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

[Ralf LÃbben <ralfluebben at gmx dot de>]

Hello,

maybe this is the better list to post the problem (see below).

Regards
Ralf

On Wednesday, 26. September 2007 18:23:34 Ralf LÃbben wrote:
> 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