Mailinglist Archive: opensuse (686 mails)

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Re: [opensuse] 64 bit vs 32 bit RAM consumption
On 7/23/2013 7:07 AM, James Knott wrote:
Felix Miata wrote:
In googling the possibility of using a 64 bit kernel on an otherwise 32 bit
openSUSE (consensus: don't try it), I found quite a bit of
mention of 64 bit apps using twice as much RAM? Is this true? Or is it just
misunderstanding arising from 64 bit using twice as many CPU
registers to increase its own speed? Or something else? My main system
running 32 bit 11.4 has 4GB RAM and is consistently using >50% of
RAM for open apps, most of the rest for disk cache, and always around 360MB
unused. Would running 64 bit on this system be consuming
virtually all RAM for apps, leaving little free for disk cache?


The same claims were made when we went from 8 to 16 and 16 to 32 bit CPUs and
reflect a lack of knowledge about how things work. For
example, a larger instruction set means you may be able to get by with fewer
instructions. It shouldn't affect data at all, as the CPUs
have instructions for working with smaller than register size data.

Take a look at file sizes in 32 & 64 bit systems. You'll likely find the
sizes are similar.


Nice theory, if you code in assembler.

But for the most of us, sitting behind a compiler, we are at the complete mercy
of the compiler
writers, and there was a lag of over a year before these were optimized for
larger instructions and larger
registers. Just shlepping large chunks of text around was still done in small
32bit chunks until
the compilers caught up.

Anyway, there is a discussion of this here:
http://en.wikipedia.org/wiki/64-bit_computing#32-bit_vs_64-bit


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