Re: Poor pow() / floating point performance of on x86_64

[Ralf LÃbben <ralfluebben at gmx dot de>]

Ok, 

the problems seems to be the pow() function. If I use instead the function 
gsl_pow_int(double x, int n) from the gsl library the performance on the 
x86_64 machine is much faster.
I call the pow function with the following values:

pow(5.0,-3.0);
pow(10.0,-3.0);
pow(15.0,-3.0);
pow(20.0,-3.0);

The problem also occurs with gcc 4.2.1, but not with the x86 Ubuntu Feity Fawn 
distribution on the x86_64 machine. 
Sorry for this misinformation, before.

Ralf

 
On Wednesday, 26. September 2007 16:35:50 Ralf LÃbben wrote:
> Hi,
>
> I just have tried two other setups on the x86_64 machine:
>
> 1. Ubuntu Feisty Fawn (gcc 4.1.2) server x86:
> - Expected performance: about two times faster than on my notebook
>
> 2. Ubuntu Gutsy Gibbon (gcc 4.2.1) server x86:
> - nearly same performance than "Ubuntu Feisty Fawn (gcc 4.1.2) server x86"
> - Expected performance: about two times faster than on my notebook
>
> Was there a change from gcc 4.1.2 to gcc 4.2.1 which could explain that?
> Or is there anything else which could explain that?
>
> Ralf
>
> On Wednesday, 26. September 2007 10:35:20 Ralf LÃbben wrote:
> > Hello,
> >
> > in the last days I ran a simulation on a x86_64 architecture:
> > ###################
> > processor       : 0
> > vendor_id       : GenuineIntel
> > cpu family      : 15
> > model           : 6
> > model name      :                    Genuine Intel(R) CPU 3.20GHz
> > stepping        : 8
> > cpu MHz         : 3192.081
> > cache size      : 8192 KB
> > physical id     : 0
> > siblings        : 2
> > core id         : 0
> > cpu cores       : 2
> > fpu             : yes
> > fpu_exception   : yes
> > cpuid level     : 6
> > wp              : yes
> > flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge
> > mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm syscall nx
> > lm constant_tsc pni monitor ds_cpl vmx est tm2 cid cx16 xtpr lahf_lm
> > bogomips        : 6390.34
> > clflush size    : 64
> > cache_alignment : 128
> > address sizes   : 40 bits physical, 48 bits virtual
> > power management:
> > #####################
> >
> > with very poor performance.
> >
> > I ran the same simulations on my notebook:
> >
> > ######################
> > processor       : 0
> > vendor_id       : AuthenticAMD
> > cpu family      : 6
> > model           : 8
> > model name      : mobile AMD Athlon(tm) XP 2000+
> > stepping        : 1
> > cpu MHz         : 797.820
> > cache size      : 256 KB
> > fdiv_bug        : no
> > hlt_bug         : no
> > f00f_bug        : no
> > coma_bug        : no
> > fpu             : yes
> > fpu_exception   : yes
> > cpuid level     : 1
> > wp              : yes
> > flags           : fpu vme de pse tsc msr pae mce cx8 sep mtrr pge mca
> > cmov pat pse36 mmx fxsr sse syscall mp mmxext 3dnowext 3dnow ts fid vid
> > bogomips        : 1596.37
> > clflush size    : 32
> > #######################
> >
> > The same simulation is about 10 times faster on my notebook.
> > The simulation was compiled with "-O3 -ffast-math", without "-ffast-math"
> > the performance of the x86_64 architecture is much worse.
> > I used gcc 4.1.2 on Ubuntu, the simulator is Omnet++.
> >
> > There was already a post about the topic:
> > http://gcc.gnu.org/ml/gcc-help/2006-05/msg00185.html
> > on AMD machines.
> >
> > I could also figure out, that one problem ist the pow() function, maybe
> > there are more functions with poor performance on x86_64 machines.
> >
> > Has anyone an idea about the reasons or how to improve the performance on
> > x86_64 machines?
> >
> > Thanks.
> >
> > Regards,
> > Ralf