Re: [PATCH 10/25] Unionfs: add un/likely conditionals on copyup ops

Erez Zadok
2007-09-26T15:44:22Z

In message <2DDDD55D-4445-4565-9384-4E4BE7B41D7D@mac.com>, Kyle Moffett writes:
> On Sep 26, 2007, at 09:40:20, Erez Zadok wrote:
[...]
> > Recently we've done a full audit of the entire code, and added un/
> > likely where we felt that the chance of succeeding is 95% or better
> > (e.g., error conditions that should rarely happen, and such).
>
> Actually due to the performance penalty on some systems I think you
> only want to use it if the chance of succeeding is 99% or better, as
> the benefit if predicted is a cycle or two and the harm if
> mispredicted can be more than 50 cycles, depending on the CPU.

*That's* the information I was looking for, Kyle: what's the estimated
probability I should be using as my guideline. I used 95% (20/1 ratio), and
you're telling me I should use 99% (100/1 ratio). The difference between
the number of cycles saved/added is very compelling. Given that I certainly
agree with you that I'm using un/likely too much. I'll re-evaluate and
update my patch series then.

Thanks,
Erez.