Re: Consistency problem on IPF

[Erich Focht <efocht@hpce.nec.com>]

Hi Marc,

usually, if you can solve the problem at user level it is improbable
that someone will provide a solution in the kernel. To my knowledge
there is no knob for optimizing memory layout in 2.6 and 2.4 results
will be similar. What plaform do you use? If it's NUMA, there are more
answers...

At user level you could:
- try an optimized matrix-matrix multiply (BLAS3 DGEMM function from
the Intel MKL (math kernel library) ?). I'd expect that to be coded in
such a way that the impact of your data layout is reduced/limited.
- try using pages from hugetlbfs and keep all data in one page.

Regards,
Erich

On Monday 12 July 2004 11:13, Marc Gonzalez-Sigler wrote:
> Hello,
> 
> Several weeks ago, I wrote a naive matrix-matrix multiply program.
> 
> int main(void)
> {
>    static double A[N][N], B[N][N], C[N][N];
> 
>    /* Initialize A and B */
> 
>    /* Main loop */
>    for (i=0; i < N; ++i)
>      for (j=0; j < N; ++j)
>        for (k=0; k < N; ++k)
>          C[i][j] += A[i][k]*B[k][j];
> 
>    /* Print the sum of all elements of C */
> }
> 
> The system:
> 
> $ cat /proc/cpuinfo
> processor  : 0
> vendor     : GenuineIntel
> arch       : IA-64
> family     : Itanium 2
> model      : 1
> revision   : 5
> archrev    : 0
> features   : branchlong
> cpu number : 0
> cpu regs   : 4
> cpu MHz    : 1300.000000
> itc MHz    : 1300.000000
> BogoMIPS   : 1946.15
> 
> processor  : 1
> [same as processor 0]
> 
> $ uname -a
> Linux c64 2.6.6 #2 SMP Thu Jun 10 18:03:20 CEST 2004 ia64 GNU/Linux
> 
> 
> I started with NQ2, which was a bad idea. I ran the same program 100 
> times on an empty system, and saw very different execution times. I 
> tried to pin the program to a single CPU, but the results were similar.
> 
> NQ2
> MIN    = 1.190000
> MAX    = 11.470000
> MEAN   = 4.686900
> MEDIAN = 1.390000
> STDDEV = 4.181866
> 
> OK. NQ2 was probably a pathological case. Let us try NP0.
> 
> NP0
> MIN    = 0.670000
> MAX    = 1.770000
> MEAN   = 1.013100
> MEDIAN = 0.670000
> STDDEV = 0.466653
> 
> Better, but still quite inconsistent...
> 
> The same experiment on a 3.0 GHz Northwood running 2.4.22
> 
> NP0
> MEAN   = 1.375200
> MEDIAN = 1.375000
> STDDEV = 0.002825
> 
> Tony Luck, an Intel engineer, told me on a different list this was a 
> page-coloring issue. Would you agree? Is there a knob in Linux 2.6 to 
> request a smarter physical page allocation policy? Do you think I would 
> get similar results if I used 2.4 instead of 2.6?
> 
> Thanks to everybody for reading this far.