[Beowulf] Q: AMD Opteron (Barcelona) 2356 vs Intel Xeon 5460

Vincent Diepeveen diep at xs4all.nl
Wed Sep 17 20:37:07 EDT 2008


How does all this change when you use a PGO optimized executable on  
both sides?

Vincent

On Sep 18, 2008, at 2:34 AM, Eric Thibodeau wrote:

> Vincent Diepeveen wrote:
>> Nah,
>>
>> I guess he's referring to sometimes it's using single precision  
>> floating point
>> to get something done instead of double precision, and it tends to  
>> keep
>> sometimes stuff in registers.
>>
>> That isn't a problem necessarily, but if i remember well floating  
>> point state
>> could get wiped out when switching to SSE2.
>>
>> Sometimes you lose your FPU registerset in that case.
>>
>> Main problem is that there is so many dangerous optimizations  
>> possible,
>> to speedup testsets, because in itself floating point is real slow  
>> to do at hardware,
>> from hardware viewpoint seen.
>>
>> Yet in general last generations of intel compilers that has  
>> improved really a lot.
> Well, running the same code here is the result discrepancy I got:
> FLOPS:
>    my code has to do: 7,975,847,125,000 (~8Tflops) ...takes  
> 15minutes on 8*2core Opeteron with 32 Gigs-o-RAM (thank you OpenMP ;)
>
> The running times (ran it a _few_ times...but not the statistical  
> minimum of 30):
>    ICC -> runtime == 689.249  ; summed error == 1651.78
>    GCC -> runtime == 1134.404 ; summed error == 0.883501
>
> Compiler Flags:
>    icc -xW -openmp -O3 vqOpenMP.c -o vqOpenMP
>    gcc -lm -fopenmp -O3 -march=native vqOpenMP.c -o vqOpenMP_GCC
>
> No trickery, no smoky mirrors ;) Just a _huge_ kick ASS k-Means  
> parallelized with OpenMP (thank gawd, otherwise it takes hours to  
> run) and a rather big database of 1.4 Gigs
>
> ... So this is what I meant by floating point errors. Yes, the  
> runtime was almost halved by ICC (and this is on an *opteron* based  
> system, Tyan VX50). The running time wasn't what I was actually  
> looking for rather than precision skew and that's where I fell off  
> my chair.
>
> For the ones itching for a little more specs:
>
> eric at einstein ~ $ icc -V
> Intel(R) C Compiler for applications running on Intel(R) 64,  
> Version 10.1    Build 20080602
> Copyright (C) 1985-2008 Intel Corporation.  All rights reserved.
> FOR NON-COMMERCIAL USE ONLY
>
> eric at einstein ~ $ gcc -v
> Using built-in specs.
> Target: x86_64-pc-linux-gnu
> Configured with: /dev/shm/portage/sys-devel/gcc-4.3.1-r1/work/ 
> gcc-4.3.1/configure --prefix=/usr --bindir=/usr/x86_64-pc-linux-gnu/ 
> gcc-bin/4.3.1 --includedir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.3.1/ 
> include --datadir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.3.1 -- 
> mandir=/usr/share/gcc-data/x86_64-pc-linux-gnu/4.3.1/man --infodir=/ 
> usr/share/gcc-data/x86_64-pc-linux-gnu/4.3.1/info --with-gxx- 
> include-dir=/usr/lib/gcc/x86_64-pc-linux-gnu/4.3.1/include/g++-v4 -- 
> host=x86_64-pc-linux-gnu --build=x86_64-pc-linux-gnu --disable- 
> altivec --enable-nls --without-included-gettext --with-system-zlib  
> --disable-checking --disable-werror --enable-secureplt --enable- 
> multilib --enable-libmudflap --disable-libssp --enable-cld -- 
> disable-libgcj --enable-languages=c,c++,treelang,fortran --enable- 
> shared --enable-threads=posix --enable-__cxa_atexit --enable- 
> clocale=gnu --with-bugurl=http://bugs.gentoo.org/ --with- 
> pkgversion='Gentoo 4.3.1-r1 p1.1'
> Thread model: posix
> gcc version 4.3.1 (Gentoo 4.3.1-r1 p1.1)
>>
>> Vincent
>>
>> On Sep 17, 2008, at 10:25 PM, Greg Lindahl wrote:
>>
>>> On Wed, Sep 17, 2008 at 03:43:36PM -0400, Eric Thibodeau wrote:
>>>
>>>> Also, note that I've had issues with icc
>>>> generating really fast but inaccurate code (fp model is not IEEE  
>>>> *by
>>>> default*, I am sure _everyone_ knows this and I am stating the  
>>>> obvious
>>>> here).
>>>
>>> All modern, high-performance compilers default that way. It's  
>>> certainly
>>> the case that sometimes it goes more horribly wrong than  
>>> necessary, but
>>> I wouldn't ding icc for this default. Compare results with IEEE  
>>> mode.
>>>
>>> -- greg
>>>
>
>

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