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

[Ralf LÃbben <ralfluebben at gmx dot de>]

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