Re: [PATCH] random: get_random_u64_below()
From: Kent Overstreet
Date: Sat Mar 15 2025 - 17:32:59 EST
On Sat, Mar 15, 2025 at 08:55:32PM +0000, David Laight wrote:
> On Sat, 15 Mar 2025 14:20:46 -0400
> Kent Overstreet <kent.overstreet@xxxxxxxxx> wrote:
>
> > On Sat, Mar 15, 2025 at 01:52:34PM +0000, David Laight wrote:
> > > On Thu, 13 Mar 2025 12:38:10 -0400
> > > Kent Overstreet <kent.overstreet@xxxxxxxxx> wrote:
> > >
> > > > bcachefs needs this, for sampling devices to read from based on squared
> > > > device latencies.
> > > >
> > > > this uses the same algorithm as get_random_u32_below: since the multiply
> > > > uses the top and bottom halves separately, it works out fairly well.
> > >
> > > Adding two separate copies of much the same code is silly.
> > > Given what the code is doing, does it ever make any sense to inline it.
> > >
> > > Inlining the original get_random_u32_below(ceil) that did
> > > (random_u32() * ((1ull << 32) / ceil) >> 32
> > > (for constant ceil) made sense.
> > > While good enough for most purposes it was replaced by the much more
> > > expensive function that guarantees that all the output values are
> > > equally likely - rather than just evenly distributed.
> >
> > Expensive!? It adds a multiply.
>
> I make it two multiplies and a loop.
> Have you looked at what happens on 32bit systems?
Not many people run 32 bit anymore.
Have you looked at get_random_bytes()?
> > That % gets constant folded, in the inlined case, and in the non-inline
> > case it's hit only a small fraction of the, time, for typical ceil.
>
> If the % is only a small fraction on the cost for the non-inline case
> (and it is over 100 clocks on many cpu that people still use) then
> why inline anything?
> A quick look shows divide being 'only moderately slow' on zen3 and coffee lake.
>
> What you might want to do is pass -ceil % ceil through to a real function
> (especially if constant).
David, the way get_random_u32_below is optimized now is just fine.
I'm not going to completely redo the inlining just for this, nor am I
going to do get_random_u64_below() differently from
get_random_u32_below().
> Oh I guess you haven't actually tested the version you submitted.
> Time to play 'spot the silly error'.
Please do share what you think you've found.