Re: pgbench - add pseudo-random permutation function

From: Fabien COELHO <coelho(at)cri(dot)ensmp(dot)fr>
To: Thomas Munro <thomas(dot)munro(at)gmail(dot)com>
Cc: Hironobu SUZUKI <hironobu(at)interdb(dot)jp>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>, David Steele <david(at)pgmasters(dot)net>
Subject: Re: pgbench - add pseudo-random permutation function
Date: 2019-07-23 07:44:09
Message-ID: alpine.DEB.2.21.1907230730150.7144@lancre
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Hello Thomas,

>>> Function nbits(), which was previously discussed, has been simplified by
>>> using the function pg_popcount64().
>
> Hi Fabien, Suzuki-san,
>
> I am not smart enough to commit this

I'm in no hurry:-)

> or judge its value for benchmarking,

I think that it is valuable given that we have non uniform random
generators in pgbench.

I'm wondering about the modular_multiply manual implementation which adds
quite a few lines of non trivial code. If int128 is available on all/most
platforms, it could be removed and marked as not supported on these,
although it would create an issue with non regression tests.

> but I have a few trivial comments on the language:
>
> + It allows to mix the output of non uniform random functions so that
>
> "It allows the output of non-uniform random functions to be mixed so that"

Fixed.

> + ensures that a perfect permutation is applied: there are no collisions
> + nor holes in the output values.
>
> "neither collisions nor holes", or "no collisions or holes"

I choose the first.

> + The function errors if size is not positive.
>
> "raises an error"

Fixed.

> + * 24 bits mega primes from https://primes.utm.edu/lists/small/millions/
>
> "24 bit mega primes"

Fixed.

> +/* length of n binary representation */
> +static int
> +nbits(uint64 n)
> +{
> + /* set lower bits to 1 and count them */
> + return pg_popcount64(compute_mask(n));
> +}
>
> I suppose you could use n == 0 ? 0 : pg_leftmost_one_pos64(n) + 1, and then...

It would create a branch, that I'm trying to avoid.

> +/* return smallest mask holding n */
> +static uint64
> +compute_mask(uint64 n)
> +{
> + n |= n >> 1;
> + n |= n >> 2;
> + n |= n >> 4;
> + n |= n >> 8;
> + n |= n >> 16;
> + n |= n >> 32;
> + return n;
> +}
>
> ... here you could use 1 << nbits(n)) - 1. I have no idea if doing it
> that way around is better, it's just a thought and removes a few lines
> of bit-swizzling code.

This would create a infinite recursion as nbits currently uses
compute_mask. The 6 bitfield operation above is pretty efficient, I'd let
it at that.

Attached a v16.

--
Fabien.

Attachment Content-Type Size
pgbench-prp-func-16.patch text/x-diff 18.0 KB

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