I have try to add iteration like this
const int num_iterations = 100;

and I get:
Average time for lrand48: 37 cycles

there is a huge gap about the order of magnitude  of cycles.
It seems like the first call for lrand48 do more thing than subsequent calls



On Tue, Apr 2, 2024 at 10:16 PM Adhemerval Zanella Netto <
adhemerval.zanella@linaro.org> wrote:

>
>
> On 01/04/24 08:47, abush wang wrote:
> > This is test:
> > ```
> > uint64_t getnsecs() {
> >     uint32_t lo, hi;
> >     __asm__ __volatile__ (
> >         "rdtsc" : "=a"(lo), "=d"(hi)
> >     );
> >     return ((uint64_t)hi << 32) | lo;
> > }
> >
> > int main() {
> >     const int num_iterations = 1;
>
> This low number of iteration makes the benchmark pretty much useless
> on modern hardware with frequency scaling.  By raising to something
> like 1000000000 I see no variation on my workstation (Ryzen 5900).
>
> >     uint64_t start, end, total_time = 0;
> >
> >     start = getnsecs();
> >     for (int i = 0; i < num_iterations; i++) {
> >         (void) lrand48();
> >     }
> >     end = getnsecs();
> >     total_time += (end - start);
> >
> >     printf("Average time for lrand48: %lu cycles\n", total_time /
> num_iterations);
> >     return 0;
> > }
> > ```
> > before:
> > Average time for lrand48: 21418 cycles
> >
> > after:
> > Average time for lrand48: 9892 cycles
>