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

On Tue, 26 Oct 2021 22:21:23 -0400 Steven Rostedt rostedt@goodmis.org wrote:

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!

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.

-- Steve