Re: Issue with the PRNG used by Postgres

Robert Haas <robertmhaas@gmail.com> writes:
> I'm not convinced that we should try to improve the RNG, but surely we
> need to put parentheses around pg_prng_double(&pg_global_prng_state) +
> 0.5. IIUC, the current logic is making us multiply the spin delay by a
> value between 0 and 1 when what was intended was that it should be
> multiplied by a value between 0.5 and 1.5.

No, I think you are misreading it, because the assignment is += not =.
The present coding is

        /* increase delay by a random fraction between 1X and 2X */
        status->cur_delay += (int) (status->cur_delay *
                                    pg_prng_double(&pg_global_prng_state) + 0.5);

which looks fine to me.  The +0.5 is so that the conversion to integer
rounds rather than truncating.

In any case, I concur with Andres: if this behavior is anywhere near
critical then the right fix is to not be using spinlocks.

            regards, tom lane