Re: [RFC PATCH] random: add get_random_max() function

[George Spelvin <lkml@sdf.org>]

On Sun, 24 Mar 2019 at 21:47:50 +0100, Jason A. Donenfeld wrote:
> I generally use a slightly simpler algorithm in various different projects:
> 
> //[0, bound)
> static unsigned long random_bounded(unsigned long bound)
> {
>        unsigned long ret;
>        const unsigned long max_mod_bound = (1 + ~bound) % bound;
> 
>        if (bound < 2)
>                return 0;
>        do
>                ret = random_integer();
>        while (ret < max_mod_bound);
>        return ret % bound;
> }
>
> Is the motivation behind using Lemire that you avoid the division (via
> the modulo) in favor of a multiplication?

Yes.  If we define eps = max_mod_bound * ldexp(1.0, -BITS_PER_LONG) as
the probability of one retry, and retries = eps / (1 - eps) as the
expected number of retries, then both algorithms take 1+retries
random_integer()s.

The above agorithm takes 2 divisions, always.  Divides are slow, and
usually not pipelined, so two in short succession gets a latency penalty.

Lemire's mutiplicative algorithm takes 1 multiplication on the fast
path (probability 1 - 2*eps on average), 1 additional division on the slow
path (probability 2*eps), and 1 multiplication per retry.

In the common case when bound is much less than ULONG_MAX, eps is
tiny and the fast path is taken almost all the time, and it's
a huge win.

Even in the absolute worst case of bound = ULONG_MAX/2 + 2 when
eps ~ 0.5 (2 multiplies, 0.5 divide; there's no 2*eps penalty in
this case), it's faster as long as 2 mutiplies cost less than 1.5
divides.

I you want simpler code, we could omit the fast path and stil get
a speedup.  But a predictable branch for a divide seemed like
a worthwhile trade.


(FYI, this all came about as a side project of a kernel-janitor project
to replace "prandom_u32() % range" by "prandom_u32() * range >> 32".
I'm also annoyed that get_random_u32() and get_random_u64() have
separate buffers, even if EFFICIENT_UNALIGNED_ACCESS, but that's
a separate complaint.)