Re: [PATCH] Athlon/Opteron Prefetch Fix for 2.6.0test5 + numbers

[Nick Piggin]

Andi Kleen wrote:

> On Wed, 17 Sep 2003 14:53:40 +1000
> Nick Piggin <piggin@xxxxxxxxxxxxxxx> wrote:
>
>
> > The conditional compilation thing is a seperate issue. This patch may
> > have just broken a few camels' backs.
> >
> > What is intriguing to me is the "Its only a 2% slowdown of the page
> > fault for every cpu other than K[78] for this single workaround. There
>
> Erm. It helps to get your numbers right first when arguing. Why did I waste the
> time on benchmarking and collecting data when people afterwards still argue with 
> bogus numbers? @)

I was using your numbers.

> It is not 2%, but <1% [~0.6% to be exact]

100 * (3.7268 - 3.65945) / 3.65945 = 1.8 (am I wrong?)

> (probably lower the statistical error for the test, LMBench results vary more
> than that on multiple runs)
>
> Then it is not for all page faults, but only for a very narrow special
> case - a page fault that is not handled, but causes a signal. These are
> not very common. Arguably there are some applications that use this
> stuff (like generational garbage collectors), but these are not exactly
> common.

I missed that, sorry.

> In summary, it causes 0.6% slowdown in an quite obscure use case.
>
> That's small enough that it is best to just always enable it, because
> the cost of processing any support request when people forget to enable
> etc. is much greater.

I agree that with the current cpu selection arrangements it has to be
always enabled. I'm talking about Adrian's patch though.

I wasn't really arguing about it based on it causing a big slowdown. I
just don't agree with your opinion (which you're very entitled to) on
the matter.

I think that messing up your config options is a user error anyway. And
obviously a big reason for config options is so you don't have to
compile everything, are we really going to try catering for people who
make the wrong choices?

Aside, I think Adrian's patch makes cpu selection simpler for users, and
if it was combined with a warning for booting a non selected cpu I can't
see it being any more of a support problem.


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