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Re: Silly question

Jason Thorpe wrote:

> On Sep 6, 2008, at 3:49 PM, Perry E. Metzger wrote:
> > Vincent <> writes:
> >> If true, what are the benefits (and drawbacks) of using AMD64 instead
> >> of I386? Increased memory space, but is there any performance  
> >> upgrade?
> >
> You also get a much faster system call interface.
> In general, I've found "64-bit user code is a total win" for almost  
> everything.

On a single core opteron system from earlier this year, a test
shows a slight benefit using 32-bit userland/kernel over 64-bit:

i386 building i386 release
     7157.711u 1477.441s 2:28:11.86 97.1%    0+0k  3268+288792io 58119pf+177w
     7162.819u 1475.024s 2:28:21.42 97.0%    0+0k 10427+288785io 58130pf+157w
amd64 building i386 release
     7064.045u 1823.976s 2:34:21.40 95.9%    0+0k 13196+295870io 68022pf+81w
     7065.928u 1842.092s 2:34:32.37 96.0%    0+0k 12758+294870io 67918pf+72w
i386 building amd64 release
     5529.617u 1149.725s 1:56:21.37 95.6%    0+0k 13885+226200io 39807pf+193w
     5528.191u 1151.156s 1:56:16.65 95.7%    0+0k 13477+227283io 39808pf+210w
amd64 building amd64 release
     5409.935u 1418.328s 2:00:08.56 94.7%    0+0k 13632+232673io 48524pf+110w
     5410.603u 1421.017s 2:00:06.11 94.8%    0+0k 13338+231756io 50470pf+115w

> > It is very application dependent. The AMD64 architecture has more
> > registers, and the registers are wider, so for some applications you
> > can get significant speedup. On the other hand, you also get wider
> > pointers and such, and those consume more cache and memory. You should
> > benchmark if you're doing any sort of performance critical application
> > to figure out which is better for your app.


My current rule of thumb is "if you need to access more that 3.5G of RAM
use amd64, otherwise use i386."


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