Re: questions on calc_delta_mine() in sched.c

Joel Schopp
2008-05-02T20:30:55Z

>> if (unlikely(!lw->inv_weight))
>> lw->inv_weight = (WMULT_CONST-lw->weight/2) / (lw->weight+1);
>>
>>
>> Q1) This code is hit often in scenarios I run, is this really unlikely for
>> others?
>
> I think it became a lot more likely recently, perhaps removing that
> unlikely is not such a bad idea.

Especially if you remove the divide in the other locations, then this might
even be likely.

>
>> Q2) The rest of the code in sched.c seems to make inv_weight ==
>> WMULT_CONST/weight and I was wondering if you could explain why this
>> instance is different.
>
> because the rest of the code is wrong, there are only 2 other sites, and
> I have a patch that removes those div64_64() with =0;
>
> The idea is to use rounding division: (x + y/2) / y
> but we can't because 'x' is touching the limits of our modulo space,
> hence we do: (x - y/2) / y
> which comes in 1 short, that fixup has been lost along the way.

Ah, that makes more sense, and the fact that it is broken explains why I
couldn't figure it out.