Re: CPU costs of random_zipfian in pgbench

On Sun, Feb 17, 2019 at 10:52 AM Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr> wrote:

> > I'm trying to use random_zipfian() for benchmarking of skewed data sets,
> > and I ran head-first into an issue with rather excessive CPU costs.
> > [...] This happens because generalizedHarmonicNumber() does this:
> >
> > for (i = n; i > 1; i--)
> > ans += pow(i, -s);
> >
> > where n happens to be 1000000000 (range passed to random_zipfian), so
> > the loop takes quite a bit of time.
>
> If you find a better formula for the harmonic number, you are welcome
> and probably get your name on it:-)
>

There are pretty good approximations for s > 1.0 using Riemann zeta
function and Euler derived a formula for the s = 1 case.

I also noticed that i is int in this function, but n is int64. That seems
like an oversight.

Regards,
Ants Aasma