Re: [PATCH v4 6/8] tracing/histogram: Optimize division by a power of 2

On Tue, Oct 26, 2021 at 8:16 PM Steven Rostedt [off-list ref] wrote:

On Tue, 26 Oct 2021 22:21:23 -0400
Steven Rostedt [off-list ref] wrote:

quoted

I'm sure there's an algorithm somewhere that can give as the real max.

You got me playing with this more ;-)

OK, I added the rounding in the wrong place. I found that we can make
the max_div to be the same as the shift! The bigger the shift, the
bigger the max!

Nice! :)

        mult = (1 << shift) / div;
        max_div = (1 << shift)

But the rounding needs to be with the mult / shift:

        return (val * mult + ((1 << shift) - 1)) >> shift;


When val goes pass 1 << shift, then the error will be off by more than
one.

Did you mean, val should be such that when we do the (val * mult) we
only get rounding errors less than (1 << shift)?

I think we also need to flip the delta now since we round down initially:

    delta =  (1 << shift) - (mult * div)

Thanks,
Kalesh

-- Steve