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

[Kalesh Singh]

On Tue, Oct 26, 2021 at 6:15 PM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Tue, 26 Oct 2021 18:09:22 -0700
> Kalesh Singh <kaleshsingh@xxxxxxxxxx> wrote:
>
> > >   delta = mult * div / 2^20
> > >
> > > That is if mult is a power of two, then there would be no rounding
> > > errors, and the delta is zero, making the max infinite:
>
> That should have been (as shown in the algorithm)
>
>   delta = mult * div - 2 ^ 20
>
> As mult is 2^20 / div; and the above should end up zero if there's no
> rounding issues, as it would be:
>
>  delta = (2^20 / div) * div - 2^20

Good catch. We're checking if we get back the exact value.

And IIUC max_div is an arbitrary value we decide on that's <= 2^shift?
Is there a rule of thumb for choosing this?

Thanks,
Kalesh
>
> -- Steve