, that's not really up-to-date anymore, but given that you've got   
   two of them, that should ease things out a bit if you're multitasking.   
      
   I would recommend going with something above 2 GHz.   
      
   > Memory		: 1015MB (556MB used)   
      
   Make sure you have at least 4 GiB, whether you'll be going with 32-bit   
   or 64-bit.   
      
   > Operating System		: Mandriva Linux 2010.2   
   >   
   > The mobo can accept up to 4 GB of DDR2 RAM.   
      
   If you're going to go for a more modern CPU, then you'll probably also   
   need to go with a more modern motherboard.  So the memory limit of the   
   current motherboard is irrelevant. :p   
      
   > The 64 bit live versions of Crunchbang 10, Mint Debian Edition 13,   
   > Mageia 2, and mdv 2011 all work on my box, although the latter is   
   > noticeably slower than the others.   
   >   
   > However, my interest is in learning a general rule of thumb to apply   
   > to hardware generally, and not simply "your box is best suited for an   
   > X bit OS".   
      
   There is no such rule anymore, unless you're really stuck with a box   
   that doesn't support more than 2 GiB of RAM, but which still for some   
   reason was equipped with a 64-bit processor.  That would be about the   
   only valid reason for running a 32-bit operating system on a 64-bit   
   processor.  For everything else, go with 64-bit.   
      
   --   
   = Aragorn =   
   (registered GNU/Linux user #223157)   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)