RS: [Beowulf] Sempron compile optimization

[Joe Landman]

Alan Ward wrote:
> I'm running a Fedora, classicaly compiled for "i686", on the aforesaid 
> Semperon 3000+ (actually a 1800MHz clock).

Hi Alan:

   This "i686" will not work with this "-m64".   You are running a 32 
bit OS, and there is no support for running a 64 bit OS on a 32 bit OS 
that I am aware of in Linux.

[...]

> So far, a quick series summation gave me:
> 
> * "vanilla" gcc (no switches):  19.1 s
> 
> * add -march=athlon: 9.9 s
> 
> * other FPU options (-sse, -3dnow) have no further effect, probably 
> incorporated into athlon.
> 
> * replace with -march=x86-64: 8.3 s
> 
> * add -m64: cc1 complains, doesn't compile. Doc says it should work (yah).

No it should not work.  The ABIs are different.  You might be able to 
run a 32 bit OS app on your 64 bit OS if you have the right libraries 
installed, and the correct patches applied to the kernel.  Basically it 
invokes a thunking layer as I remember to interface to the rest of the 
system.

> 
> No exactly rocket science, but a 130% speedup that doesn't cost 
> anything. Disclaimer: probably depends quite a lot on your code, as usual.
> 
> The bad news is: you also get to recompile all your dynamically linked 
> libraries, or you don't win anything. Happened to me with libgmp.
> 
> Best,
> Alan Ward
> 
> 
> 
> 
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Joseph Landman, Ph.D
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