32bit vs 64bit Linux

Fri May 31 11:56:10 MST 2013

64-bit execution helps to speed up encryption and decryption as well as 
file compression.
This aids in archiving, secure shells, secure sockets, and memory to 
memory copy,
not to mention compiler (gnu) and linker (ld) speed-ups.

There are also hardware considerations besides memory size, such as 
multi- channel
memory configuration and use of matched memory sticks. Upgrading to 
64-bit can
get an even better throughput increase if all the hardware and BIOS 
factors are in accord.

If your computer is using all available memory and a significant portion 
of swap space, then
you have a situation known as "running from the disk".    If that is 
case, your system will be
rather pokey and the remedy is to add more memory.  This can happen 
regardless of whether
you are running a 32-bit kernel or a 64-bit kernel.

On 05/31/2013 02:08 PM, Lisa Kachold wrote:
> Hi Keith,
>
> Excellent subject!  See my inline comments below:
>
> On Fri, May 31, 2013 at 9:28 AM, keith smith <klsmith2020 at yahoo.com 
> <mailto:klsmith2020 at yahoo.com>> wrote:
>
>
>     Hi,
>
>     Even though I have 64bit hardware I always install the 32bit
>     version of Linux.  I do so because of the past discussions on this
>     list that made me believe the 32bit OS was better because 64bit
>     caching is actually slower due to the requirement that the cache
>     be filled to a certain point before it is moved.  I think I recall
>     something about the amount of RAM having some effect here also.
>
>
> The full questions of whether to use 64 bit OS verses 32 bit are 
> included below.  For instance with a Python based application, CPU 
> performance gains (possible 10%) verses greater memory requirements 
> (30%)  (in 64bit OS).
>
> It is really going to depend on what the server is doing?   Is that 
> application actually optimized for 64 bit registers?
>
> Reference: 
> http://askubuntu.com/questions/7034/what-is-the-difference-between-32-bit-and-64-bit-and-which-should-i-choose
>
> If you are running a database application, with more than 4 GB RAM, 
> you might find great speed improvement with memcached under 64bit.
>
> *From a technical perspective, a 64-bit OS gives you:*
>
>  *
>
>     Allows individual processes to address more than 4 GB of RAM each
>     (in practice, most but not all 32-bit OSes also limit the total
>     usable system RAM to less than 4 GB, not just the per-application
>     maximum). While a PAE kernel also does this; it is significantly
>     slower!
>
>  *
>
>     All pointers take 8 bytes instead of 4 bytes. The effect on RAM
>     usage is minimal (because you're not likely to have an application
>     filled with gigabytes of pointers), but in the worst theoretical
>     case, this can make the CPU cache be able to hold 1/2 as many
>     pointers (making it be effectively 1/2 the size). For most
>     applications, this is not a huge deall.
>
>  *
>
>     There are many more general-purpose CPU registers in 64-bit mode.
>     Registers are the fastest memory in your entire system. There are
>     only 8 in 32-bit mode and 16 general purpose registers in 64-bit
>     mode. In scientific computing applications I've written, I've seen
>     up to a 30% performance boost by recompiling in 64-bit mode (my
>     application could really use the extra registers).
>
>  *
>
>     Most 32-bit OSes really only let individual applications use 2 GB
>     of RAM, even if you have 4 GB installed. This is because the other
>     2 GB of address space is reserved for sharing data between
>     applications, with the OS, and for communicating with drivers.
>     Windows and Linux will let you adjust this tradeoff to be 3 GB for
>     applications and 1 GB shared, but this can cause problems for some
>     applications that don't expect the change. I'm also guessing it
>     might cripple a graphics card that has 1 GB of RAM (but I'm not
>     sure). A 64-bit OS can give individual 32-bit applications closer
>     to the full 4 GB to play with.
>
> *From a user's perspective:*
>
>  *
>
>     Application speed is usually faster for a 64-bit application in a
>     64-bit OS compared to the 32-bit version of the application on a
>     32-bit OS, but most users won't see this speed-up. Most
>     applications for normal users don't really take advantage of the
>     extra registers or the benefits are balanced out by bigger
>     pointers filling up the cache.
>
>  *
>
>     If you have any memory hog applications (like photo editors, video
>     processing, scientific computing, etc.), if you have (or can buy)
>     more than 3 GB of RAM, and you can get a 64-bit version of the
>     application, the choice is easy: use the 64-bit OS.
>
>  *
>
>     Some hardware doesn't have 64-bit drivers. Check your motherboard,
>     all plug-in cards, and all USB devices before making the switch.
>     Note that in the early days of Windows Vista, there were lots of
>     problems with drivers. These days things are generally better.
>      nVidia for instance is one of the drivers optimized for 64bit
>     which is extremely faster.
>
>  *
>
>     If you run so many applications at a time that you're running out
>     of RAM (usually you can tell this because your computer starts
>     getting really slow and you hear the hard disk drive crunching -
>     indicating your caching), then you'll want a 64-bit OS (and
>     sufficient RAM).
>
>  *
>
>     You can run 32-bit applications (but not drivers) in 64-bit land
>     with no problems. The worst slowdown I've measured for a 32-bit
>     application in 64-bit Windows is about 5% (meaning that if it took
>     60 seconds to do something in 32-bit, it took at most 60/0.95 = 63
>     seconds with the same 32-bit application in 64-bit).
>
>
>     All in all, personally, Keith, I would install 64bit CentOs 6
>     unless you have a better reason not to?
>
>
>
>     Using a 32bit version over a 64bit version seems counter
>     intuitive, however that is what I have taken away from these
>     conversations about 32bit vs 64bit Linux.
>
>     I'm using CentOS 6.x on a LAMP server that gets a low amount of
>     traffic.   However I may make the jump to Linux on my desktop this
>     summer. (this will be my 3rd attempt to become M$ free except one
>     VM so I can use IE for testing) I think all of my hardware is 64bit.
>
>     So that begs the question, is 32bit better than 64bit or do I not
>     understand the issue?
>
>
> The issue is complex, but now in this new moment of time, a greater 
> number of drivers are optimized for 64 bit registers.
>
>
>     Thank you for your feedback.
>
>     Keith
>
>     ------------------------
>     Keith Smith
>
>
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